Have 2002 Duramax/Allison 324000 miles on the allison. At idle speed when backing up it will drop out of gear and the prindl R will flash (blink). Sometimes when going forward at idle same thing but prndl D will flash.

1) changed the NSBU switch - no help
2) changed the internal and external transmission filters - no help
3) have codes po0746 and po0776

GM Dealer thinks it might be valve body
Allison dealer checked the clutchs by computer said they are not slipping but he hasn't seen the blinking R problem and thinks its electrical

I would be mighty grateful if someone could say they have seen this before or could tell me what the problem is.

I would say check with Mike L. at Inglewood Transmissions.

maybe a sticky valve???

I just noticed tonight that if I torque the engine with the brake on just before backing up, the allision stays in gear and does its job perfectly.

If the solenoids/valves are dirty will they get better with the new oil and filters?

Thanks!

I would change the spin on filter before I did anything. When was the last time it was changed?

Just changed both the internal filter and the spin-on 3 days ago. I noticed that the magnet was missing from the spin-on so I went back to allison dealer and got one & put it in.

Changed to Dexron VI ATF. I resisted the suggestion to put Lucas Transmission Oil treatment in.

Old oil was somewhat discolored but not bad. The internal filter was
really clean. Bottom of pan had that dark film material coating but nothing else.

If I knew it was the valve body I'd change it out. Can you change out just the solenoids?

yes, you can just change out solenoids. i am still thinking its a valve...

Just changed both the internal filter and the spin-on 3 days ago. I noticed that the magnet was missing from the spin-on so I went back to allison dealer and got one & put it in.

Changed to Dexron VI ATK. I resisted the suggestion to put Lucas Transmission Oil treatment in.

Old oil was somewhat discolored but not bad. The internal filter was
really clean. Bottom of pan had that dark film material coating but nothing else.

If I knew it was the valve body I'd change it out. Can you change out just the solenoids?

I hate to tell you but you need to drain that Dex VI out immediately and put in a Dex. III or TES-295 fluid. All though Dex VI is supposed to be backwards compatable there is a problem with using it in 2005 and earlier Allisons due to seal compatability. Allison/GM put out a bulletin inregards to this. If you do a search on this website more information will come up.

Yeah, I agree, Dex VI is out , Dex III is back in. GM and Allison probably have the same statistical probability chart regarding that front seal getting hard and leaking with Dex VI but Allison has a name and reputation to uphold, GM...well....

Allison dealer told me he could get me a valve body for around $500 but I hate to just guess, especially since he still thinks its electrical. This dealer doesn't work on 1000s, only heavy trucks

Since you said it holds in gear when you raise the engine speed I would get a gauge on it. There is a test port next to the spin-on filter. If the pressure is out of spec. the valvebody isn't going to fix it. If the pressures are in spec. there is a small filter in the valvebody that might be causing an issue but I'm not sure that it was there in the 2002 model year. It's a small plastic filter than can be washed out with solvent. Also the A & B soloniods have small filter in the front of them that can be washed too.

Great info diesel tech. Thank you.

What about Castrol TransSynd? Thats what should be in it.

Hey Everybody
Thanks for the info. We're going to work on it again this week-end.

One thing that I have found today is that the batteries were not in good shape, in fact one of the positve terminals was corroded pretty bad. I bought two new batteries and I can't believe the difference in the starter RPMs. Wow!

Could this be part or all of the problem? I've got to haul some horses next week so I hope its ready to go to work.

I've owned 8 chevy trucks(one was that 5.7 converted diesel thing) and a couple of GMCs and this Duramax is just plain awesome but I've got to get this Allison problem cleared up.

OK, I think I got it solved.

The rear ABS sensor (the one in the transfer case) had a micro-crack in it. Changed it ($58 from dealer) and no problems since. :)

Thanks everyone for the help!

Dan