PlethoraOfGuns
Adventurer
Oh, the good ole' days with steel transfer cases and gears...
Stoopid Chevy Transfer Cases!
- Thread starter snowblind
- Start date
1stDeuce
Explorer
In the trucks with the "auto" 4wd button, the front shaft does spin at road speed AT LEAST when you run with the t-case in auto mode... And it does spin all the time on trucks that don't have a CAD system, but I'm not sure which years do and which don't... But if you run them in Auto 4wd, the front is spinning, and the clutches in the t-case just engage when needed to send power that way.
That said, crap does happen. And u-joints on the front do wear out, and you get a nice squeak and a vibration... usually. Which 90% of people ignore until it fails catastrophically. Not saying the OP ignored obvious signs, just saying everyone needs to be aware that a repetitive squeak or new vibration is cause to start checking things out.
My 2006 2500 (with CAD) has 214k on it and I've never replaced a u-joint. No pump rub either. These trucks aren't junk, but metal parts do occasionally fail. Be interesting to see the offending parts... Were the caps dry, if the u-joint broke, was the break partially rusty, were there any inclusions in the forging, etc. There is a REASON it broke, and that reason isn't that GM designed a "stoopid" t-case. (Actually, Chevy just buys the T-case, and the design is either Borg Warner or New Venture Gear)
That said, crap does happen. And u-joints on the front do wear out, and you get a nice squeak and a vibration... usually. Which 90% of people ignore until it fails catastrophically. Not saying the OP ignored obvious signs, just saying everyone needs to be aware that a repetitive squeak or new vibration is cause to start checking things out.
My 2006 2500 (with CAD) has 214k on it and I've never replaced a u-joint. No pump rub either. These trucks aren't junk, but metal parts do occasionally fail. Be interesting to see the offending parts... Were the caps dry, if the u-joint broke, was the break partially rusty, were there any inclusions in the forging, etc. There is a REASON it broke, and that reason isn't that GM designed a "stoopid" t-case. (Actually, Chevy just buys the T-case, and the design is either Borg Warner or New Venture Gear)
snowblind
Adventurer
Your line about "torque taking the path of least resistance" is I think the root of the issue. Add to that a fragile magnesium case and bad things happen. No offense taken on your thoughts and I appreciate the input.
You are right about how hidden the front driveshaft is. I couldn't get a look this weekend. I will investigate more in the future and report back.
Matt
snowblind
Adventurer
Slop in the steering parts can apparently mask the symptoms. The vibration only became noticeable above 50mph AFTER I replaced my front wheel bearings and tie rods. I had the tires re-balanced as the first step in curing the problem. You might want to double check the condition of your front driveshaft. I had mine "inspected" by GM Service and they never found anything.
The U-joint didn't break. It developed enough play to allow the shaft to rotate in an oval motion. The driveshaft yoke HIT the transfer case by moving upwards and over towards the passenger side. This was a long-term wear issue that coincided with the wear of the transfer case front output bearing. It could have been that the bearing caused the U-joint to fail or vice versa. At the moment of driveshaft impact we were driving on a worn out section of I-80 with a lot of bumps and undulations. The frequency of the bumps caused the front driveshaft to begin some sort eccentric rotation that it had never done before and with each rotation it made contact with the transfer case housing.
It is important to note that the vehicle wasn't disabled. AFTER the impact I pulled over and looked for a problem. Finding nothing apparent I began driving slowly and finished the last 50 miles @ 70mph with no vibrations or noises. It was only after I pulled into town and hosed the truck and trailer off I started hearing NASTY noises coming from the transfer case and headed for4 the shop. With the case removed we realized that something had cracked the top of the case from inside! The direct cause is that the worn output bearing wore out the magnesium T-Case housing.
GM makes mistakes in design and spec all the time. Sometimes they correct. Other times it's just business. GM went to magnesium T-case housings to save weight. On a 1500 SUV maybe that makes sense for fuel economy and performance. On a 2500 SUV/Truck I think it makes a lot less sense. Hell, maybe with a stronger case I would have felt the output bearing eating it away!
I'm back on the road now just a few $$ poorer. Going to look into rebuilding the rear driveshaft and what other things I might need to proactively replace to preserve this investment.
Matt
snowblind
Adventurer
Found an interesting GM Bulletin on the NVG 246 and how it handles the different 4WD modes. It has more complicated processes than I expected. I'm guessing a lot of the software controlled torque biasing is only on the 2003 and later trucks.
The front shaft IS NOT spinning at road speed in "2WD" mode. (But probably is rotating).
The front shaft IS spinning at road speed when in "Auto" mode but gets no torque. (If the rear spins faster THEN torque is sent to the front driveshaft.)
T-Case uses a wet clutch to vary the amount of torque to the front driveshaft.
It's sort of like as close to AWD as you can get while still being "part-time".
I'm going to mess around with the "Auto" and "2WD" buttons in a controlled test. I am interested to see if I can hear or feel a difference when the front driveshaft "disconnects."
