Chevrolet Express Van - AWD to 4x4 conversion

[zuren]

I wanted to take a moment to update this thread, answer some questions for archival purposes, and apologize for being MIA. Life happened and my time had to be focused in other directions.

Forum member jknight sent me a private message and with his permission I am posting my response here for the benefit of all:

Ive been reading your post about the conversion, because i literally plan on doing the same thing soon. I actually work on cars but am not familiar with certain things, such as specific models of transfer cases and gear rations on differentials. I just never got into it, and reading your post has made me realize that you had to know a little something about it to decide on a transfer case.

So my first question is in relation the the transfer case. You used a NP241, and talked about some worries with using a NP241c. After the install and everything, do you still hold that same view, as far as the cross member and the exhaust getting in the way with the NP241c. I guess in short, my question is, do you think it would be possible to use the NP241c with no problems.

It appears that I wasn't clear in my thread so hopefully I can shed some light. I used the term NP241 and NP241C interchangeably in my post. There are NP241C (Chevy), NP241D (Dodge), NP241OR (Jeep/Rubicon), and possibly other NP241(x) transfer cases. I used the older model NP241C because it had that extension flange on the front output that very closely mimicked the output on the AWD transfer case. The newer version NP241C has a stubby output and I was afraid the angles would be thrown off and the cross-pipe for the exhaust would become an issue. I did not explore any possibilities beyond that since the only way to truly know was trial-n-error and I didn't have the budget to buy transfer cases simply to try. Sorry for the confusion.

My second question is concerning the frond differential being locked all the time. Is there any way to get around that?

From my research, the front AWD Express differential is a GM 7.25" that has axle tubes specific for the van. The 7.25" differential is used on a number of different GM vehicles so I'm guessing each distinct platform has specific axle tubes. The Chevy S10 is included in this list but does have a differential disconnect. Early in this project, I thought that this would be like a S10/Astro axle swap but that is not the case. The passenger-side axle tube on the Express is longer and the point where it bolts to the frame is different front the S10. I have not proven this, but it is my hypothesis that you could take a S10 axle tube (with the diff. disconnect housing/parts) and an Express axle tube (with the proper attachment point and length for the van), cut both axles, then weld the disconnect housing/parts onto the Express axle tube, and bolt it to the Express diff. giving you a disconnect. You would really want someone who knows what they are doing performing the cutting and welding since tolerances and alignment would be critical. This thread shows the front S10 axle and the disconnect hardware the Express axle lacks: http://www.s10forum.com/forum/f220/...explained-and-grinding-from-front-end-506164/

Then you could use a manual disconnect for the front diff. since most shifters in the cab are linked to a switch and electric actuator to connect/disconnect the differential: http://4x4posi-lok.com/

And what is the difference between the diff on the newer AWD expresses vs say a silverado or something? I know you mentioned a disconnect, which is a newer term to me But after some research i think you are talking about the ability for the axle to not be locked. You said that you didnt see a place to bolt it on. So would the issue with replacing the stock diff on the AWD van with one from say a silverado, be a place to bolt it on?

The frames on a Silverado vs. the Express are different and so are the front differentials. The Express frame is slightly wider. As I mentioned above, the front diff./axle on an AWD Express is most similar to a S10 truck and their AWD SUVs (Envoy, Acadia, etc.) utilizing the GM 7.25" diff., not the Silverado. The connecting point at the far end from the diff. differs between the S10 and Express variants and the frame widths all differ (S10 vs. Express vs. Silverado), so sadly, you can't just bolt anything on. If I'm not mistaken, Quigley 4x4 Conversions designed a custom subframe to get around this issue so they could use a stock Silverado front axle. It's part of the reason they charge $10,000+ for a conversion.

Another question being how the stock set up actually works on those vans. Im thinking that the front diff is not locked all the time with the stock set up. Is that right? If so, do you think there would be anyway to keep that set up and add a option somehow to select or put the van in 4wd mode without changing the transfer case?

