GMT-800: Intermittent "Battery Not Charging" only while towing?

[Martinjmpr]

So now another error message has popped up. A couple of times a day, usually when starting up, I get the "Service 4wd" message on the DIC.

I replaced the entire encoder motor 2 years ago and I should stress that everything works exactly as it should.

I periodically test the 4wd system to make sure it's working and it appears to be working fine: Auto (no real way to test it other than I can hear the axle actuators going on and off if I roll the windows down), 4hi (can hear/feel the transfer case shift) 4lo (ditto) and back to 2hi.

The only other easily replaceable component I can swap out at this point is the switch itself which is about a $25 part on Amazon.

https://www.amazon.com/dp/B01L1H20V...colid=29PHGCD6GGEP&psc=1&ref_=lv_ov_lig_dp_it

The only thing that concerns me is that this might be an issue with the front axle disconnect. Is there a way to test that? What are the replaceable components?

This time of year I don't use 4wd much but my one concern is getting it stuck in 4lo while on a long camping trip. That would suck trying to limp home.

So what are the GMT800 Mafia group's thoughts? Should I replace the switch, or Is this likely just a "phantom" message on the DIC that I shouldn't be worried about?

 

[XJLI]

I higher end code reader will be able to tell you the exact code and give you more information.

 

[Martinjmpr]

UPDATE: Just got back from another long camping trip. As expected, on the way down to Ridgway (in the San Juan Mountains of SW Colorado) I was frequently annoyed by the "battery not charging" message and alternator light flashing. I was running the camper fridge on battery power, and it sucks up a lot of battery.

My assumption was that the camper fridge was the culprit, so on the way home, I put all the perishables in the TruckFridge and turned off the camper fridge. I figured this would likely cure the "battery not charging" issue.

Well, you know what they say about ASS-umptions! I kept getting the message, if anything MORE frequently. At this point I figured it has got to be a problem with the alternator. I had replaced the alternator in April 2017 with a brand new (not rebuilt) Duralast unit from AutoZone.

So after unpacking the truck, I pulled the alternator, drove over to AutuZone to have them test it. They were able to find my previous purchase record in their computer (lucky for me as I had thrown away the receipt!) and confirmed the "lifetime warranty." Then they discovered that their "alternator testing machine" wasn't working. Since the alternator had a "lifetime warranty" they decided to simply replace it for me. I reinstalled it in the driveway and it seems to be working great.

I'm hoping that the alternator was the culprit, and also may have been the cause of other intermittent error messages. I know from experience that a bad alternator on a modern vehicle can cause mystifying and inconsistent electrical glitches.

 

[zoomad75]

We just went through something similar with a customer's truck at work. 2006 (still GMT800) truck with a slide in pop-up camper. Aux battery installed in the camper with an 150 amp isolator under the hood. All it came in for was just an oil change. The customer didn't mention the charging issue during the write up. She had taken it after we completed the maintenance and a couple hours later I stepped out to grab some iced tea with my lunch and she comes stumbling through the gate to the yard and it stalls out right in front of me. I ask what the problem was and it was a dead stick. No battery charge at all and she reported the incident starting with the "battery not charging" message. Then the volt gauge did drop, multiple other warnings started coming across and ultimately it stumbled and stalled out with the dash blanking out completely.

We got it inside and started the diag on the charging system. Keep this in mind on the GMT800 charging system. It's not a straight voltage on the field wire to the alternator to get it to charge. There is a charging system module the PCM controls and the field wire runs with pulse width modulation to control the charge output of the alternator. Just jumping 12v to the F terminal won't make it charge. We could monitor the charging system through the Tech2, but it was not putting out amperage/voltage. We confirmed the control side was working, but had nothing more than weak battery voltage at the alternator output. We confirmed the need for a replacement alternator. We got one and installed it. Funny thing was we had output at the alternator now, but still had the alternator not charging message and system voltage in the scan tool data matched the volt gauge both were under 12v. The volt gauge in the cluster is just getting information via serial data so in theory the scan tool data and voltage should match. We began to backtrack and found the original charge output wire from the positive cable had been re-routed to the owner's installed battery isolator. The wiring took the heavy gauge wire and ran it to the input of the isolator from the alternator and the output of the isolator looped back to the factory positive cable. We did not have the same voltage at the input of the isolator as we did at the output back to the factory cable. As a test we divorced the isolator and hooked the factory cable back to the alternator. Bingo, the alternator is charging. The DIC message is gone and the voltgauge and scan tool data were showing 14.5v.

Turned out the Isolator was shorted internally. It had heavy signs of water pooling in the center section where the terminal studs poked out of the potting. They had located it right where the stock 2nd battery would go on the passenger side right at the base of the cowl, so I'm certain it got wet with water runoff the windsheild and cowl. The new isolator worked perfectly and we added a shield to deflect water off of it.

If your isolator has 100% of the alternator output routed to it and then back to the stock truck electrical system I'd double check the isolator. The isolator in our case was rated for 150amps. I am totaly unsure of the loads the customer had running off of the aux battery beside the horribly undersized wire they had running from the isolator back to the camper. But the loads may have been too much for the isolator AND loads of the truck like recharging the main battery much less the control systems. It's something to look at for sure.

 

[Martinjmpr]

If your isolator has 100% of the alternator output routed to it and then back to the stock truck electrical system I'd double check the isolator. The isolator in our case was rated for 150amps. I am totaly unsure of the loads the customer had running off of the aux battery beside the horribly undersized wire they had running from the isolator back to the camper. But the loads may have been too much for the isolator AND loads of the truck like recharging the main battery much less the control systems. It's something to look at for sure.

