M1089 wont downshift, stalling engine.

[thetruckguy]

I just picked up my m1089. After I replaced the starter, and got I to run, she took right off. Shifted up tight at first, but then smoothed out. For the first few intersections, it was fine. Then it started to not downshift on its own, and stalled the truck. I had to manually downshift the rest of the way home. It downshift fine that way. The only gear that wouldn't stall out the truck, is 2nd and reverse. (Didn't try 1st). Has anyone else experienced this?

 

[NDT]

What year is it.

 

[thetruckguy]

95

 

[Skyhawk13205]

It could be your TC lock up is not disengaging causing a stall. I think the lock up is at 3rd gear.

you could check the transmission fluid to make sure it is not disappearing or contaminated. Replace the fluid, pull and inspect filters for contamination. Lastly you can take to shop to check the sump, valves, clutch pressures and outputs to see if the transmission is operating correctly hydraulically and electrically.

 

[NDT]

You have a WTEC II. Pull the codes to see why it's not working right, then go from there. Attached is the Allison book.

 

[Ronmar]

Yep, read codes. Pay attention to the mode on indicator while reading the code memory locations, it being lit indicates a current active fault. The fact you can downshift manually is a good sign. This could be as simple as a Throttle Position Sensor(located on to- of the engine) out of cal or failed...

As mentioned the torque converter lockup is engaged in 3rd and above, so if you roll up to a stop in 3rd or above it will stall the engine...