Allison 6sp: Po872 problem

[clrussell]

Hey guys I've been working on this truck for 3 weeks on and off trying to figure out what's wrong. Customer truck, 08 lmm efi live delete tune and stock everything else. No trans tuning. Truck will drive and shift perfect until trans temp reaches 100+ degrees. While driving it will start shifting between 5th& 6th gear after fluid warms up. Pressure switch one is showing fluid pressure off and on as the problem is occurring.

Will also do it in first rolling and braking hard as if in a parking lot. It's like it has a valve shifting back and forth. I've pulled the valve body twice and don't see any valves scored or sprigs broke, tried new solenoid and pressure switch with no change.

I know I'll probably get flamed for trying to have y'all help me with diag but I'm lost as where to go from here. I don't want to throw a bunch of parts at it and still not fix it.

Thanks in advance

 

[clrussell]

Just realized my thread title is wrong..

Po842 : shift sol c stuck on

 

[Mike L.]

I would replace pressure switch manifold and make sure the connector on the internal harness is not leaking.

 

[clrussell]

Forgot to post it has a new pressure switch manifold and internal wiring harness. The old harness was leaking through the connector.

What's weird is it failed consistently at about 100-120 degree or within 5 miles until I took the tcm off the fan shroud to back probe the pink wire of pin 17 for ps1 signal. I was checking to see that the tcm was actually seeing a ground when it said it was (which it was correct). After I put the lab scope in and was watching for ground on the pink wire I drove it 30 miles and never had a problem. Went back and test drove later with lab scope on it again and it did failed within the first 2 miles.

I'm not sure if anything makes sense of what I'm posting. It's hard to describe some of it.

Thanks for helping Mike, I know your busy

 

[clrussell]

Also removed tcm from problem truck and put it in my daily driver and drove about 15 miles with no problem and trans temp up.

 

[Mike L.]

Change the C solenoid with either D or E and see if it throws the same code or a different one. If it throws a different one you have a bad C solenoid. If it throw the same code start looking at your main harness leading to the TCM for a short.

 

[clrussell]

Thanks mike, I'll check into external harness for short. Already moved solenoids around.

Am I on the right track by thinking it's something to do with shift sol 1 or shift valve 1? Looking at the fluid diagrams for 1st 5th and 6th pressure switch 1 is off and is what shows me that something is changing with shift valve 1 correct? However if shift valve 1 was actually moving while in say 6th gear wouldn't it try to apply some clutches that didn't need to be applied and do some really aggressive things?

 

[Mike L.]

The TCM thinks there is a stuck valve or a glitch. It will try up to 15 times to set things straight then it will throw a code when it doesn't work. Your problem sounds like an intermiten short in the harness.

 

[clrussell]

Ok I'll pick through the wiring harness today, thanks!

 

[clrussell]

Found the problem, was in the wiring harness up by the speed sensor on the trans. Pressure switch 1 signal wire had the smallest rub spot I've ever seen. I looked at it for a couple minutes before I could tell it was actually bare. Repaired it and I've drove it about 100 miles now and it's been great.

Pink wire, I tried getting the best picture I could such a small area it was hard to get a quality picture..

Mike thanks for helping. I really appreciate it! If I'm ever around your area I'd like to buy you a beverage

 

[Mike L.]

Found the problem, was in the wiring harness up by the speed sensor on the trans. Pressure switch 1 signal wire had the smallest rub spot I've ever seen. I looked at it for a couple minutes before I could tell it was actually bare. Repaired it and I've drove it about 100 miles now and it's been great.

Pink wire, I tried getting the best picture I could such a small area it was hard to get a quality picture..

Mike thanks for helping. I really appreciate it! If I'm ever around your area I'd like to buy you a beverage

I'll buy you a beverage my friend. You are good. Glad you fixed it.:thumb:

 

[clrussell]

Thanks again for the help mike.