ht 740 d transmission

[haroldmc8]

I have a retran from detroit allison that is out of warranty by a few weeks. The problem is with reverse. When it is in neutral with the 871 running, the trans tries to go into reverse. The driveshaft starts jumping and jerking. If I speed up the engine the trans goes to neutral it seems,driveshaft stops moving and all looks ok. When I put it in reverse it does the same jumping and jerking for a few seconds and then goes into reverse. It will pull in reverse once it goes all the way in and pulls forward good. I have replaced the shift cable thinking it may be the problem, but was not. The transmission does the same thing with the shift cable unhooked and shifting the lever by hand. Any ideas what the problem is ? It seems like a clutch dragging for some reason. I have only about 2000 miles on the retran.

[luvrbus]

Harold, do you have a air or manual throttle.you know you can go to the Allison web site and download a trouble shooting guide for a few bucks and you will always have one .but if your engine speeds up I would check the modulator adjustment first   have a great day


sorry but I missed read about the engine speeding up I thought it doing it by its self

[haroldmc8]

I have just installed an air throttle with an air modular. I did not think the modulator went into action until you were at highway speed. Thanks for your help

[luvrbus]

Harold, some of the guys that have installed the air throttles can help you on this one, the 740 will do strange things if the pin is missing or the wrong pin in the modulator.

Mohave Steve was having this problem when he installed a air throttle on his bus and it was the wrong pin If you like go to the archives on BNO if its working and read about it

[TomC]

To me sounds like a blown valve body gasket where some of the trans fluid is leaking into the reverse circut causing the jumping.  I think you're going to have to take it back to the shop for work-maybe they'll cut you a break on the repair.  Good Luck, TomC

[compedgemarine]

the first question is did this show up as soon as you installed the air throttle or was it there before. if it showed up with the change then I would tend to look at the modulator. if it was there first then maybe something in the trans.

[haroldmc8]

It started acting up a couple of weeks before I installed the air throttle. It is definately worse now for whatever reason.I removed the cable modulator and stuck the air modulator in real quick to not drain much trans fluid. The rod did not come out,I have removed the cable unit before to replace the seals on the old transmission.

[luvrbus]

Harold, if your pin has a lots of play in the transmission side or the modulator side it will give you these problems I would pull the modulator and check for excess play.( you don't have but a couple of gallons of oil over your head) not funny huh

[haroldmc8]

Are there different sizes of the pins available? I know there are different air modulator part #s, I may not have a matched set of parts. I ordered the modulator from an allison detroit dealer,but no gurantee that I have what I need . I will call Allison folks in the am and check on the parts. Thanks for all of you bus nuts help. I will let you know how I make out.

[luvrbus]

Harold, there are several different pins for the 740 and the modulator     Good luck