Diablosport Forums

1. U7180x
2. 9R08
3. Stock 2002 GMC 2500HD LB7 / Alison

I installed the 85HP tune. I drove it like a baby for the first 100 miles based on what the vendor told me. I then started to put a little more foot into it gradually, but nothing like foot to the floor, burnouts, etc. Mainly just acceleration. When accelerating on the freeway from about 60-80 MPH the truck accelerated nicely and then acted like the governor kicked in, power dropped off completly so I took my foot off the accelerator, the truck would not shift out of a lower gear and the check engine light came on. I dont recall the code unfortunately, but it had something to do with the transmission.

I cleared the code reset the truck back to stock and am afraid to do anything else with the tuner until I understand what is happening. Is the Alison tranny not strong enough for the 85 HP/TQ increase? Is the 65HP OK? What's going on? What do you recommend?

Well, it certainly sounds like the tranny went into limp mode, which is generally caused by the tranny slipping.

It may not have relearned enough, may simply need servicing, or may not like the extra 175 ft/lbs of torque.

Try the 65 tune and drive it easy for a day or two, then see how it works.

Thanks Mike. In your experience is a normally functioning stock Alison capable of handling the 85HP tune? Its kind of a waste of $$ if I am limited to one tune (65HP) on this vehicle. I was under the impression that the Alison was a beast of a tranny capable of handling more than stock power. Am I wrong?

Our stock allison handles the 120 tune no problem...but as the trans gets older and higher in mileage, it can wear out.

Do you service it regularly?
When was the last time the fluid and filters were changed?

I dont know if Id call it a beast of a tranny, it will start to slip with excessive torque output, though they can be built to be monsters...

Thanks Mike. It has been serviced regularly, and driven like a baby for the most part. It has 90K miles on it. However, I will change the filters and fluids again. Can you tell me why the tranny would not shift out of low gear once it went into limp mode and I had to clear the error code to get it to shift up again?

If you can think of anything else please let me know. I really like the feel the Predator adds, but hate to think I might be damaging the tranny.

Also, can a decent shop do diagnostics on the tranny to see if it is functioning correctly?

Limp mode forces it to stay in 2nd gear and will generally limit throttle input as well until the truck is shut off.

The TCM is pretty good at forcing limp mode before you can hurt anything....

Any GM dealer or quality repair shop can diagnose the Allison trans for sure.

Hi torchrider. I've got the same problem, and I'm brand new on this forum, and since this topic is kind of old, have you solved the problem? When I put it in tow/haul mode, there's no limp mode even if I put the pedal to the metal? 85 BHP mode. Without it, It goes limp passing aprox. 80mph.

if it does not limp in tow haul, it indicates you are slipping the overdrive gear...

Thanks for reply. Solution?

Start with a fluid/filter change to see if it brings some grip back to the clutches.

Thanks again, will do.

I've changed oil and both filters. Drove nicely for a while, and made sure the trans. and engine where good and warm. Same thing happened, limp mode if I push it, unless I put in Tow/Haul? Read that this trans. will wear as everything else, so now I wonder if the problem is just that?
I've got 126 000 miles on it now, so what say you?

Look into a soft part rebuild, fresh clutches should add some life and more holding power

Thanks. I'll see if I can find someone with knowledge in my part of the world.

The Allison tranny is a beast one of the best out there IMO, but once you start increasing the HP/TQ your C3 and C4 clutches start slip and will go into limp mode, once you have limped the trans you should just go out and get a kit to handle the added HP/TQ. Good rule of thumb is not to go over 100hp from stock but I have also heard of people limping the tranny at that setting all depends on how heavy your foot is. There are many forums out there that talk about this subject just search it and you will find them, lots of good info out there.