Matt
Transfer Case Description and Operation
General Operation
The New Venture Gear model NVG 246 RPO NP8 transfer case is a two speed
automatic, active, transfer case. The NVG 246 EAU provides 5 modes, Auto
4WD, 4HI, 4LO, 2HI and Neutral. The Auto 4WD position allows the capability
of an active transfer case, which provides the benefits of on-demand torque
biasing wet clutch and easy vehicle tuning through software calibrations.
The software calibrations allow more features such as flexible adapt ready
position and clutch preload torque levels. The technology allows for vehicle
speed dependent clutch torque levels to enhance the performance of the
system. For example, the system is calibrated to provide 0-5 ft lb of clutch
torque during low speed, low engine torque operation, and predetermined
higher torque for 40 km/h (25 mph) and greater. This prevents crow-hop and
binding at low speeds and provides higher torque biases at higher vehicle
speeds, in order to enhance stability.
Transfer Case Shift Control Switch
The NVG 246 EAU transfer case features a 4 button shift control switch
located on the instrument panel. When the ignition key is in the RUN
position, the transfer case shift control module monitors the transfer case
shift control switch to determine if the driver desires a new mode/range
position. At a single press of the transfer case shift control switch, the
lamp of the new desired position will begin flashing to inform the driver
that the transfer case shift control module has received the request for a
new mode/range position. The lamp will continue to flash until all shifting
criteria has been met and the new mode/range position has been reached, or
has been engaged. Once the new mode/range position is fully active, the
switch indicator lamp for the new position will remain ON constantly.
During normal driving situations, the transfer case can operate in the Auto
4WD mode. In the Auto 4WD mode, the transfer case shift control module
monitors rear wheel slip speed, based on the inputs from both the front and
rear propshaft speed sensors. When the vehicle experiences a rear wheel slip
condition, the transfer case shift control module sends a pulse width
modulated (PWM) signal to an electronic motor, which is the transfer case
encoder motor. This motor rotates the transfer case control actuator lever
shaft, applying a clutch pack. This clutch pack is designed to deliver a
variable amount of torque, normally delivered to the rear wheels, and
transfers it to the front wheels. Torque is ramped up to the front wheels
until the front propshaft speed sensor matches that of the rear propshaft
speed sensor. Torque is ramped down to the front wheels. The process would
repeat if rear wheel slip is detected again.
The NVG 246 EAU transfer case has the added feature of also providing the
driver with 3 manual mode/range positions:
a.. 4HI - 4 Wheel Drive high range
b.. 2HI - 2 Wheel Drive high range
c.. 4LO - 4 Wheel Drive low range
The driver may choose to select any of these mode/range positions while
driving the vehicle. However, the transfer case will not allow a shift into
or out of 4LO unless the following criteria has been met:
a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
This transfer case also has a Neutral position. A shift to the Neutral
position allows the vehicle to be towed without rotating the transmission
output shaft. Neutral position may be obtained only if the following
criteria has been met:
a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
d.. The transfer case is in 2HI mode.
Once these conditions have been met, press and hold both the 2HI and 4LO
buttons for 10 seconds. When the system completes the shift to neutral, the
red neutral lamp will illuminate.
The NVG 246 EAU case halves are high-pressure die-cast magnesium. Ball
bearings support the input shaft, the front output shaft, and the rear
output shaft. A thrust bearing is located inside of the input shaft gear to
support the front of the rear output shaft. The transfer case requires Auto
Trac® II Fluid GM P/N 12378508 (Canadian P/N 10953626) which is blue in
color. The fluid is designed for smooth clutch application. An oil pump,
driven by the rear output shaft, pumps the fluid through the rear output
shaft oil gallery to the clutch and bearings.
There are two versions of the NVG 246 EAU, which depend on the transmission
applications and vehicle applications. If the vehicle is equipped with a
transmission RPO M30, the transmission splines in the input gear will have
27 teeth. With this application the planetary carrier assembly will have 3
pinion gears. If the vehicle is equipped with transmission RPO MT1 or MN8,
the transmission splines in the input gear will have 32 teeth. The planetary
carrier assembly on this application will have 6 pinion gears.
The front shaft IS NOT spinning at road speed in "2WD" mode. (But probably is rotating).
The front shaft IS spinning at road speed when in "Auto" mode but gets no torque. (If the rear spins faster THEN torque is sent to the front driveshaft.)
T-Case uses a wet clutch to vary the amount of torque to the front driveshaft.
It's sort of like as close to AWD as you can get while still being "part-time".
I'm going to mess around with the "Auto" and "2WD" buttons in a controlled test. I am interested to see if I can hear or feel a difference when the front driveshaft "disconnects."
Matt
Transfer Case Description and Operation
General Operation
The New Venture Gear model NVG 246 RPO NP8 transfer case is a two speed
automatic, active, transfer case. The NVG 246 EAU provides 5 modes, Auto
4WD, 4HI, 4LO, 2HI and Neutral. The Auto 4WD position allows the capability
of an active transfer case, which provides the benefits of on-demand torque
biasing wet clutch and easy vehicle tuning through software calibrations.
