So I had a similar problem several months ago, took it to a dealer and they found the code P0875 and told me I needed a new NSBU switch. So I bought a new tan NSBU switch and replaced the connectors as well. Truck was fine for about 500 miles and then the problems got worth. After driving it for atleast 30 minutes it would have a hard 2-1 shift when slowing down and would feel like someone bumped me from behind. Also when doing this, if I shifted it from Park to Drive it would have a loud clunk.

It's done this 3-4 times. Works fine when driving less than 30 minutes. I took it to a dealer and they pulled these two codes....

P0875 - Reverse Pressure Switch Circuit Malfunction

P0846 - Pressure Switch Solenoid D Circuit Stuck Open


What's causing this? How can I fix it?

Could be low on fluid or a couple of open circuits. Start with the fluid, warm the transmission up to 150-180 F and the fluid should be between the mid and top of the Hot Band on the dipstick on level ground in Park. Also, when was the last time the spin on filter was changed?

If the fluid looks good I'd check the transmission connector for broken or corroded wires or pins. Pin F is for the D pressure switch circuit and Pin K is for Reverse. They're both grounded through the case.

Would a new transmission fluid pressure switch be the answer to codes P0875 & P0846?

I heard from a very reliable allison mechanic that its likely a bad switch causing the problems and triggering those codes? True or false?

The last dealer said a P0875 is a bad NSBU switch, which turns out isn't the case.

What do you think?

One other thing I just discovered. The transmission fluid level is way to high. Checking it cold (about 110 degrees) the fuild level is over half way up the HOT marks on the dipstick. When it was having problems I checked it hot and it was WAY above the hot mark.

So I got to thinking, a while back I took it to a local dealer to have the oil changed and told them to top off the fluids if needed (never had a tranny problem before this). Then a while after the tranny problems started. So I took it back to the same dealer and they're the ones who told me the code P0875 and it needed a new NSBU switch. So I installed a new switch but it didn't fix the problem.

So my question is, is it possible that the levels being way to high could cause these tranny problems and trigger these codes? Do you know how I can lower the transmission fluid level? Or do you still think its the pressure switch?

Thanks
Tom