Neil 6.0

So my brother finally got a new transfer case in his 1999 2500 Silverado with Autotrac 4x4. Got it home and look underneath and the front driveshaft still rotates when it is in 2Hi/2wd. We put it up in the air on jackstands and the front wheels do not spin until you select one of the 4x4 modes, but the front shaft rotates 100% of the time no matter what. I looked at two other friends Autotrac trucks and their front shaft doesn't rotate in 2wd. So what the hell is going on here?

TG02Z71

iTrader: (2)

Not realy sure but with Auto 4 it works with a viscous coupling, there may be enough friction in the coupling with the fluid to make the shaft turn. I don't think it actually putting power to the front shaft, just residual fluid in the coupling making it turn when you have it up on stands. JMO.

Neil 6.0

Trust me, it's putting power to the front shaft. I grabbed it with my hand to see if I could hold it back....no way.

smooth sierra

no offense here but are you retarded???? why would you try to grab a driveshaft that is moving.....sounds like a good way to loose an arm

Neil 6.0

I am not retarded, I am just that damn good.
The truck only moved like 3 inches, not even a 1/2 turn of the shaft.

WhippledHD

Couple of things here, don't get mad just asking because there is a lot of misconceptions out there about Auto trac. I just went through a similar debate getting some work done on my truck. According to Chevy directly, any vehicle with Autotrac, the front drive shaft will always turn, the front diff is were the locking and un-locking occurs. There is some confusion about what Autotrac is, it does not mean the 4 wheel drive is engaged with the push buttons, it specifically refers to a vehicle with the 2wd, Auto 4 wheel drive, 4 high, and 4 low. Just because it has push button four wheel drive does not mean it is Autotrac. On my truck I have the push button 4 wheel drive and the front drive shaft does not turn in 2 wd, my buddy with the same truck has the Auto trac and his driveshaft turns all the time. I'm not an expert but this is what I got from Chevy directly when I recently was trying to diagnose a 4 wheel drive problem on my truck.

Chris

Mort

If I crawl under mine I can easily turn the front drive shaft by hand with the engine off. I have not tried anything with the engine on and the truck in gear yet but I will sometime. I do have the Autotrack transfer case and Auto 4WD.

Mine has a yellow sticker under the hood on the drivers side near the headlight that says never to dyno the truck on a two wheel drive dyno because the truck has an automatic transfer case. A safety item in case the the truck decides to engage the front diff while it is on the dyno.

So it does look like the front drive shaft does turn all the time when in gear. When the Auto 4X4 is engaged it looks like it engages the front diff when wheel slip is sensed at the rear wheels thereby engaging 4HI automatically.

2000SIERRA

iTrader: (2)

I just spent 4 hours 2 weekends ago doing my front gears and installing my new front driveshaft, and will confirm what everyone else is saying. If you have autotrac, the front shaft spins whenever the truck is in gear. There is a actuator on the front diff that engauges and disengauges the 4wd. When the auto 4wd is turned on and the rear wheels slip, the actator on the front diff locks it in 4 hi until the rear wheel slip is stopped.

qwank

iTrader: (4)

I'm a tech at a chevy dealer and yes the front drive shaft is supposed to turn on an auto 4x4 transfercase.

Neil 6.0

Thanks guys. I was 99% sure that is how it is supposed to work, but after seeing the two other Autotrac trucks with shafts that were not turning it got me worried.

How do you explain the other two trucks then? One is a '01 Z71 and the other is a '04 1500HD.