This is not correct. In its stock, AWD configuration, the front diff. is always locked. There is no provision to disconnect it, as I discussed above. It is the job of the transfer case to determine where power needs to go and that happens instantly so the diff. always has to be ready. I'm not sure what retaining the AWD transfer case but disconnecting the front diff. would gain you since in normal driving conditions, most of the torque is going to the rear wheels. If the rear wheels slipped, the AWD transfer case would still try to engage the front diff. The point of this conversion is to install hardware that ensures the constant transfer of torque to all tires so you maintain forward motion if the front or rear tires slip on slippery or uneven ground.

Another question is, is the gear ratio of the rear diff connected to the front? Refering to the van not shifting well in 4low because of the gearing. In other words, can you regear your diffs with out considering what you have up front, say i wanted to put a different rear end on my chevy express AWD, would i have to consider the front 7.5? Or can i just swap and not have to worry about it?

To my knowledge, yes, the gearing in the front diff. always has to match the gearing in the rear diff. Otherwise you have hardware spinning at different rates and that is a recipe to destroy.......something. Here is a link to explore what diff. gearing you may have: http://www.sierragear.com/gm-rpo-axle-ratio-identification-codes-3/. So if you want to switch out your rear diff. gears, plan on doubling your cost and work involved. My 4LO transmission shifting issue is not related to gearing, it is related to the Express computer (aka ECU) not knowing it now has a transfer case with reduced gearing. The same issue exists with those who have done conversions with Chevy Astro vans. I have instructions on how to possibly fix my issue (tricking the ECU) but it has been a low priority.

I saw another post of yours about some axle wear on your diff. Do you think that is connected to the swap?

I'm not certain but I doubt it. I did replace the front diff. but the remanned unit had very similar play so unfortunately I paid a chunk of money to learn that the play I felt was normal.

There were some other questions a had rolling around in my head last night after i sent that message. Because the diff is locked now on your van, does that mean the wheels spin all the time via the transfer case. Like what is the difference between the stock AWD and the affect of you get now with the thing being locked all the time. And then how does that differ from 4wd? Thanks for your time. Any other questions i come up with i will shoot your way.

No, the only time the front wheels are receiving power is if the transfer case is engaged in 4HI or 4LO so when I'm on pavement in 2HI, no torque is being transferred to the front. The front wheels and connected hardware spin only because the tires are touching the ground. In normal, pavement driving conditions, there is really no difference between the stock AWD transfer case and a selectable transfer case in 2HI. If you plan to stay on pavement or gravel, especially during rainy or snowy driving, the AWD setup may be preferred. If the rear wheels start to slip, the viscous coupling in the transfer case "detects" the difference in wheel speed and transfers torque to wheels that potentially have more grip to keep you in control. When you are offroad and you KNOW you will have a lot of tire spin, a selectable transfer case that keeps all of the hardware locked together is preferred. This being said, I will add 2 things:

1. There are a number of people who have stressed the AWD hardware and have nothing bad to say. So it seems that the stock AWD setup may be very capable and a conversion is largely unnecessary especially if you plan to spend most of your time on pavement.
2. In 2HI and the fact that the front diff. is always locked, the front driveshaft is turning which is turning the chain in the transfer case. It is reasonable to assume that is causing wear and I believe I can hear the chain turning. It's a white noise that I have had trouble diagnosing but I'm 95% certain that is the source. I suppose an easy test is to disconnect the front driveshaft and go for a drive.

 

[zuren]

Zuren , it`s been over 3 years now .....

Does the front differential still does the job being 4x4 now ? ( more stress on the 7.5" ) ?

Dan ,

Since the conversion, I've had no complaints. For the record, the front diff. is a GM 7.25".

As I mention in my lengthy response to jknight, it is the same front diff. as they put in the S10 and other light GM SUVs so I haven't been rock crawling. I have been through plowed fields, rutted roads, 3-4 ft. snow drifts, and yanked a loaded,Express 3500, FedEx box truck that skidded 20+ ft. off an icy road in 2 ft. of snow back onto the road and it never blinked.