The software calibrations allow more features such as flexible adapt ready
position and clutch preload torque levels. The technology allows for vehicle
speed dependent clutch torque levels to enhance the performance of the
system. For example, the system is calibrated to provide 0-5 ft lb of clutch
torque during low speed, low engine torque operation, and predetermined
higher torque for 40 km/h (25 mph) and greater. This prevents crow-hop and
binding at low speeds and provides higher torque biases at higher vehicle
speeds, in order to enhance stability.
Transfer Case Shift Control Switch
The NVG 246 EAU transfer case features a 4 button shift control switch
located on the instrument panel. When the ignition key is in the RUN
position, the transfer case shift control module monitors the transfer case
shift control switch to determine if the driver desires a new mode/range
position. At a single press of the transfer case shift control switch, the
lamp of the new desired position will begin flashing to inform the driver
that the transfer case shift control module has received the request for a
new mode/range position. The lamp will continue to flash until all shifting
criteria has been met and the new mode/range position has been reached, or
has been engaged. Once the new mode/range position is fully active, the
switch indicator lamp for the new position will remain ON constantly.
During normal driving situations, the transfer case can operate in the Auto
4WD mode. In the Auto 4WD mode, the transfer case shift control module
monitors rear wheel slip speed, based on the inputs from both the front and
rear propshaft speed sensors. When the vehicle experiences a rear wheel slip
condition, the transfer case shift control module sends a pulse width
modulated (PWM) signal to an electronic motor, which is the transfer case
encoder motor. This motor rotates the transfer case control actuator lever
shaft, applying a clutch pack. This clutch pack is designed to deliver a
variable amount of torque, normally delivered to the rear wheels, and
transfers it to the front wheels. Torque is ramped up to the front wheels
until the front propshaft speed sensor matches that of the rear propshaft
speed sensor. Torque is ramped down to the front wheels. The process would
repeat if rear wheel slip is detected again.
The NVG 246 EAU transfer case has the added feature of also providing the
driver with 3 manual mode/range positions:
a.. 4HI - 4 Wheel Drive high range
b.. 2HI - 2 Wheel Drive high range
c.. 4LO - 4 Wheel Drive low range
The driver may choose to select any of these mode/range positions while
driving the vehicle. However, the transfer case will not allow a shift into
or out of 4LO unless the following criteria has been met:
a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
This transfer case also has a Neutral position. A shift to the Neutral
position allows the vehicle to be towed without rotating the transmission
output shaft. Neutral position may be obtained only if the following
criteria has been met:
a.. The engine is running.
b.. The automatic transmission is in Neutral.
c.. The vehicle speed is below 5 km/h (3 mph).
d.. The transfer case is in 2HI mode.
Once these conditions have been met, press and hold both the 2HI and 4LO
buttons for 10 seconds. When the system completes the shift to neutral, the
red neutral lamp will illuminate.
The NVG 246 EAU case halves are high-pressure die-cast magnesium. Ball
bearings support the input shaft, the front output shaft, and the rear
output shaft. A thrust bearing is located inside of the input shaft gear to
support the front of the rear output shaft. The transfer case requires Auto
Trac® II Fluid GM P/N 12378508 (Canadian P/N 10953626) which is blue in
color. The fluid is designed for smooth clutch application. An oil pump,
driven by the rear output shaft, pumps the fluid through the rear output
shaft oil gallery to the clutch and bearings.
There are two versions of the NVG 246 EAU, which depend on the transmission
applications and vehicle applications. If the vehicle is equipped with a
transmission RPO M30, the transmission splines in the input gear will have
27 teeth. With this application the planetary carrier assembly will have 3
pinion gears. If the vehicle is equipped with transmission RPO MT1 or MN8,
the transmission splines in the input gear will have 32 teeth. The planetary
carrier assembly on this application will have 6 pinion gears.
underdrive
jackwagon
Well, for the record Ford has been using magnesium transfer cases since the late 80s. They are not weak by any means, actually it's quite amazing what kind of abuse they can tolerate provided they remain lubricated thru the ordeal. We had a BW1356 die once due to the typical for that model oil pump failure - pump keeper arm rubs thru the case internally and allows pump to spin with the rear output shaft till said keeper arm hits the shift forks, it's all downhill fast from there on out and unlike GM's pump rub this all happens inside the t-case with no visible signs of trouble on the outside till all hell lets loose. Some fierce carnage was found inside, still case did not split.
The type of failure you experienced is something that no t-case can be designed to handle, and typically it would never come to it in the first place as the driveshaft will vibrate quite noticeably well before the bearing lets go. This should have been caught early on while the U-joint was still wearing out, your front end issue masked it - no fault of your own there, it was a pure coincidence. You as an owner and operator of the truck actually did your part by noticing the vibration afterwards and bringing it up with GM Service. And this is who failed you, it wasn't the engineers who designed the t-case, but rather the "techs" who were supposed to service the truck. If you want to place blame place it on the untrained monkey who missed the loose U-joint - idk what they got taught in "tech" school, or how hard their boss hawked them to turn and burn, but that driveshaft should have been removed from the truck and then any U-joint issues would have become apparent...
So basically, you got screwed by someone who was supposed to help you. Any chance you could stick the dealership with the repair bill?
