4 wheel drive not working

Jminter6001 · September 26, 2018

Yeah it's manual. Oh ok. Thank you so much for your time and your help! I'm going to look into whether or not it should have a 4wd indicator light on the cluster. Have only had the truck a few years and have only used 4 wheel a couple times. So I cant remember if there actually was an indicator light.

casey.mikl · September 26, 2018

Being that it is manual I don’t believe they do. When it engages, atleast for 4low the traction control and stuff will all pop up and say they are deactivated.

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davester · September 26, 2018

If it's similar to the GMT800 manual setup (99-07 Sierras), when you move the shift lever, it rotates a gear in the differential to move things to the appropriate place (hi/lo, 2wd/4wd), and on that gear is a sensor, that notices when you switch between 2wd and 4wd. That sensor controls the front actuator (whether it is retracted for 2wd, or extended for 4wd), and the front actuator has a sensor that notices it's setting and that sensor is what turns the 4wd light on/off in the cab (well, it's hooked up to one of the computers, and that computer forwards the setting to the IP).

So, I would say it's either the sensor in the transfer case or a wiring problem (maybe the fuse blew?).

Your truck could be different than the setup I've described (yours is a GMT900, and I'm only familiar with GMT800), so I would suggest getting a subscription at alldatadiy.com for your truck, to get access to the full-service manual for it, and go through the testing procedure for this, and that will quickly sort out where the problem is.

carkhz316 · September 26, 2018

Actually, the manually shifted 4WD trucks do have a indicator on the dash, not the push button ones though. When you mention the front driveshaft is turning, is it actually being driven or just parasitic drag? With the truck off the ground, the front shaft will often spin, even in 2WD because of the viscous drag in the Tcase. I would verify that the Tcase is actually shifting into 4WD first, then go to the front axle for issues. If it is indeed a issue out at the front axle, pull the actuator out and manually shift the front axle (just poke something in the access hole to engage the shift mechanism) to see if the axle locks in.

casey.mikl · September 26, 2018

Actually, the manually shifted 4WD trucks do have a indicator on the dash, not the push button ones though. When you mention the front driveshaft is turning, is it actually being driven or just parasitic drag? With the truck off the ground, the front shaft will often spin, even in 2WD because of the viscous drag in the Tcase. I would verify that the Tcase is actually shifting into 4WD first, then go to the front axle for issues. If it is indeed a issue out at the front axle, pull the actuator out and manually shift the front axle (just poke something in the access hole to engage the shift mechanism) to see if the axle locks in.

This. Didn’t think of that. Shift it in 4x4 with the tires off the ground and the truck off. See if your able to spin the front drive shaft freely or if it causes the rear tires to spin and is very hard to turn. If it spins free with the 4wd engaged then the issue lies in the transfer case. Not the front axle

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Jminter6001 · September 26, 2018

Thanks everyone for the info!! I'll definitely try all this. I did get a refurbished transfer case put on my truck about 6 months ago. I've been wondering if the transfer case could be causing this issue. But the transfer case is turning the front drive shaft, so I thought everything was working as it should. But if there is a sensor in the transfer case not working, then that could be a problem.

Jminter6001 · October 1, 2018

So...something else I just thought of. I had put a new hub on the front left earlier this year. Bought it from AutoZone. Is there a chance I could have bought a hub that is not an "automatic locking hub"? Guess that could be why it's not engaging. I searched autozones website again for a hub and it looks like that's all they sell.

(Trying to get all my ducks in a row before I go back to the shop that installed my Transfer Case, and start a war)

But the 4x4 indicator light not coming on on my instrument cluster, I'm guessing, is one of my problems.

Edited October 1, 2018 by Jminter6001
Forgot something

davester · October 2, 2018

I believe (again, not certain, as I have a GMT800 truck, and your truck is GMT900) yours works the same as mine, so there is no "auto-locking" hub, like say, Fords have. The hub is solid, and splined, so the short axles with CV joints at each end, always rotate with the wheel.

The way GM engineered locking/unlocking the front end, is by using the actuator in the front axle to engage/disengage the passenger side stub axle (inside the front diff assembly) from the diff.

So, when the actuator connects the passenger side axle to the diff (ie, 4wd is engaged), then everything works the same as the rear end.

When the actuator disconnects the passenger side axle (ie, 2wd), then the passenger side axle just spins as the passenger wheel spins, and the driver's side wheel still spins the axle into the diff, but generally this just spins the spider gears (as it's an open diff).

casey.mikl · October 2, 2018

I believe (again, not certain, as I have a GMT800 truck, and your truck is GMT900) yours works the same as mine, so there is no "auto-locking" hub, like say, Fords have. The hub is solid, and splined, so the short axles with CV joints at each end, always rotate with the wheel.

The way GM engineered locking/unlocking the front end, is by using the actuator in the front axle to engage/disengage the passenger side stub axle (inside the front diff assembly) from the diff.

So, when the actuator connects the passenger side axle to the diff (ie, 4wd is engaged), then everything works the same as the rear end.

When the actuator disconnects the passenger side axle (ie, 2wd), then the passenger side axle just spins as the passenger wheel spins, and the driver's side wheel still spins the axle into the diff, but generally this just spins the spider gears (as it's an open diff).

This is correct. How my 2011 operates.

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HawkScot · October 3, 2018

Unless they completely changed the design, the floor shift is for the transfer case, but the actuator at the front diff is electrical. There is a switch that gets activated by the floor shift when you shift transfer case into 4wd. On my 94 pickup, the indicator light was on the floor (part of the trim for the shift lever). Move it to 4wd, light comes on when actuator is energized... It was easy to mod to get 2wd low range by putting switch in actuator wire so you chose when front axle is being driven. Not so easy on more current models for that mod.

You likely have a bad wire or switch... perhaps a blown fuse.

Jminter6001 · October 3, 2018

Does anyone have a diagram for my fuse box. I dont have my owners manual, and I can not find a diagram anywhere online. Closest diagram I can find online is a GMC Denali. It's a 2008 GMC 2500HD 6.6 4x4 SLE

The transmission shop I took my truck too couldn't find a diagram either, so we were just playing a guessing game

Edited October 3, 2018 by Jminter6001

casey.mikl · October 3, 2018

Unless they completely changed the design, the floor shift is for the transfer case, but the actuator at the front diff is electrical. There is a switch that gets activated by the floor shift when you shift transfer case into 4wd. On my 94 pickup, the indicator light was on the floor (part of the trim for the shift lever). Move it to 4wd, light comes on when actuator is energized... It was easy to mod to get 2wd low range by putting switch in actuator wire so you chose when front axle is being driven. Not so easy on more current models for that mod.

You likely have a bad wire or switch... perhaps a blown fuse.

This here I think may help you a lot

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Jminter6001 · October 3, 2018

Thanks everyone. May have to put this project on the back burner for now. Truck is showing me the check engine light now. Code: P122A. Got to figure this one out quick.