[midieselguy]

Yesterday I went to merge on the highway and the truck shifted 1-2 but didn't make the 2-3 shift with the throttle down 3/4. It seemed like it was staying in second gear and having trouble getting through the RPM range. The rpm's were moving very slowly above 2500. As soon as I let off the throttle, it shifted into 3rd and then 4th fine.
If I try to move through the gears at a light throttle, no problem. It goes through all 5 fine.
Tranny was serviced 7000 miles ago. Fuel filter was changed last weekend.
I'm stumped. I run a 30 hp tune on the truck and nothing more. There are no check engine lights.

 

[Chevygirl12]

Mines been doing that to. Except its if I stomp on it after being at a stop, the rpms go up but it doesn't shift until I let off the throttle.

 

[dan0617]

do 2 acceleration runs at 1/2 throttle, then again at 3/4 throttle, then again at WOT. Will relearn the trans. Should fix it. If you have more or less babied your truck or drove for good fuel mileage for a while the trans can relearn itself like that, then hang a shift or 2 when you go heavy throttle. If this clears it up, do the same thing again in tow/haul mode. If not, it is almost always a fuel filter or a slight fuel leak.

I have had this happen to me twice already. Once it needed to relearn, second time it needed a fuel filter. Acted almost the same both times.

You mentioned you just changed the fuel filter last week. Possibly something isn't right there?? I'd replace it again before going to a dealer. Keep the old one so if it doesn't fix the problem you can re-use it.

 

[kman2468]

Mine doesn't either ... Don't tell me change fuel
filter cause i did lol

 

[bigdaddydiesel]

You may want to check the fluid level to make sure that it is right. The Allison is snesitive to fluid levels.

 

[midieselguy]

now I notice there's even a sticky about this question.
It was the fuel filter, put a new one on and truck drives like a top.:gearjamin

 

[DMAXchris]

I was gonna say check your fuel filter even though you just replaced it. One tank of crappy fuel will do ya in.