The type of failure you experienced is something that no t-case can be designed to handle, and typically it would never come to it in the first place as the driveshaft will vibrate quite noticeably well before the bearing lets go. This should have been caught early on while the U-joint was still wearing out, your front end issue masked it - no fault of your own there, it was a pure coincidence. You as an owner and operator of the truck actually did your part by noticing the vibration afterwards and bringing it up with GM Service. And this is who failed you, it wasn't the engineers who designed the t-case, but rather the "techs" who were supposed to service the truck. If you want to place blame place it on the untrained monkey who missed the loose U-joint - idk what they got taught in "tech" school, or how hard their boss hawked them to turn and burn, but that driveshaft should have been removed from the truck and then any U-joint issues would have become apparent...
So basically, you got screwed by someone who was supposed to help you. Any chance you could stick the dealership with the repair bill?
snowblind
Adventurer
I never said magnesium was weak. I said it was used to save weight. If not designed/used properly problems can occur. EG: Replacement tail cones for these transfer cases are aluminum.
Front end sort of masked it. In reality everything wore out at about the same point. 105K equaled new wheel bearings, new tierods, new pitman, new idler and... new front drive shaft and transfer case. None of the front end parts were "that" bad.
I'm thinking the bearing was going out since before I bought the truck at 80k. I have confirmed that the system has the front driveshaft rotating (via direct drive or indirect torque) when the truck is in motion. The driveshaft angles and overall design can minimize vibration so well that unless the shaft is out of balance it may be very hard to notice the worn U-joint. The soft magnesium case further masked the vibrations as it wire away. The U-joint, output bearing AND transfer case were already severely worn when the problem was finally revealed.
I'm not looking to stick anyone with the bill. I got "screwed" by a prematurely failing U-Joint. BUT... put that same failing U-joint in a differently designed system and maybe it doesn't take the transfer case with it when it develops play.
GM made a number of design compromises with their IFS trucks. Some will never buy a GM IFS truck because of these choices. I used the term "stoopid" to in part jest (I DO know how to spell) but only partly. Spend some time researching transfer case failures in GMT800 trucks and you will find plenty of problems. There is no reason to think that these problems have ZERO relation to design.
In the end I'm hoping that others will benefit from my problem. Check those U-Joints people!
Matt
snowblind
Adventurer
What is the front driveshaft really doing in 2WD?
Hey Guys.
I performed a few tests this weekend to see if I could perceive any engagement/disengagement using shift-on-the-fly 4WD. I was stuck in the city so I did not do any true 4WD testing but I did play with the Auto/2WD settings and discovered some interesting things.
In "Auto" mode the front wheels and drive shaft ARE spinning at the same speed as the the rear wheels and drive shaft. This allows almost instant auto switching to 4WD.
In "2WD" mode the front wheels and drive shaft ARE NOT spinning at exactly same speed as the the rear wheels and drive shaft. They are spinning though. Just not at exactly the same speed.
When switching from 2WD to Auto there is a variable time to engagement:
Based on the above I think the front drive shaft is spinning at 50-75% of the speed of the rear drive shaft.
Matt
Hey Guys.
I performed a few tests this weekend to see if I could perceive any engagement/disengagement using shift-on-the-fly 4WD. I was stuck in the city so I did not do any true 4WD testing but I did play with the Auto/2WD settings and discovered some interesting things.
In "Auto" mode the front wheels and drive shaft ARE spinning at the same speed as the the rear wheels and drive shaft. This allows almost instant auto switching to 4WD.
In "2WD" mode the front wheels and drive shaft ARE NOT spinning at exactly same speed as the the rear wheels and drive shaft. They are spinning though. Just not at exactly the same speed.
When switching from 2WD to Auto there is a variable time to engagement:
- When stopped the switch happens almost instantly.
- When driving the switch can take anywhere from a 1/2 a second till almost 3 secs.
- The switch time decreases significantly on multiple, subsequent shifts. eg: 2WD-Auto-2WD-Auto
- There is no perceptible change in drive train noise, vibration, etc when switching from 2WD to Auto
Based on the above I think the front drive shaft is spinning at 50-75% of the speed of the rear drive shaft.
Matt
PGW
Observer
That sucks about your transfer case. I wonder why the driveshaft is spinning when not in 4wd. If the diff actuator is working correctly the driver side axle should just be spinning the spider gears in the diff.
When I was doing research on the GMT800 platform the biggest deficiencies I noted were the 4wd system (electronic diff actuator, pump-rub issues, and electronic t-case shifter) and the steering. That is why I have a 32 spline NP241 in my garage waiting to be rebuilt. I'm also going to install a GMT800 floor shifter (the center console must be trimmed) and a posi-lok in the front diff to completely delete the electronic 4wd system. I have a 2003 Yukon XL 8.1L and the one time I took it off road so far (I'm building it up now) the diff actuator decided to take a nap and I got stuck in sand on the beach. Can't wait to throw out my autotrac case and electric diff actuator.
