Truck won't shift into gear after sitting for a few days

rszimm · July 14, 2021

2002 Silverado 2500 HD w/ 6.6L diesel and allision transmission.

Whenever I leave the truck idle for a few days and jump in, I'll start it up, shift it into gear, and it won't move. I'll rev a bit and still nothing. I'll then move it to a different gear back and then things work just fine. I thought it was only in reverse (as it's almost always in my driveway when this happens), but today I put in in drive first and it did the same thing...nothing. I revved a bit, shifted to reverse and it went fine.

I don't have any other problems with the transmission. Fluid level in the transmission seems fine.

Any thoughts?

OAKLANDPUNX · July 15, 2021

I'm having a similar problem with my transmission. Please let me know if you find out any helpful info.

I've got the a 2001 with 8.1 gasser mated with an Alison.

My ses light came on simultaneously while on the freeway stepping on the gas to pass another vehicle and having massive power loss.

Got off the freeway and engine sounded fine as far as it revving but I wasn't getting any torque delivered to the wheels.

I barely gimped up the hill to coast down the street to my house. My reverse gear works just fine, but I have no forward drive gears whatsoever.

I'm hoping someone can recommend a diagnostic scanner that will pull Allison transmission codes because my friends scanner only pulled a generic code

(? 0800 ?) and I need a little bit more detail.

Even though it's a 2001, I only have 48000 miles on it. I checked the transmission fluid and it seems to be within the right range, the color seems fine and doesn't look like there is any metal/particles in it.

I got in touch with my local Alison certified transmission shop and found out the hard way that GM was afraid of losing too much revenue to Allison and decided to force vehicles under the 4500 series into the GM dealerships for service. My 3500 has the allison 1000 series

I was told by the rep at the Allison certified shop that even if they wanted to work on my vehicle they couldn't because GM somehow was able to remove data on those lighter vehicles from the Allison proprietary diagnostic software.

It was my intention to take my truck to the most capable transmission shop, but I'm being told that I must take it to the dealership to have it scanned with a tech 2 scanner ( I'm not certain if it will pull detailed codes from the transmission) then have to pay inflated rates for (most likely very inefficient) diagnostic troubleshooting by their service department.

A lot of big dealerships in the San Francisco Oakland Bay area have closed down and the ones that are still in business rarely see/work on medium duty trucks and probably even less have much experience with less common vehicles like my 1 ton chassis cab 4x4 dually equipped with the 8.1 liter Gas engine mated with an allison transmission.

I've been reading up on some other forums (Duramax Diesel Forum and thedieselpageforums) and have noted several posts that have gone through exhaustive diagnostic work (without much success) but I keep noticing a reoccurring theme/issue being suspect and that is electrical problems with a wire rubbing and grounding out or loose connections with plugs in a number of locations (inside the trans or somewhere along the wiring harness) all of which cause poor communication between the ECU and the TCU by sending wrong/incorrect data which screws things up.

Aside from that, I have no idea what's going on and I am feeling hesitant about taking it to the Stealership and start hemorrhaging my wallet on a potentially inefficient diagnosis process or get subjected to the modern corporate business practice of trying different parts until they figure out the problem !