4wd not engaging

[Frank R]

I was hoping maybe someone could give me a few ideas of what to look for here...

The 4wd on my 2003 Tahoe (5.3L, 4-button 4wd) stopped engaging today. The lights on the switch still come on, and you can hear the transfer case shift when 4hi or 4lo is selected - however the front axle is not engaging. If it helps at all, the last few times 4wd was used there was a low hum coming from what sounded like the driver side front, which was slightly more noticeable under deceleration.

I really need to get this back up and running ASAP, so any advice on what to look at will be greatly appreciated.

Thank you.

 

[PG01]

Just curious, how do you know its not 'engaging'? Did you put it in 4wd, hit the gas and the front tires didnt spin?
there is a solenoid on the front axle, if its really not going in, maybe start there.

 

[Frank R]

My wife got it stuck in the snow in my driveway. I'm at work and haven't been home to check it out yet. However, my father went over to help her and reported to me that neither front wheel was turning even though 4wd was engaged.

 

[PG01]

going home and checking it out for yourself would be the first step. Its hard to diagnose or give a definitive answer 1st- over the internet, 2nd-when you arent exactly sure what the truck is doing yourself.
Personally, i would get in it myself and hit a few buttons, listen, put it in neutral, try putting it in 4wd. Could be a loose, corroded electrical connection, or any of the electrical components in the 4wd system, switch could be bad, solenoid shot, any number of things, i dont know. Maybe someone knows better than myself but thats what i got. This is my opinion but u know what they say about those. Sorry bud

 

[Frank R]

Had a brief chance to check it out this morning before work. My observations:

- Front wheels definitely not turning in 4hi or 4lo
- 4-button switch seems to be functioning fine - lights coming on as they should and I can hear the TC shift into and out of 4lo. Can't hear anything from 2wd to 4hi, but I couldn't hear that even when everything was working fine.
- 4wd fuse in cab and TREC fuse under hood are both intact.
- No "service 4wd" error message. When 4wd fuse or TREC fuse is pulled, "service 4wd" message comes on immediately and goes away when fuse is put back in.

I was not able to check to see if front driveshaft was turning. I will check this out tomorrow morning. If the driveshaft is turning, what do I need to check next?

Thank you.

 

[rockola1971]

Wont matter if your front driveshaft is turning or not if your front axles arent locked in at the differential.

 

[Frank R]

Ok, from what I'm seeing it seems that the front axle is locking in, but the TC is not putting power to the front driveshaft. In 4hi or 4lo, if I pull it up to a snow bank so that the back tires spin, the front driveshaft is not turning. However, when the Tahoe is moving, the front driveshaft is turning when in 4wd, and not turning when in 2wd.

Am I correct in thinking that the front axle is turning the driveshaft passively when the vehicle is in motion (being driven only by the back wheels), but that the TC is not providing power to the driveshaft to turn the front axle? If so, where do I look for a problem, noting that I can hear the TC shifting into and out of 4wd.

 

[rockola1971]

If you 100% sure that your front axle is locked and your front driveshaft is NOT turning under power of the TC and your TC is correctly shifted into 4H or 4L then your problem is in your TC. Could be your chain is busted or other hard parts failed in your case. Drain it and see what comes out. Any metal chips?