When I was doing research on the GMT800 platform the biggest deficiencies I noted were the 4wd system (electronic diff actuator, pump-rub issues, and electronic t-case shifter) and the steering. That is why I have a 32 spline NP241 in my garage waiting to be rebuilt. I'm also going to install a GMT800 floor shifter (the center console must be trimmed) and a posi-lok in the front diff to completely delete the electronic 4wd system. I have a 2003 Yukon XL 8.1L and the one time I took it off road so far (I'm building it up now) the diff actuator decided to take a nap and I got stuck in sand on the beach. Can't wait to throw out my autotrac case and electric diff actuator.
snowblind
Adventurer
If everything is working correctly then there is limited direct torque but I think it is a mistake to think there is NO rotation. If you imagine a truck going down the road with engine spinning, trans spinning, 1/2 the transfer spinning, rear drive shaft spinning, rear axles spinning, all four tires/wheels/brakes spinning, etc, etc, it makes sense that a smooth spinning drive shaft could be rotating sympathetically. As Underdrive pointed out the design of this system "it on torque generated by the wheels taking the path of least resistance, which as you found out isn't always the one the engineers intended."
That was the point behind my testing of the AutoTrac system engagement times and posting the differences. The front drive-shaft is NOT at the exact same speed as the rear in 2WD but the engagement happens quickly enough to indicate that it is not motionless either.
FYI - There is also an "on-demand torque biasing wet clutch" in the autotrac transfer case.
Dude. That old case will get you $300+ as a core!
That Posi-lock looks cool. Basically the modern equivalent of locking/unlocking hubs. I couldn't see if the posi-lock could be actuated when moving? Do you know?
Either way, looks like posi-lock will keep the front drive shaft spinning at the same speed as the rear and allow SOTF 4WD while removing some failure prone components.
Do yourself a favor and rebuild those driveshafts too! Mine was <$100 for bearings and balance on the front.
Matt
1stDeuce
Explorer
Well, I'm glad you got your truck fixed. I'm going to second what Underdrive said, and add a bit of engineering insight... Well, I'm an engineer, so it'll probably be more information than you wanted... This insight is based on my experience from 20 years in the auto industry, mostly working with 4wd systems. I do not wish to offend or start a pissing contest, only to provide useful information.
First, and very importantly, cast magnesium is NOT soft. At it's worst, it is very comparable to cast aluminum, but usually it's slightly harder and a little stronger. It is also lighter, which is EXACTLY why it's used instead of cast aluminum or iron. It certainly didn't mask any failure by "absorbing vibrations"... Not anymore than an aluminum or iron case would have, anyway. The t-case being of magnesium instead of aluminum or even iron was NOT your problem, and the material of the housing likely didn't contribute AT ALL to the failure you experienced.
I've seen similar failures in Jeeps (with aluminum t-case housings!) so I'd bet my dog (and I do love her dearly!) that your failure was due to one of two things... Either a really bad u-joint let go at road speed, causing much carnage, or more likely, you had a bad bearing/seal on the front bearing of the t-case, which would have allowed all the fluid to leak out (The seals won't last at all if the bearing is even just a little bad...) and thus your t-case was eventually (and likely fairly quickly) REALLY low on fluid. Chain drive t-cases won't go 100 miles once the fluid is gone before they pretty much grenade, and it doesn't matter who made them, or what they're made of. With no fluid, the chain stretches, and then piles up between the bottom sprocket and the housing, and BANG. Busted case, u-joints, driveshaft, wiring, fuel lines, etc... Basically big mess...
Definitely sucks for you, and anyone else it might happen to, but it's REALLY uncommon, and piling on to GM for using magnesium is pretty far off base. Your failure was, one way or another, crappy timing, or a missed maintenance issue. I'm not saying it's your fault at all, and as Underdrive pointed out, if the U-joint or seal was bad, and a dealer or other mechanic missed it, you should at least let them know you won't be coming back to their business. Ever.
As for the spinning front driveshaft, the CAD actuator slides a splined collar that engages the right half shaft. Because it's just splines, it cannot engage until the driveshaft is spun up by the t-case. So when you hit the auto button from 2wd, the process is: Clutches in the t-case spin up the front driveshaft to road speed (on an older truck, you usually hear a "thump" if you're going faster than 45mph or so), CAD actuator gets power and runs its motor to slide a splined collar and connect the right half shaft into one piece, t-case disengages clutches and begins monitoring for slip. If you shift to 4wd instead of auto, it just leaves the t-case clutch engaged. In a truck that doesn't have auto, or with a manual shifter on the floor, there is a synchronizer in the t-case just like in a manual trans that spins up the front driveline so it can engage 4wd. Once the t-case is in 4wd, the CAD actuator is energized to engage the front axle. The CAD engages best when you're actually moving. At a stop it may not engage at all if the splines collide instead of line up. That's why you should engage 4wd before you need it while you're still moving, or at least rock the vehicle GENTLY a few times if stopped to ensure full engagement of the splines before you resort to "a whole lot of flooring" to extricate yourself. A partially engaged CAD can strip splines, and then it's big money to fix!