Interesting - I hadn't considered that it might be my dual battery system. However, I am not using undersized cables, in fact the + cable that runs from the main battery to the isolator, then from the isolator to the house battery, is a 2AWG welding cable with properly crimped-on connectors (thanks to forum member Dave In Denver.) Ditto for the - cable from the house battery which runs to the same chassis ground on the firewall that the main battery grounds to. Also my isolator shows no signs of unusual wear. It is bolted the firewall with jack nuts but as near as I can tell it is not exposed to weather.





And before someone says it, no, this is not what it looks like now. This photo was taken while I was still installing the dual battery system. The cables are properly secured, not just draped over the engine cover.

 

[rayra]

Pretty much the same as my isolator setup. And right down to the ground hookup for my isolator. I hooked my isolator power feed cable (1/0, went 1/0 for the secondary and the extension to the rear cargo power box) to an aftermarket terminal post on my primary battery's positive post. And my secondary battery's negative cable to a convenient hole on the top of the adjacent frame rail, pretty much emulating the negative connection of the primary battery.
But I have been thinking to instead route the isolator cable to the factory jumping terminal in the red enclosure located between the alternator and primary battery. That's where those two things properly connect, anyway. I'm thinking maybe the Secondary / isolator setup should maybe be fed from there. But I can't really explain why. Since the cable hookup at the primary positive terminal is essentially the same thing.
Maybe I'll hang my future front bumper SB175 connector (for front hitch mounted winch) and cutoff from there. Since both my Primary and Secondary terminals are already getting cluttered up pretty bad.

 

[rayra]

...
Turned out the Isolator was shorted internally. It had heavy signs of water pooling in the center section where the terminal studs poked out of the potting. They had located it right where the stock 2nd battery would go on the passenger side right at the base of the cowl, so I'm certain it got wet with water runoff the windsheild and cowl. The new isolator worked perfectly and we added a shield to deflect water off of it. ...
.

Yep, you can see it in Martin's pic, center of that cowl bracket where the curved fender support brace used to attach. That's where that plastic top and rubber hood gasket on the cowl runs out. The last 4-5" on both sides dump into the engine bay and basically down the back side of the whee tubs. It's a real bad place to put a additional fuse box or anything electrical.
Got a good look at that when I was routing the flex conduit for my cable runs to the rear. I ran both pos and negative extensions to the rear and they run right down that gap

 

[zoomad75]

The isolator that was in use on the customer's ride was like this one:

The big difference I found from your setup and my dual battery setup is that this one intercepts the charge wire from the alternator to main harness and routes it to the isolator. Your setup and mine (most really) use an isolator that is in the positive cable between the two batteries. Which I think is the key to why my customer's setup failed. It really means 100% of the loads from both the vehicle and second battery/camper are being put through this isolator. Add moisture intrusion and it was doomed for failure. When the isolator is only in the positive cable linking the two batteries, only the load of the items on the second battery are on it when the isolator is allowing the two batteries to be joined.

 

[rayra]

I think you might have that backwards. When the isolator is keyed on, the two batteries are joined and any draw could conceivably draw on both / either. Whichever has greater potential, IIRC. Or joined to both be charged by the alternator. Only with the key off are the 'house' loads only drawing from the Secondary. Providing that's how things are wired up.

With my roof solar / PWM connected at the rear, my Secondary battery is always topped up and consistently has a 'state of charge' at least half a volt higher than my Primary battery. In fact on short errand runs (my typical week-day driving), I'm probably dumping power from the secondary to the primary when they are briefly connected.
Maybe some day I'll get a much fancier isolator / combiner and run my solar feed forward where it can top off both batteries when the vehicle is parked.

I still need to re-wire my factory 'Power Ports' to my Secondary battery. I left a peltier six-pack cooler plugged in for 24hrs plus while parked a couple years ago, primary stone dead and not even enough voltage to trigger the solenoid when I keyed it on. Tried to jumper from the Secondary to the hot on the solenoid and backfed and burned out the aux fuse the solenoid was connected to. Had to self-jump with jumper cables between my two batteries. Which is yet another reason to like having both batteries under the hood. And fried the piece of small wire I was using as a jumper. Stupid mess.
I've since cleaned up some corrosion (some idiot put a steel washer between the copper cable lugs and copper studs on the isolator), used a toothbrush to coat the top of the isolator with dielectric grease, got some copper washers and once assembled I coated the connections with liquid electrical tape. So no temptation to 'jumper' anything, anymore.

Took a couple pics after my earlier post, just to illustrate the alternator / primary battery positive cable juncture. The vehicle manuals says attach here when you are jumping another vehicle.

 

[zoomad75]

Yeah, it's backward. The way this customer's setup is arranged they are interrupting 100% of the charge output and routed it to the isolator. From there with the key on the batteries are joined together. Which means all the current loads for the aux battery and vehicle are pulled through the isolator with the isolator linking both batteries together (key on).

In your setup pictured above (like mine too) the alternator is able to charge the vehicle battery and power the vehicle systems without being routed through the isolator. When engaged (both batteries linked) the aux battery getting power via the isolator, but only the loads of the aux system/house is placed on the isolator.

To show what they did can be described by using the 2nd picture in the last post as a reference. From that jump point you can see the second smaller gauge cable come off of the stud. Follow that up and to the left and you'll see its connected directly to the charge output of the alternator. So what they did was remove the charge output wire from the alternator and hook it up to the output wire of the from the isolator. They just used a bolt and nut through two ring terminals and wrapped it in electrical tape. A 2nd wire was connected to the output of the alternator and over to the input of the isolator. This is why in my mind it is backward to normal 2nd battery (aux/house) isolator setup.