Allison Problems

highroller152 · February 28, 2010

Hey all, I have a new problem with my truck and can't seem to figure it out for the life of me. I am really worried about this one.

2001 2500HD 4x4 8.1, Allison, 94k miles. I just finished performing all the maintenance items up to 100k miles except for the transfer case fluid change.

I just started getting this problem:

When the tranny is cold, it shifts just fine. Once is warms up to operating temps, the dash indicator will flash and eventually (in 3-5 minutes) disappear from the D. Then as you continue driving (assuming you slow down and the tranny downshifts), it will not shift out of 3rd gear (ie. you come to a stop light, it will downshift all the way to 1st just fine, then as you accelerate it will not come out of 3rd).

If you come to a complete stop and put it in P, sometimes the indicator will light up in P and other times it will not. Then as you slowly run through the gears, it is hit and miss whether it will engage into R (if it does not engage, it will stay in N) and the same for D. Once you get to 3rd, there's a huge thunk and the truck lurches forward as it engages into 3rd.

Sometimes, the indicator will show solid, other times it will flash. If you continue, the indicator will again disappear as you enter 2nd and reappear in 1st. If you go back up in gears again, you get a hit and miss situation whether it will engage into D.

There is no check engine light and again, all fluids are at spec. This started happening before I changed the trans fluid and filter but the fluid levels had been fine before I changed it (and they had been changed around 50k miles).

One thing that I have not done is replace the sensor on the side of the transmission. When I changed the fluid, it was used (off-pink) but not burnt (brown).

Any ideas would be greatly appreciated.

Course, these trannies are sooo cheap, maybe I should just put a new one in and call it good

highroller152 · February 28, 2010

Bah, would an admin mind moving this post to the HD forum please? Sorry about that - it's late and I'm stressed about my tranny

highroller152 · February 28, 2010

Something else I just found (on another forum... shhh):

Could be either the Neutral Saftey Switch or the Reverse Pressure Switch.

I'll give those a shot tomorrow and see if it's either of those. Anyone else have any other suggestions?

dewfpo · February 28, 2010

The NSBU sensor has been a known problem on the early Allison's. It's not expensive if you replace it yourself and it's fairly easy to replace.

DEWFPO

highroller152 · February 28, 2010

Thanks DEWFPO. That's two votes for the Neutral Safety Switch. I'll have to see if there's anyone open today that has these. RockAuto's got them for around $120. I guess 120 is better than 4500.

highroller152 · March 2, 2010

OK, as a follow-up, I replaced the Neutral Safety Switch and drove over 100 miles today (about 50 of those I was pulling an empty flatbed). I didn't have a single issue with the transmission. Keeping my fingers crossed, but it looks like that was the issue.