The CAD is fairly slow to disengage if you're moving, as drag makes it hard to slide the splined collar, and it's only disengaged by a light spring. So, if you engage, then dis-engage, but then re-engage right away, the CAD doesn't have time to actually dis-engage, and the t-case can see that it's already spinning at speed, so the process is pretty much done already. (Much faster than starting the process with the CAD disengaged.)
As for reliability, it is my experience that the CAD actuator on the ~'98+ 400 and all 800/900 series trucks is hardly EVER the issue. The old style (~'97 down) heated gas chamber unit on 400 trucks was known to have issues though, particularly in cold climates, or on older trucks. There is a retrofit kit to use the motor driven unit on older trucks that I HIGHLY recommend if you have an older 400 series truck. There is also a solid billet unit that will just keep it engaged all the time. Good for ranch trucks, snow plowing, etc, but probably not for full time daily driver use.
Installing a Posi-Lock in place of the electric CAD does make it a manual process, and allows you to just leave the front engaged, or leave it dis-engaged for 2wd low if you need low speed to maneuver, but don't want the binding that happens in 4wd. (Useful for backing trailers) In 2wd, the Posi-lock can ONLY be engaged if you are stopped. You can engage on the fly by shifting to 4wd first, then engaging the Posi-lock.
As for the driveline spinning, in 2wd, the driveshaft is spun only by the friction/oil drag that's in the front differential and t-case, not by any mechanical means. (Just like on any older 4wd truck, even with true lockout hubs...) The ones I've watched on chassis rolls start spinning at about 25mph, and spin fairly slowly at pretty much all speeds. It may spin a bit more with the "auto" t-case, as the clutches also add some to the rotation, particularly on a new t-case, which is what you have now... it'll slow as you get some miles on the t-case and the clutches loosen up. Regardless of the speed, since there's no load, there's relatively little wear anywhere, and on the plus side, it serves to keep u-joints, seals, and differential parts oiled up and moving freely for people who seldom engage 4wd.
Lastly, the most common problem with any electric shift t-case is that people NEVER use it, then expect it to work once in a blue moon. I recommend that you cycle through all the shift positions a few times once a month. Mostly, this keeps the range sensor in the actuator working, so the shift controller knows what range it's shifted to. As the grease in the range sensor gets old, it can get a little stiff and cause an intermittent contact, which results in the t-case not knowing what range it has entered. Usually shifting it several times will get it working again, but using it occasionally will also keep the grease spread out, and provide you with trouble free operation. NEVER using it will almost certainly result in disappointment when you actually need it to work... (Read into the sinking of the Bounty for a good example of how not using equipment to keep it "fresh" is a terrible way to ensure it'll work when you need it...)
Whew. My fingers hurt... I hope everyone finds this information to be helpful, and not hurtful, as that's how I meant it... And hopefully my facts (and experience based opinions) are not too offensive.
And as usual, Your Mileage May Vary...
Chris
First, and very importantly, cast magnesium is NOT soft. At it's worst, it is very comparable to cast aluminum, but usually it's slightly harder and a little stronger. It is also lighter, which is EXACTLY why it's used instead of cast aluminum or iron. It certainly didn't mask any failure by "absorbing vibrations"... Not anymore than an aluminum or iron case would have, anyway. The t-case being of magnesium instead of aluminum or even iron was NOT your problem, and the material of the housing likely didn't contribute AT ALL to the failure you experienced.
I've seen similar failures in Jeeps (with aluminum t-case housings!) so I'd bet my dog (and I do love her dearly!) that your failure was due to one of two things... Either a really bad u-joint let go at road speed, causing much carnage, or more likely, you had a bad bearing/seal on the front bearing of the t-case, which would have allowed all the fluid to leak out (The seals won't last at all if the bearing is even just a little bad...) and thus your t-case was eventually (and likely fairly quickly) REALLY low on fluid. Chain drive t-cases won't go 100 miles once the fluid is gone before they pretty much grenade, and it doesn't matter who made them, or what they're made of. With no fluid, the chain stretches, and then piles up between the bottom sprocket and the housing, and BANG. Busted case, u-joints, driveshaft, wiring, fuel lines, etc... Basically big mess...
Definitely sucks for you, and anyone else it might happen to, but it's REALLY uncommon, and piling on to GM for using magnesium is pretty far off base. Your failure was, one way or another, crappy timing, or a missed maintenance issue. I'm not saying it's your fault at all, and as Underdrive pointed out, if the U-joint or seal was bad, and a dealer or other mechanic missed it, you should at least let them know you won't be coming back to their business. Ever.
As for the spinning front driveshaft, the CAD actuator slides a splined collar that engages the right half shaft. Because it's just splines, it cannot engage until the driveshaft is spun up by the t-case. So when you hit the auto button from 2wd, the process is: Clutches in the t-case spin up the front driveshaft to road speed (on an older truck, you usually hear a "thump" if you're going faster than 45mph or so), CAD actuator gets power and runs its motor to slide a splined collar and connect the right half shaft into one piece, t-case disengages clutches and begins monitoring for slip. If you shift to 4wd instead of auto, it just leaves the t-case clutch engaged. In a truck that doesn't have auto, or with a manual shifter on the floor, there is a synchronizer in the t-case just like in a manual trans that spins up the front driveline so it can engage 4wd. Once the t-case is in 4wd, the CAD actuator is energized to engage the front axle. The CAD engages best when you're actually moving. At a stop it may not engage at all if the splines collide instead of line up. That's why you should engage 4wd before you need it while you're still moving, or at least rock the vehicle GENTLY a few times if stopped to ensure full engagement of the splines before you resort to "a whole lot of flooring" to extricate yourself. A partially engaged CAD can strip splines, and then it's big money to fix!