 

[zuren]

the back end is always easy .......swap the 1500 diff , heard the 1500 is not really doing the job if treated rough ....( bigger tires )
concerning the fornt IFS , I need to know too ...... If a pick up diff could fit !
there aint seem to have lots of people who knows about these .....

From what I know, the Chevy vans are not as DIY 4x4 conversion friendly as the Ford vans. I believe Quigley 4x4 fabricates a custom subframe in order to use a Silverado front axle. In stock form, nothing to knowledge, is easily swapped to one of these Express AWD vans.

I think there is 1, maybe 2, model years where GM made a 2500 AWD Express. They are unicorn rare and I have no idea what hardware is underneath. I have some paperwork somewhere describing the different frames (Express 1500 vs. Express 1500 AWD vs. 2500 vs. 3500). I'll see if I can find that. It may have only been Silverado 1500 vs. the Express AWD.

 

[jknight]

2500 awd

Thank you for responding to the questions i had. I really appreciate that. I actually have the 2003 2500 awd van. The GVW is 7300lbs just like the 1500, the brakes are the same and probable everything else. Im sure the shocks are different and probably the leaf springs. I dont have a 1500 to compare it with, but i doubt there is any difference in the frame. Im not sure why it is even a 2500 with virtually the same GVW, im not sure what makes it different. Mine has the 4.11 rear end, do you think that is good or bad? I have a lift at my house, so if anyone wants to see pics of the thing lifted in the air to compare let me know. But im sure they are the same.

 

[Herbie]

In its stock, AWD configuration, the front diff. is always locked. There is no provision to disconnect it, as I discussed above. It is the job of the transfer case to determine where power needs to go and that happens instantly so the diff. always has to be ready.

Just a minor correction to the nomenclature here: The OEM front differential does not lock, it is an open differential, allowing the two front wheels to turn at different speeds. The function of the front axle disconnect on vehicles like the S10 is to separate the axle shaft from the differential. This is normally automatically controlled by vacuum and the purpose is to create a "path of least resistance" when operating in 2WD mode. (The S10s that include the axle disconnect use clutch-based transfer cases so there's always a small amount of power sent to the front driveshaft due to the clutch preload. Disconnecting the axle from the diff means that this only has to turn the spiders, instead of trying to spin the axle/wheel.) In comparison, the AWD vans and trucks (Bravada, later S-10s, Astros, Express, etc.) always have the front axles connected to the differential, but the difference in the transfer cases means that the front driveshafts are not powered, because those transfer cases can opt to send zero power to that shaft.

Now, the option of a front axle disconnect could enable the option to use a locking or limited slip front differential, since you'd want to decouple this when driving 2WD on pavement, but the standard OEM arrangement is that the front diff is open. This means if you lift a front tire while wheeling, that wheel will get all the power. Some S10 owners (and I believe at least 1 Astro owner) are pursuing the path of using a Torsen or other limited slip diff in the front. This will definitely require using a front-axle disconnect so that on-pavement behavior will not be compromised. At this time, I do not know of ANY selectable locking option for the GM 7.5 differential. Your only possibility is either a limited slip or automatic locker and a manually controlled axle-disconnect, but this will be all aftermarket. GM does NOT make any sort of locking arrangement for the front diff.

Note that unless you have the G80 option code, your rear diff is open also, so depending on terrain you could end up with one two wheels spinning and very little power to the wheels that need it. The Express van uses the larger GM rear-end, though, so at least there are a variety of aftermarket lockers available. (The Astro/S10 platforms use the 7-5/8" rear diff, for which there are no selectable lockers available, only limited slip or autolockers.)

Hope this helps clear some stuff up.

 

[jknight]

Note that unless you have the G80 option code, your rear diff is open also, so depending on terrain you could end up with one two wheels spinning and very little power to the wheels that need it. The Express van uses the larger GM rear-end, though, so at least there are a variety of aftermarket lockers available. (The Astro/S10 platforms use the 7-5/8" rear diff, for which there are no selectable lockers available, only limited slip or autolockers.)

