As long as I don't go over half throttle the truck drives fine, but if I go past that the truck seems to get hung up and won't shift. The first time it happened the check engine light came on and the truck acted really strange, almost like it was a motor problem. Later that day I hopped on the highway and when I hit the gas to get off the on ramp, the truck fell flat on its face. I let off the gas and the truck shifted but hard. Then when I got up to speed easing on the throttle the truck wouldn't go over 65, and if I did it felt like the trans brake came on and the truck would slow was down to like 35. If I shut the truck off and let it sit for a while it would act fine again until I went over half throttle. The trans temp never went over 150* and the fluid level is right where it should be. Any ideas?

Edit:The truck seems like it just doesn't want to shift in the lower gears, if I let up on the peddle or kinda flutter it it will shift. This isn't the dreaded allsion limp mode is it?

What year, make, model, engine and aftermarket goodies? IE Juice etc. The more details the better.

2001, 2500 ext cab, 6.6L duramax, allison tranny, box stock as far as I know. I can't see anything aftermarket so I'm assuming its stock. I wish I had a datalogger or code scanner.

How long has it been since you changed the fuel filter? It should be done about every 10,000 miles. If you get a couple of bad loads of fuel they really hurt performance when they start to plug.

It was just done :(

The TPS couldn't make it do that could it?

a friend of mine with more experience with automatics said it feels like the torque converter may be slipping. Could that cause me to go into limp mode? The truck has absolutley no mods done to it and as far as I know never has (I've looked and couldn't find any evidence). Could my TC just have gone bad?

I have the same exact problem. That is why I'm on the board. Mine started like this... A type of sensor (fuel pressure sensor? on top of the motor right next to where the fuel line connects) had backed off a bit and was leaking diesel like crazy. Truck ran fine while it was leaking. I tightened the sensor and it quit leaking. But ever since then the shifts will "hang" under heavy throttle unless you back off the gas, then it will shift. If you just stay on the gas and let it hang, it will throw the check engine light on and lose power. After that the truck will not turn over about 2200 rpm. Some days the problem is totally gone. But most days, especially the last few days, it is there.