Allison Question

Bart2500HD · July 30, 2004

After my truck sits for awhile, like overnight, I have a weird noise that comes from the tranny. A second or two after starting the engine, I hear a clunk and feel the truck move backwards briefly as if I have it in reverse with the parking brake applied and try to back up. This is with the lever in Park. does this happen to anyone else? Is this common for the Allison? I seem to have read someplace that after sitting awhile, the tranny fluid drains down into the pan and this happens when the fluid pressure builds back up. The tranny works great otherwise and it has had the NSBU switch update done.

Chev8.1 · July 30, 2004

Yes this is normal. I have watch the info video they sometimes give with new trucks, It explains all the going ons with ally. Dad got one with new truck(info video). Also mine does this also every so often.

BlackSilver · July 31, 2004

.... it has had the NSBU switch update done.

What is an "NSBU switch update"?

Firetiger(MN) · July 31, 2004

The NSBU switch in some '03s and older would confuse the truck as to whether it was in gear or not. If a truck sat for a long period of time and you tried to put it into gear, the gear indicator would flash and nothing would happen. There are two models, tan and black colored. I can't remember right off hand which is the newer version. I believe I also heard of some people saying that if it got wet it also could cause the same symptoms. The new version has a small shield to deflect water.

There is also an issue with tranny fluid drainback that will cause a short delay until the pressure comes up and then the tranny will engage.

The sound you hear when first starting the truck (hollow thump) is the hydraulic pressure coming up to running spec. Normal and described in the Allison video.

Bart2500HD · August 2, 2004

The newer NSBU switches are the tan colored ones. Thanks for the info guys. I bought the truck used, so no video about the Allison. Anyone want to send me a copy?