My savana has the 4.11 rear end. You think with me doing the same mod the zuren did it will work out nice? He has the g80 locker, i dont really want that rear end, i was just wondering what the difference in performance would be between mine and his

 

[Herbie]

Those two questions are independent. 4.11 rear end is the sweet spot for Astro vans with ~31" tires and our smaller engine. Your best final drive ratio will be largely determined by what size tire you wish to run and whether or not you'll be towing. Whether or not you made the 4x4 conversion will have very little to do with the final drive ratio. The question for that is whether you'd prefer to have low-range with the locking center, or whether the AWD is better for you. Do you drive in snow or sand frequently? AWD might be better. I like having the low range so I can go a little slower under power and not have to ride the brakes.

The G80 is generally considered a plus. On the road it doesn't affect anything, and offroad it's certainly better than nothing. It's certainly not stout enough for hard core crawling, but then again neither are several of the other components in the van. If I had the G80 in my van, I'd probably leave it. Since I have the 3.42 gears and an open diff, I'm going to be doing a full setup anyhow, so I'm going to change the ratio and put in a detroit locker at the same time.

 

[jknight]

Those two questions are independent. 4.11 rear end is the sweet spot for Astro vans with ~31" tires and our smaller engine. Your best final drive ratio will be largely determined by what size tire you wish to run and whether or not you'll be towing. Whether or not you made the 4x4 conversion will have very little to do with the final drive ratio. The question for that is whether you'd prefer to have low-range with the locking center, or whether the AWD is better for you. Do you drive in snow or sand frequently? AWD might be better. I like having the low range so I can go a little slower under power and not have to ride the brakes.

Im planning on running stock tire size. LT 245/75/16. My goal is not to create a rock crawler or anything close, i just want the van body with 4x4 capability. Basically a stock 4x4 pick up except with the room of a full size van. Not so much sand, but deffinetly mud and some snow, the AWD is an awesome thing to have in a full size van, however i need to be able to control it.

 

[Accrete]

Glad to hear from you zuren!

A couple observations for any who may travel the awd path.

On GVWR. Our rig is basically set up like one might a Syncro-Westy build. Simple interior (bed/bath). The one thing we were missing was an easy camp-kitchen set up so we went with some custom Aluminess bumpers/boxes that have a galley box on the camp side and a water/fuel jerry can box on the street side. I was concerned about the additional weight though was told it would be close. I'm happy to report the weight difference is 80 pounds over stock for front bumper, rear bumper, and the two boxes. Overall the difference will actually be closer to 50 pounds because we don't have to include the old galley box we were lugging out from under the platform bed.

Also on tire size. We were running E rated stock size though did go up last year after installing lift to E rated 265/70/r17. There is no sag in rear and we run very close to GVWR all the time with full tanks and two occupants.

: ) Thom

 

[TDIJ]

I have done this as well, and would like to share what I have learned about the low range shifting issues. For reference I did it to a 2007 awd 5.3 flex fuel.

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I attempted to see if the low range programming was in the stock pcm as the astro guys have been fortunate enough to find. The pin that signals the low range gearing for the pcm is the green 16 if I remember correctly. I added the wiring and grounded... No luck. This is only one van tested... so I can't say it will never work but it definitively does not work on mine.

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Next I worked with a tuning company who felt confident that they could swap the transmission segments out of a silverado 4x4 truck and make it work. This did not work. Surprised... he came to me and we test drove my van while he played with many things. Turns out the programming for the low range gearing is in the main system segment. This creates a challenge because that segment can't just be swapped as easily as a transmission segment. This segment controls much of your body stuff. We could make the low range work for me... but at the expense of the a/c and possibly other things. It can surely be done... but it will take someone actually custom modifying/programming the code within the segment... not just full segment swaps.

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One other thing we learned earlier in the process... We had to use a pcm with the same code on it... or it would mess with my a/c. Not just any silverado pcm will work.

