Allison 6sp: 2008 GMC 2500 Shift Problems

K

Kilerat

New member
Dec 28, 2011
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I am hoping someone on this site might be able to help point me in the direction of a problem with my trans.

The problem started with a random issue when accelerating from a stop and the RPM running up and then the trans slamming in gear. (almost like it was in neutral then dropped into gear) This happened 3-4 times over the past year.

Now recently when I was coming to a stop the truck acted like it stuck in high gear and had to run the RPM up like it had a huge stall converter to get it rolling but once you were up to speed it was fine. (Until I stopped again) Nothing seemed to work, the manual shift didn't do anything, I shut the truck off and re-started it...nothing. Then I shut the truck off for about 5 minutes and everything has been fine for a few months.

Tonight I fired the truck up to move it out of the driveway and it went into reverse fine but when I put it in Drive it acted like it was in neutral again. I shut it off for a couple minutes and now it's fine.

Is there some sort of electronic shift module that might be going bad? The dealer had no idea what it might be and said I should take it to a trans shop to be re-built. I find that hard to believe because it's fine for months on end and when it acts up it seems that just shutting the truck off for at least 5 minutes solves the problem.

Any ideas?

Thanks in advance for any assistance....
 
K

Kilerat

New member
Dec 28, 2011
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Yea I've checked the fluid level and it's full.

I'm thinking it has to be something electrical. When I pulled it out of the driveway this morning it went into reverse fine but when I put it in drive...nothing. I tried manually shifting it but still nothing...then when I went back to reverse...nothing.

I shut the truck off and let it sit for a couple minutes and everything is fine.

I get to my first stop and this time it doesn't go into reverse but drive is fine. I tried going from park to reverse a few times but nothing... then I shut if off for about 60 seconds and everything is fine.

When I leave my second stop reverse works fine...it goes into drive fine.. but when I get on the road it won't shift out of first gear. (Manual shifting didn't work either. I shut if off for about 60 seconds and then everythign is fine again....

I have a check engine light now that I haven't had before so I guess I'll take it in Monday to have it checked out.
 
K

Kilerat

New member
Dec 28, 2011
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Forgot to mention... when it was acting up the last time it flashed a message in the odometer area that said something about shifting to neutral to re-set....??
 
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