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Gary
Gary, ASE Certified Technician
Category: GMC
Satisfied Customers: 15723
Experience:  GCM master tech over 25 years
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I have a 2008 GMC Sierra 4x4 with 177k miles. last couple of

Customer Question

I have a 2008 GMC Sierra 4x4 with 177k miles. For the last couple of months, there is a problem with it popping out of gear when taking off. The motor revs a little and when it revs down it will make a grinding sound and pop back into gear. Or I just put it in neutral and it will pop back in gear. It just does it every now and then, around the 20th - 40th time I take off. Took it transmission mechanic that rode with me when it did it he listened underneath and said it was transfer case. Put used transfer case with only 103k miles and did the same thing. Thought it was module or switch so unplugged motor and did same thing. Different transmission mechanic thinks could be coincidence that both transfer cases could be bad or have weak fork but the odds of that seem long to me. Supposed to let them take the transfer case apart next week to see what's wrong. No one can figure it out.
Submitted: 1 year ago.
Category: GMC
Expert:  Gary replied 1 year ago.

when the transfer case was replaced was the encoder motor actuator re used from the old original case ?

Customer: replied 1 year ago.
The encoder motor and speed sensor that came with the used transfer case were used instead of the one that was originally on my truck
Expert:  Gary replied 1 year ago.

ok thank you for that information

i agree it is very odd that 2 cases would do the same thing

will it do this if you drive it in 4HI ?

can they duplicate this issue with the truck up in the air ?

Customer: replied 1 year ago.
Have not tried it in 4H and have no tried to duplicate it in air
Expert:  Gary replied 1 year ago.

i would first see if it will do this in 4HI

if it does NOT do this in 4HI then the odds are it is in the transfer case but we have had some transmissions cause this type of noise and issue

the best bet will be see if it will do this in 4HI or not and also see if you can duplicate the noise

since either way the transfer case has to come out with the issue being a case or the transmission have the case removed and the back 1/2 removed for inspection if NOTHING is found then the transmission is the issue with a possible planetary gear set issue

Customer: replied 1 year ago.
Transmission mechanic thought the only possible way that the transmission could do that was the sprag. So tried to drop the transmission into 1 or 2 when it popped back. If it immediately went into gear he would think it was the sprag, but it didn't affect it
Expert:  Gary replied 1 year ago.

those sprags are tough to nail down usually when you get the grinding its in the transfer case or something in the planetary gear set

its just going to have to come down to old school with this issue of pull the case and inspect it and if the inside of the case looks good go after the transmission taking it out and apart

Customer: replied 1 year ago.
Wondered if the module could be mis-aligning the gears in either transfer case so that any slack can allow it to pop out of gear. So that any transfer case installed, the module would set the encoder motor position wrong where the gears are barely aligned
Expert:  Gary replied 1 year ago.

no that should not be the issue since it was driven with the encoder motor unplugged this issue comes down to a hard internal issue in the case or transmission

Customer: replied 1 year ago.
No one can figure it out and I'm at wits end. Two dealers didn't know. Didn't want to replace transfer case again when once didn't work unless weird coincidence with both transfer cases having same problem. Understand this year model uses a plastic fork that could get weak and not hold gears aligned?
Expert:  Gary replied 1 year ago.

yes that is true but since it still did this with another case and another encoder motor and with the encoder motor unplugged it is a physical issue in the case or the transmission

they are on the right track that the case needs to come out and the back 1/2 removed to inspect it next

these issues come down to the direction of removing and visually inspecting next

there is no 100 % it is this or its that with these type of noises but its one of the 2 and things need to start get eliminated next or they will do nothing at all for you keep guessing and throwing parts at it

Customer: replied 1 year ago.
You would think that two transfer cases having the same exact issue would be a common enough problem that someone else has heard about it
Expert:  Gary replied 1 year ago.

the rare chance of 2 cases doing the same thing would lean us more so to an internal transmission issue causing this noise and issue

Customer: replied 1 year ago.
May be the transfer case since the first mechanic jumped under the truck and the grinding sound was coming from transfer case
Expert:  Gary replied 1 year ago.

then pull the case and take the 1/2 off the back and inspect it next

Customer: replied 1 year ago.
I think it has to be in the transfer case since that was where the sound was coming from. Just kept thinking that something else could be causing it to do that. I should pay you for your help since you tried
Expert:  Gary replied 1 year ago.

you can always get me back through this post for FREE for any follow up anytime with this question even after you rate us here at Justanswer

but if they ran it up in the air and 100 % the noise was from the case and it did this noise with the encoder motor unplugged it would have to be in the case i agree

Expert:  Gary replied 1 year ago.

how are you making out ?

Customer: replied 1 year ago.
Ok now but they will open it up Monday. I suspect that the plastic fork is weak
Expert:  Gary replied 1 year ago.

Okay sounds good let me know

Customer: replied 1 year ago.
It was the low sprag in the transmission. Took the transfer case apart and it was like new with no slack. Took the transmisssion apart and saw the low sprag was coming apart. Rebuilt transmission while they were at it.
Expert:  Gary replied 1 year ago.

great to hear they found it

dont forget to rate us here at Justanswer as i do not get credit for my work until you rate us here