Allison automatic gurus, help!

[96bulldogpower]

Hello guys! It's been a while since I've posted but I came back to my old friends for some help.   A few years ago we completed an Allison HT740 swap into a 1991 Mack Ch613 that was equipped with a 9-speed originally.  This is a dump truck, originally this was only supposed to be an experiment to see how we would like an automatic, but we have had such a great experience running this truck that it grew on us quickly.   

Now for the problem, two days ago the driver reported that the truck would no longer go above about 15 miles per hour. Upon a test drive I have concluded that the transmission will (manually selected on the shifter) go into first and second speed but will remain in second speed when shifter to third on the shifter.   That transmission refuses to go into third and fourth now.  While I drove the truck I played with the shifter until I got it to go into third at which point I began to slow down and I noticed that once in third speed the truck began to buck and jerk (I was coming to a stop) and it almost stalled the engine right before I tossed the shifter in neutral. I was not able to replicate third speed again and will only go in 1st and 2nd.    It is worth noting that sometimes the truck will not want to shift into reverse unless you play with the shifter back and forth until it "grabs", also the back up alarm and back up lights do not come on until the transmission does finally fall into reverse.     

I have adjusted the shifter cable but it did not appear to be the issue to begin with. 

 

Please help guys I need to find a fix for this truck quick. i will trailer this truck from the job site to my shop this weekend and i already want to start brainstorming what to look for. 

[TeamsterGrrrl]

Sounds like the problem is with the shifter, which is cheaper to fix than the transmission... Thus you want to rule out shifter problems before you go into the transmission!

[96bulldogpower]

With the shifter? But this is an older mechanical, cable operated transmission, the only thing electronic is the modulator. I'm not sure if that is what you meant, but the shifter has already been adjusted without a change in performance.    

[TeamsterGrrrl]

Yup, I know it's a mechanical Allison. On the ones I've driven, if you don't move the lever beyond the "2" position it won't upshift any higher. Thus if the shifter or cables travel was restricted by binding, out of adjustment, etc. it wouldn't shift beyond 2nd. Hopefully that's the problem, if you have to go into the transmission it gets expensive.

[96bulldogpower]

I just got under the truck and had the driver flip through the shifter, it looks like it is functioning 100% correctly

[Maxville Truck]

Check the governor , 4 bolt cover on driver side near the output shaft, pull the cover off, grab the governor with the weights in your hand and the gear facing up, squeeze the weights in and out the spool valve inside should move up and down freely if not clean it thoroughly ( look for tiny metal filings) put it back in. try it again, also check operation of your kick down mechanism ( electric, pneumatic or cable) make sure that it functions properly.

now for the bad news...... if none of the above helps pull the pan, note how much "gobbledygook " is in the bottum, clean the pickup screen and stuff out of the pan.. "if" there was excessive stuff (metallic, and clutch friction material) be prepared for an overhaul...