The CAD is fairly slow to disengage if you're moving, as drag makes it hard to slide the splined collar, and it's only disengaged by a light spring. So, if you engage, then dis-engage, but then re-engage right away, the CAD doesn't have time to actually dis-engage, and the t-case can see that it's already spinning at speed, so the process is pretty much done already. (Much faster than starting the process with the CAD disengaged.)
As for reliability, it is my experience that the CAD actuator on the ~'98+ 400 and all 800/900 series trucks is hardly EVER the issue. The old style (~'97 down) heated gas chamber unit on 400 trucks was known to have issues though, particularly in cold climates, or on older trucks. There is a retrofit kit to use the motor driven unit on older trucks that I HIGHLY recommend if you have an older 400 series truck. There is also a solid billet unit that will just keep it engaged all the time. Good for ranch trucks, snow plowing, etc, but probably not for full time daily driver use.
Installing a Posi-Lock in place of the electric CAD does make it a manual process, and allows you to just leave the front engaged, or leave it dis-engaged for 2wd low if you need low speed to maneuver, but don't want the binding that happens in 4wd. (Useful for backing trailers) In 2wd, the Posi-lock can ONLY be engaged if you are stopped. You can engage on the fly by shifting to 4wd first, then engaging the Posi-lock.
As for the driveline spinning, in 2wd, the driveshaft is spun only by the friction/oil drag that's in the front differential and t-case, not by any mechanical means. (Just like on any older 4wd truck, even with true lockout hubs...) The ones I've watched on chassis rolls start spinning at about 25mph, and spin fairly slowly at pretty much all speeds. It may spin a bit more with the "auto" t-case, as the clutches also add some to the rotation, particularly on a new t-case, which is what you have now... it'll slow as you get some miles on the t-case and the clutches loosen up. Regardless of the speed, since there's no load, there's relatively little wear anywhere, and on the plus side, it serves to keep u-joints, seals, and differential parts oiled up and moving freely for people who seldom engage 4wd.
Lastly, the most common problem with any electric shift t-case is that people NEVER use it, then expect it to work once in a blue moon. I recommend that you cycle through all the shift positions a few times once a month. Mostly, this keeps the range sensor in the actuator working, so the shift controller knows what range it's shifted to. As the grease in the range sensor gets old, it can get a little stiff and cause an intermittent contact, which results in the t-case not knowing what range it has entered. Usually shifting it several times will get it working again, but using it occasionally will also keep the grease spread out, and provide you with trouble free operation. NEVER using it will almost certainly result in disappointment when you actually need it to work... (Read into the sinking of the Bounty for a good example of how not using equipment to keep it "fresh" is a terrible way to ensure it'll work when you need it...)
Whew. My fingers hurt... I hope everyone finds this information to be helpful, and not hurtful, as that's how I meant it... And hopefully my facts (and experience based opinions) are not too offensive.
And as usual, Your Mileage May Vary...
Chris
Last edited:
Watt maker
Active member
Well, I'm glad you got your truck fixed. I'm going to second what Underdrive said, and add a bit of engineering insight... Well, I'm an engineer, so it'll probably be more information than you wanted... This insight is based on my experience from 20 years in the auto industry, mostly working with 4wd systems. I do not wish to offend or start a pissing contest, only to provide useful information.
First, and very importantly, cast magnesium is NOT soft. At it's worst, it is very comparable to cast aluminum, but usually it's slightly harder and a little stronger. It is also lighter, which is EXACTLY why it's used instead of cast aluminum or iron. It certainly didn't mask any failure by "absorbing vibrations"... Not anymore than an aluminum or iron case would have, anyway. The t-case being of magnesium instead of aluminum or even iron was NOT your problem, and the material of the housing likely didn't contribute AT ALL to the failure you experienced.
I've seen similar failures in Jeeps (with aluminum t-case housings!) so I'd bet my dog (and I do love her dearly!) that your failure was due to one of two things... Either a really bad u-joint let go at road speed, causing much carnage, or more likely, you had a bad bearing/seal on the front bearing of the t-case, which would have allowed all the fluid to leak out (The seals won't last at all if the bearing is even just a little bad...) and thus your t-case was eventually (and likely fairly quickly) REALLY low on fluid. Chain drive t-cases won't go 100 miles once the fluid is gone before they pretty much grenade, and it doesn't matter who made them, or what they're made of. With no fluid, the chain stretches, and then piles up between the bottom sprocket and the housing, and BANG. Busted case, u-joints, driveshaft, wiring, fuel lines, etc... Basically big mess...
