allison transmision not shifting from stop

I have a 1994 American Eagle on a Sparton chassis. The 6 speed allison goes down the road nicly but when trying to go from reverse to forward or the other way, sometimes it will not shift. If I turn off the rig and restart it then it will shift. The RPM of the 300 HP Cummins is at 700 RPM. The unit has been sitting for the last 3 years with regular exercising of it except in the winter.
Does anyone have any ideas of what to look at.
Re: allison transmision not shifting from stop

You need to know if the transmission control system is getting its throttle signal from a throttle position sensor dedicated to the transmission, or via engine to transmission communication.

If it's got a TPS for the transmission, the linkage might be sticking making the transmission ECU think you're on the throttle. This is a pretty common malfunction on vehicles of this vintage.

It may have some other input sticking. For example, some vehicles have a brake pressure switch wired into the system so you have to be on the brakes, closing the switch, before the ECU will allow a direction change.

There' also a very slight possibility the transmission sump thermistor could be reading extremely cold, preventing range from engaging. I've also seen false output speed readings cause this kind of complaint. There's also a slight possibility the turbine speed sensor could be faulty, making the ECU think the engine is turning faster than it really is.

Just a few ideas to get you started. The best way to check it of course, is to plug in a Pro-Link or Allison DOC to see what the ECU is thinking. That usually leads to the root cause pretty quickly, assuming the operator of the tool knows what he/she is doing.
Re: allison transmision not shifting from stop

Thanks for your responce, that sure gives me some places to look. I do think is is one of those simple switches because the unit has primarily sat for the last 3 years. the jake brake isn't working either and same same. I will start with the linkage and the brake sensor. I hope it is one of the simpler items. I just bought this rig and am pretty good tinkering with that kind of stuff. I will have to take it into the dealer for a couple of items and I will see if they can pull the codes but I'm not sure if they can to that here in the Northeast they good dealers are far and few between. I will try to find a shop that works on Allison if I can find it. I may wait til spring as it is turning real cold here, low 20's as I type. It may sit most of the winter again but I will at leat look on a warm day and spray some lubricantes on the linkages. I'm acually picking the unit up this week-end from a private sale and wanted to start working on it right away. I had a Hawkins pusher prior to this but sitting doesn't help any of them as you must know. I will let you know how I make out.
Thanks for your help, it is awesome to get advice from people in the know. :)
Re: allison transmision not shifting from stop

From just what you say, I'd bet your idle is a tad too high. Mine was at 400 when I got my coach (stalled every time I let off the pedal at a stop). Factory spec is 700, but that seemed a bit high to me. I settled on around 550 and that seems to work well.

Re: allison transmision not shifting from stop

He indicated engine idle is 700. If it has neutral to range inhibit based on engine speed, the inhibit comes in at 900. That's why I didn't mention it.
Re: allison transmision not shifting from stop

Hmmm. Interesting ... inhibit at 900 huh? I wondered just WHAT the limit was (but didn't want to take (waste) the time to figure it out :). When I was adjusting mine, 400 "sounded" good to me, but it obviously wasn't (since it died everytime I took my foot off the pedal). While adjusting it up, I played off what it "sounded" like against how much I guessed it would pull at idle at stop lights. My gut feeling was that about 550 would work (I hope it does - seems fine in the driveway).

900 sounds high, but if the Allison allows it, it must not be :).

I keep learning stuff from all these forums I'm perusing :).