Chevy Express AWD issue ?

MisterBishi

New member
Hi all.

I have an Express AWD that sometimes doesn't seem to get any power to the front axle. I have seen it put power to the front on occasion but have also had the rear spinning with no activity up front at all, it seems intermittent. This happens at a dead stop say when the truck is dragging it's belly in snow. I understand how the viscous coupling is supposed to work (different axle speeds warm and solidify silicone in the TC which tries to link the axles together ) , which apparently takes some time. But what if the truck is dead cold , say your snow plowed into a parking lot, like the last time the front axle did nothing . I spun the rear for a half dozen 5+ second attempts but the front never budged . Is that because the TC is cold ? I then backed up and slowly drove straight through. It seems as though it was using the front with a bit of forward motion but if it's standing still it is very hesitant to power the front .

It feels unstoppable when it's rolling but once the rear loses traction it seems to be a 2wd at least for a while.

I have full on snow tires on it but do not have the "G80" locker.

I know it's not a true 4x4, I have those also, but I also know it's supposed to put power to the front with a spinning rear in a reasonable period of time. Any thoughts ? Thanks !

Dan
 

Slicky72

Adventurer
I am not sure about the express transfer case , hopefully someone will chime in but the Astro van moved from a Viscous tc to an electronic tc in the late 90s and the actuator in electronic tc can fail without giving you any warning other than a spinning rear tire
 

MisterBishi

New member
Thanks Slickey . This TC is basically the same as the earlier Astro with the viscous coupler, no electronics at all. The coupler can fail but usually does so by locking solid, basically putting you in 4wd.
 

86scotty

Cynic
How many miles? We had AWD Chevys at work for a long time and they really liked frequent Tcase fluid changes, or tranny fluid, not sure how it's done under there. I just remember mine was magic when right but needed a fluid change every 60k or so or it would start behaving really strangely. Our fleet mechanic said, for some reason, the fluid just broke down quickly in them.
 

MisterBishi

New member
Thanks Scotty .

I had the local trans shop change diff and t-case fluids. It was obvious to them that the silicone from the viscous coupler had leaked into the t-case lube . In other words I was correct that I didn't have awd in certain situations (when the rear would spin). The viscous coupler is non-servicable and the t-case is being replaced under warranty.

These GM awd t-cases use two separate but overlapping "systems" to put power to the front axle. One when the rear has traction in which the van is almost in "4 hi " which gives it that rock solid highway traction even on ice. Another when the rear loses traction where it can "lock" the t-case and send almost all torque to the front - this is what was lost when the viscous coupler blew.

Other than looking at the fluid there is a procedure in the service manual where they use a torque wrench on the t-case to test the coupling if you have any doubt it's working.

Lastly, the t-case should be overfilled by a quart when lubed per factory service bulletin.

Dan
 

MisterBishi

New member
I don't have a PDF. I first read about it on another forum. When my first replacement t-case arrived it came with an instruction to over fill by 1 quart. (Apparently the input bearing can be starved if you don't. Then the seal can go and eventually the trans fluid can pump into the t-case and will start coming out the vent tube.) Something about the filler hole being too low in this application. My second replacement came with a higher fill plug.

The owner of the Trans shop is very experienced and reputable. He doesn't put much faith in these t-cases , says they almost always fail. Once the rear end breaks loose it puts tremendous strain on the viscous coupling. He thinks a lot of folks are driving with a blown VC and don't know it because it works fairly well without it. I wonder if that's why I've never heard anyone talk about it on these forums.

If this t-case goes I'll be doing a 4x4 t-case conversion. First I'm going to try a True Trac LSD or maybe a Gov Lock auto in the rear to try and keep the rear from spinning. Rear diff is open now. We do lots of snow driving.

Dan
 

blazen71

New member
Thanks Dan. I added a G80 in my van and it does help. But if I hammer on it and the rear spins a lot, the front will basically disengage, which it's supposed to do. I've found that if I go slow and take it easy, the awd works it's best. Slow and steady wins the race.

"When traction is lost at the front wheels, the viscous coupling (3) works in conjunction with the planetary differential (4) to bias the torque more to the rear wheels. The rear torque goes higher than 65 percent, up to 100 percent torque to the rear output shaft (5). The torque at the front output shaft (9) is lowered from 35 percent, to as low as 0 percent torque."
 
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