Allison Shifting Issues

613Duramax · 07-21-2007, 10:07 PM

2001 GMC 2500HD
5 speed Allison
~183000 kms
No mods of any kind...yet.

During a full throttle accelerations away from a light, my truck shifts into 2nd (or 3rd) and redlines, but won't shift up to the next gear. I am stuck at 60km/h. If I blip the throttle, it will make the shift and take off.
Same thing happens if I try and pass, it will shift into 3rd (or 4th) and redline at 80km/h and just sit there w/o shifting.
I had the truck scanned but don't have the codes with me, something about communication issues between the computers?? Fuel filter has about 80k on it.
Any thoughts or do I need to get the codes? (wrote them down but left it at work)

**ob_1jr** · 07-22-2007, 02:10 AM

Definately change your fuel filter. You should only be putting 15000 miles max on the fuel filter. that will most likely fix your problem.

613Duramax · 07-22-2007, 06:11 AM

Then it will be changed tomorrow and I will be more careful to change it more often. So you are suggesting like every third or fourth oil change, swap out the fuel filter?

Will the codes clear themselves after driving with the new filter, or do they have to be cleared manually?

**05_LLY** · 07-22-2007, 08:19 AM

Oh yeah , change the filter, i change mine every other oil change on both trucks, thats 7 k on mine and 10 k on the wifes!

The codes will clear over time as long as what ever caused them to set dosent happen again befor it goes out!

rollincoal · 07-22-2007, 03:30 PM

Your fuel filter is a very important part to change and 80k is way over due. However I don't think this would be a shifting problem. You should take it into a Chevy dealer or anywhere you can get the tranny flushed. As high of kms are on that thing, and if it's never been done that tranny fluid is really dirty. Which would be causing those pistons that direct the flow from your 2nd and 3rd rail to be sticking and keep it in second.

613Duramax · 07-22-2007, 03:41 PM

Cool,

I had the transmission fluid changed along with the filter when I bought the truck with 145000kms on it. So a flush is the right way to go, or is just changing the fluid and filter enough?

rollincoal · 07-22-2007, 03:52 PM

No just changing the fluid is not going to get it. Believe me I saw it first hand. When they change the fluid they're not getting any of the fluid from above the pan. The magnet does pretty good in the bottom of the pan, but it doesn't catch it all!

When they flush the tranny they take a machine and basically wash out all the passages and vanes of the tranny with new and clean tranny fluid. You'll see your fluid go from a dark dirty red to a bright clean red. It's amazing what the magnet and fliter don't pick up.

**ob_1jr** · 07-22-2007, 04:37 PM

I would just worry about the fuel filter. Fuel filters do cause shifting problems and is most likely the cause of your problems.

613Duramax · 07-23-2007, 06:33 AM

Quote:

Originally Posted by ob_1jr

I would just worry about the fuel filter. Fuel filters do cause shifting problems and is most likely the cause of your problems.

Here are the two codes which came up:
U1000: Class 2 Serial communication problem
U1041: Loss of EBCM/EBTCM communication

Would these two codes come up from an old fuel filter?

**ob_1jr** · 07-23-2007, 07:48 AM

I don't think so. Those are electrical communication trouble codes. Weak batteries could cause it or they could just be hicups in the programming.