03 Sierra AUTO 4WD works but not in 4Hi or 4LO

[THolscher]

2003 GMC Sierra 1500HD

Push button 4WD

 

When Auto 4WD is active and the wheels slip 4wd engages like it should. When trying to select 4hi the indicator light flashes and lights up like it is in 4hi but no power is transmitted to the front axle. When going to 4LO light again flashes like normal during transition, you have hear the transfer case shift but still no power to front axle. when driving in 4 hi or LO there is no grinding noises just nothing at the front.

 

I don't know how the system works but I assume that the front axle actuator is working since auto 4wd works. but I would think that if the indicator light says that it is in 4 high or lo that the encoder motor should be in the proper position. Didn't know if the switch in 4 hi or lo just isn't activating the actuator on the front axle or if that is even a possibility.

 

Any help would be appreciated.

 

Thank you,

[THolscher]

I forget to note that I have not received any error codes.

[THolscher]

I was at a lake and when I went to leave I had it in 4hi and spun down the back wheels without the front turning at all. Tried 4 lo with same result. Tried the auto selection because the previous winter I had used it and it worked. Sure enough once the back axle started to slip the front engaged and I came out of the hole in was digging.

 

I will check fuses. Is it possible for the indicator light to show that it is in 4hi or 4lo and not actually be? Always thought it just blinked and then indicated still in 2wd.

 

Thank you for the reply.

[00Silverado4x4]

Not to be insulting, but how did you confirm that it's working in Auto 4wd?

 

Just for giggles I would still check the fuses, there is two of them, one is labeled "4wd" and the other will be "ATC". A check that has worked for me is the turn the key to "ON" (but do not start engine) and then crawl under the truck and listen while someone else selects the different options. See if you can hear both the encoder motor on the transfer case and the actuator on the front diff shifting. That should give you a better idea of where to look next.

[efritz]

sounds alot like the encoder took a dump (or possibly the TCCM), however, even if it did you can still check that the front axle is engaged by lifting one front wheel off the ground and spinning it by hand in 2wd and then in 4wd, to see if it remains engaged. Id it does, you very likely have a sensor ring failure in the encoder. Best diagnosed with a scantool with bidirectional controls so not to waste a possibly good encoder if it is another problem.

[kstruckcountry]

Have you hooked a real scanner up? Should be able to diagnose by commanding it in/out.