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My backup plan from the beginning was to add my speed sensor between the transmission and the transfer case. This would make it always read the transmisson output speed regardless of what ratio the transfer case would be in. Price paid would be the speedometer would read 2+ (whatever the ratio of the low range in the case you use) times as fast in low range, but the transmission would shift because the computer would not know it was in low range.

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The unknown in this situation is if it would operate quite as smooth as a 4x4 truck in low range, as the mapping and control of the transmission in a stock 4x4 truck may be slightly different in low range to account for the different requirements.

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Sadly... my shop/garage was destroyed by fire before I got a chance to try that, so I am being forced to sell my van in the stock configuration without a properly shifting low range. I no longer have the means to try this but this was my planned route.

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There are ways to hack the speed sensor into the transmission to transfer case adapter, but this is the best and only non-hack solution I have found... and ultimately the way I was going to go. It consists of two parts:

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A: a speed sensor adapter from advanced adapters
http://www.advanceadapters.com/prod...5e-4-speed-automatic-transmissionadapter-kit/

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B: a stock 700R4 or early 4L60E 3" long 4 bolt to 6 bolt adapter casting for a NP208 or NP241

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The advanced adapter part will make your 4l60 mimic the 4 bolt output that the 700R4 had so then you need the 700r4 adapter to go to the six bolt case.

 

[TDIJ]

For those of you wondering about the front end holding up. I live in the Colorado Rockies, have taken my van anywhere I would ever want to take it, and been surprised how well it has done. I have had tires off the ground, and driven down rocky trails that were about as rough as I could have taken the vehicle because of ground clearance. (for reference I am running 265/75/16s... so a little oversized, and a 3/2 inch torsion bar and block lift) Definitely gotten many weird looks and funny conversations from people out on their dirt bikes or 4x4s when they see me coming down the trail. I have always been empty while doing so though... I haven't ever pushed it under a heavy load. I have put about 15000 miles on in mixed driving since I swapped it early last summer. Lots of 4wd highway time as I daily drove it most of this winter. I don't personally have much fear about the front holding up for the average person for recreational use. Also, the manual shifting low range and only having first and second hasn't really bothered me. As the OP said... anytime I am wanting low range, it is for very slow speed stuff where 2 gears is plenty.

 

[jknight]

Which transfer case did you use? I guess a better question would be; what if any are the difference between your build and zurens? Sorry to hear about your shop...

 

[TDIJ]

I used a newer 241c with the other style front output yoke. It worked with no issues. I used a cable shifter designed for the 241c so it was pretty simple. I used the adapter that came with my 241. It was slightly different... But I have no doubt the one from the awd case would have worked just fine. Nothing really different.

 

[jknight]

Can you post a pic of your shifter? How about the front drive shaft? Custom right?

 

[TDIJ]

I have some pictures of the shifter installed on another computer I never use and am not around often... I will try to upload one if EP lets me... not sure the requirements to upload pics on this forum.

This is the shifter I used:
http://jb-custom-fabrication.myshop...-shifter-single-np241-billet-style-p-n-np241c

Installed the bulk of it under the floor with the angle brackets upside down from those pictures. All that sticks through the floor is the shifter, the boot, and the silver trim piece that holds the boot to the floor.

I installed it right next to the drivers seat. I believe I got the 48" cable... but my receipts are in ashes at this point. As I installed it, you need enough cable to come off the front of the case, make a 180 degree bend and into the front of the shifter. when mounted with the cable of the shifter pointing forward, your pattern will be 4hi-2hi-n-4low forward to back.

As for the driveshaft, it is nothing special. I only brought it up because zuren mentioned he wasn't sure if the newer, shorter, yoke style would have clearance issues. It doesn't, so a case with either style front output could be used for this swap with the same ease. My shaft was custom made but it isn't special, same as zuren's except it ends in a u-joint instead of a flange like his. Just a different way of mounting to the case. I know he provided a good pic of the two different front outputs on the case. This van being IFS means the shaft doesn't articulate at all... so minimal clearance is needed since the only movement of the shaft is rotational.