Loud Grinding in reverse and now stuck in 4lo

armgmc · January 2, 2014

Hey guys, first post here. I have a 2007 GMC SLE (new body style) crew cab z71. I've owned the truck a little over a year and theres been a few occasions since i've owned it where while reversing in 2wd or 4wd that something would slip and cause an extremely loud grinding noise almost everytime the wheels spin a little bit (hard acceleration). After realizing this happens I always try to easily accelerate when reversing but sometimes it will do it anyways. I've never had anyone close by to tell exactly where it's coming from but I suspect the transfer case. Tonight I went down to my hunting spot to just check it out and before leaving I thought I'd put it in 4low (turn knob) and maybe keep from spinning as I backed up the hill. after I got out of the spot I was in I tried going back into 2hi or 4 hi but the tcase never would shift back out and the service 4x4 light was flashing on the dash. Wasn't any loud clanks or grinding during all of this so I'm not really sure what went wrong or where to start looking. I'm gonna take it to my shop tomorrow and see what I can find but just wondering if anyone has ever dealt with the problem before. If it weren't for the grinding I'd suspect maybe an actuator but I feel like that's part of my problem. Thanks in advance for any help.

armgmc · January 2, 2014

To elaborate more on the reverse grinding it's not constant. When it happens it's loud as hell and kind of winds down like it's slipped a gear and will eventually catch I believe but I usually slip back up into neutral to stop it. Don't do it all the time only on heavy acceleration in reverse. First time it done it I was backing a trailer in my yard and slipped a little on wet grass.

gmtech4 · January 2, 2014

welcome ,,,,,get a scanner tomorrow and pull the code(s) from the TCCM

do the battery disconnect thing and see if that sorts it out

Edited January 2, 2014 by govtech4

armgmc · January 2, 2014

welcome ,,,,,get a scanner tomorrow and pull the code(s) from the TCCM

do the battery disconnect thing and see if that sorts it out

I have a scanner here, not sure if it'll read the TCCM or not but I'll try in a few minutes. Getting it out of 4 low would be nice but I stil lneed to find the problem for future use.

russellb · January 2, 2014

If I'm not mistaken, the transfer case is chain type with clutch packs. I suspect the chain drive is roasted. Check fluid level. If your transfer case takes automatic transmission fluid then its a chain drive. Sounds like you need to rebuild your t case. Good luck.

armgmc · January 2, 2014

If I'm not mistaken, the transfer case is chain type with clutch packs. I suspect the chain drive is roasted. Check fluid level. If your transfer case takes automatic transmission fluid then its a chain drive. Sounds like you need to rebuild your t case. Good luck.

Thanks, I'm probably gonna tear into it tomorrow and see what I can find. I'm off of work for now so I can afford to not have it for a few days anyways. Not sure about the tcase and what kind it is. Never really paid attention to it at all. I'll either look for a scrap yard tcase or rebuild this one if it's feasable.

Kinda strange that it would go bad with only 130k miles but I dont know the trucks history from before me too well. Checked for DTC's and my innova 3130 didn't show anything wrong. It may not read tcase problems not sure. Got the battery unhooked now, about to go fire it back up and see if I can get back to 2wd.

russellb · January 2, 2014

Thanks, I'm probably gonna tear into it tomorrow and see what I can find. I'm off of work for now so I can afford to not have it for a few days anyways. Not sure about the tcase and what kind it is. Never really paid attention to it at all. I'll either look for a scrap yard tcase or rebuild this one if it's feasable.

Kinda strange that it would go bad with only 130k miles but I dont know the trucks history from before me too well. Checked for DTC's and my innova 3130 didn't show anything wrong. It may not read tcase problems not sure. Got the battery unhooked now, about to go fire it back up and see if I can get back to 2wd.

If you rebuild, don't reuse parts, especially roller chains and components in the roller chain assembly. A worn out chain can destroy new sprockets in seconds, and worn out sprockets will do the same to a chain. Be thorough and do it right the first time.

armgmc · January 2, 2014

Battery off for close to an hour didn't help anything. No noises when switched to 4 hi. Doesn't seem like the motor even tries to switch it. Usually you can hear it click to try and change it from 4 lo to 4hi but nothing at all.

armgmc · January 2, 2014

If you rebuild, don't reuse parts, especially roller chains and components in the roller chain assembly. A worn out chain can destroy new sprockets in seconds, and worn out sprockets will do the same to a chain. Be thorough and do it right the first time.

Will do, never been into a tcase before so I'm not sure how competent I am with rebuilding it. Got some good friends with experience though so maybe they can help me with the rebuild.

russellb · January 2, 2014

Take pictures, make a sketch, label everything and clean everything. Clean hands do a good clean job.

armgmc · January 2, 2014

Take pictures, make a sketch, label everything and clean everything. Clean hands do a good clean job.

Any idea why the encoder motor isn't even trying to shift? Any fuses or relays to check? possible bad TCCM?

russellb · January 2, 2014

If your putting the transmission into neutral and then turn your dial to 2wd or 4wdhi and its not engaging, then it could be a fuse or relay, if its mechanical failure though its just stuck. Check your fuse box and its connections before you get too excited about tearing your case apart. Make sure all electrical is ok first. Get a scan done.

armgmc · January 2, 2014

If your putting the transmission into neutral and then turn your dial to 2wd or 4wdhi and its not engaging, then it could be a fuse or relay, if its mechanical failure though its just stuck. Check your fuse box and its connections before you get too excited about tearing your case apart. Make sure all electrical is ok first. Get a scan done.

I'm gonna look for fuses or relays tomorrow, any idea what they'll be labeled as? If it's somewhat easily accessed I'll probably pull the shift motor and see if it will attempt shifting when not bolted onto the tcase.

armgmc · January 2, 2014

What I'm really hoping for is that the gears inside the shift motor are what's grinding or slipping when reversing. Seems to be a fairly simple piece to open up and inspect once off of the tcase. This thing has the stupid auto 4wd BS with stabilitrack and traction control so I'm thinking that when I spin in reverse it's trying to engage the auto 4wd .

russellb · January 2, 2014

My truck has the same auto system, however when its in 4 low the traction control and stabilitrak are off and it doesn't try to auto 4 when the dial is not in that position. I don't know off the top of my head what the fuse is labelled as. Once my truck is in 4 low I can't simply switch to other power transfer modes without putting it into neutral first so I have my doubts about it trying to auto 4 when its locked in 4 low.