M931 shifting issue

[SouthwindFarmer]

I have a m931 with Allison automatic that doesn't stay in the gear selected, it shifts up through as though the selector is on 5. I've poured over the TM's plus numerous searches and can't seem to find a cause or fix. I've changed the 5th gear lock-out, tested voltage following the tech manual, filter and fluid change, new shift cable and modulator and just can't seem to pinpoint the problem. Thanks for any help.

 

[sandcobra164]

Is the engine turning faster than 2,100 rpm when it shifts up?

 

[SouthwindFarmer]

Is the engine turning faster than 2,100 rpm when it shifts up?[/QUOTE

Hmmm. I think yes, but I'm not 100% sure I can answer that. Because we only use the truck during the summer and fall I would have to do a test drive to verify.