Definitely sucks for you, and anyone else it might happen to, but it's REALLY uncommon, and piling on to GM for using magnesium is pretty far off base. Your failure was, one way or another, crappy timing, or a missed maintenance issue. I'm not saying it's your fault at all, and as Underdrive pointed out, if the U-joint or seal was bad, and a dealer or other mechanic missed it, you should at least let them know you won't be coming back to their business. Ever.
As for the spinning front driveshaft, the CAD actuator slides a splined collar that engages the right half shaft. Because it's just splines, it cannot engage until the driveshaft is spun up by the t-case. So when you hit the auto button from 2wd, the process is: Clutches in the t-case spin up the front driveshaft to road speed (on an older truck, you usually hear a "thump" if you're going faster than 45mph or so), CAD actuator gets power and runs its motor to slide a splined collar and connect the right half shaft into one piece, t-case disengages clutches and begins monitoring for slip. If you shift to 4wd instead of auto, it just leaves the t-case clutch engaged. In a truck that doesn't have auto, or with a manual shifter on the floor, there is a synchronizer in the t-case just like in a manual trans that spins up the front driveline so it can engage 4wd. Once the t-case is in 4wd, the CAD actuator is energized to engage the front axle. The CAD engages best when you're actually moving. At a stop it may not engage at all if the splines collide instead of line up. That's why you should engage 4wd before you need it while you're still moving, or at least rock the vehicle GENTLY a few times if stopped to ensure full engagement of the splines before you resort to "a whole lot of flooring" to extricate yourself. A partially engaged CAD can strip splines, and then it's big money to fix!
The CAD is fairly slow to disengage if you're moving, as drag makes it hard to slide the splined collar, and it's only disengaged by a light spring. So, if you engage, then dis-engage, but then re-engage right away, the CAD doesn't have time to actually dis-engage, and the t-case can see that it's already spinning at speed, so the process is pretty much done already. (Much faster than starting the process with the CAD disengaged.)
As for reliability, it is my experience that the CAD actuator on the ~'98+ 400 and all 800/900 series trucks is hardly EVER the issue. The old style (~'97 down) heated gas chamber unit on 400 trucks was known to have issues though, particularly in cold climates, or on older trucks. There is a retrofit kit to use the motor driven unit on older trucks that I HIGHLY recommend if you have an older 400 series truck. There is also a solid billet unit that will just keep it engaged all the time. Good for ranch trucks, snow plowing, etc, but probably not for full time daily driver use.
Installing a Posi-Lock in place of the electric CAD does make it a manual process, and allows you to just leave the front engaged, or leave it dis-engaged for 2wd low if you need low speed to maneuver, but don't want the binding that happens in 4wd. (Useful for backing trailers) In 2wd, the Posi-lock can ONLY be engaged if you are stopped. You can engage on the fly by shifting to 4wd first, then engaging the Posi-lock.
As for the driveline spinning, in 2wd, the driveshaft is spun only by the friction/oil drag that's in the front differential and t-case, not by any mechanical means. (Just like on any older 4wd truck, even with true lockout hubs...) The ones I've watched on chassis rolls start spinning at about 25mph, and spin fairly slowly at pretty much all speeds. It may spin a bit more with the "auto" t-case, as the clutches also add some to the rotation, particularly on a new t-case, which is what you have now... it'll slow as you get some miles on the t-case and the clutches loosen up. Regardless of the speed, since there's no load, there's relatively little wear anywhere, and on the plus side, it serves to keep u-joints, seals, and differential parts oiled up and moving freely for people who seldom engage 4wd.
Lastly, the most common problem with any electric shift t-case is that people NEVER use it, then expect it to work once in a blue moon. I recommend that you cycle through all the shift positions a few times once a month. Mostly, this keeps the range sensor in the actuator working, so the shift controller knows what range it's shifted to. As the grease in the range sensor gets old, it can get a little stiff and cause an intermittent contact, which results in the t-case not knowing what range it has entered. Usually shifting it several times will get it working again, but using it occasionally will also keep the grease spread out, and provide you with trouble free operation. NEVER using it will almost certainly result in disappointment when you actually need it to work... (Read into the sinking of the Bounty for a good example of how not using equipment to keep it "fresh" is a terrible way to ensure it'll work when you need it...)
Whew. My fingers hurt... I hope everyone finds this information to be helpful, and not hurtful, as that's how I meant it... And hopefully my facts (and experience based opinions) are not too offensive.
And as usual, Your Mileage May Vary...
Chris
Excellent info, thank you!
highdesertranger
Adventurer
ah yes modern doo dads. highdesertranger
underdrive
jackwagon
We know of a 1-ton truck with a windshield banner that reads "Can't shift this!" (MC Hammer's "Can't touch this" anyone?) - it's got a manual trans with the factory shift knob replaced by a Road Ranger shift valve with a "range" paddle and a "splitter" slider and an aftermarket lower cover sleeve allowing installation of an electric rocker switch
And all three of them control something driveline-related. Your fleet guy would probably have a stroke if he had to deal with something like that :Wow1:And nowadays we can't even get a stick-shift in a non-Dodge truck... what a sad state of vehicular affairs.