Transmission Goes into Neutral and Looses TCM Communication

Matt Technical Support Specialist Texas Posted   Latest   Edited  
Case Study
Network Communications
2009 Ford F750 6.7L (C ISB 200) — 3FRPF75AX9V131919
P0880 — TCM Power Input Signal
Intermittent Neutral Trans
Intermittent Communication Loss With Tcm
Trans Light
Heavy Duty
Medium Duty

This is a write up I did back in March-

2009 F650 Cummins ISB/Allison 2000 series 4th Gen. 

This was be the 4th Allison TCM I have seen replaced in a year. (This was the 4th truck with a failed TCM) I was really expecting a wiring issue being as this fleet is full of driver induced wiring issues. With this one I was already trailing after the first tech told me he put 3 VSS's in it and it still has the same code.

First, thing I did was scan it with the Autel, then again with factory tools. I get down to the harness throw an ohm meter on my VSS pins @ the TCM with it unplugged. Do the wiggle and jiggle, no easy opens found here. 

Second, I go through my preliminary checks. Meanwhile I do a cooling system pressure test on the cooling system due to the ECT temp codes and the engine is stone cold at this point. No leaks, holds rated cap pressure for over 20 minutes. Meanwhile I go through and load test each battery. I have one that's great one in that grey area. I go ahead and replace just one do to the goofy contract we have set up. No coolant leaks found, held pressure. Test charging - 13.9 volts DC loads on - 98mV AC. That's about double the AC voltage I want to see but I'm not getting to excited at that. Inspect fan clutch, no visual issues, definitely moving air.

Third, I clear all codes. Voltage drop powers and grounds to the TCM, no issues found yet. I hook up T-pins to my TCM powers and label my grounds with tape. I then run leads to the VSS terminals and into the cab. I go for drive with a scope on the VSS and a laptop. No real excitement of the first go around. The ECT would creep up 220 but no codes, never really stayed there. I even began to wonder if the battery was the issue.

Forth, I let it sit for lunch. Then lunch turned into meetings. So, it got a good 3-hour soak.

Fifth, I re-scan for codes after restart, none found. Take out for a drive again. ECT is creeping around 250 this time. Still not seeing any excitement. About 4 miles into this drive I lose TCM communication. CHECK TRANS indicator flickers then lights solid. I go back over to Wabco, VSS & WSS's look happy in there, now I see no VSS activity in Cummins.

Sixth, I notice I'm now in limp in (2nd gear only) and my ECT is now pissed and setting codes. I pull over, KOEO check powers & grounds @ TCM with an old school test light-all is well there. Then no restart due to no communication. DOH! Now I'm really hoping the comm lines are happy. I plug in the an old TCM I have with a bad solenoid driver, and truck starts. I now have communication and the old code from my use TCM. So, I drive the truck back and write it up for a TCM and an engine coolant thermostat.

I would have really liked to scope my data lines during the event but at this point I'm lying under a truck on the side of runway (airport) and just don't feel like dying today.

Well that got long winded! Anyone see these fail a lot? Hydraulically these seem kung foo!

Shout out to Dean Fox, the average Joe ;) for breaking the ice!

Michael Diagnostician

Enjoyed your writeup on this vehicle. So this is the 4th tcm in this truck if so there is likely a hidden issue. Did you happen to scope the A/C Ripple. Also is it the same type failure each time or something different each time. I have not seen many of these fail yet but I dont deal with alot of heavy duty.

Ð Awarded
Matt Technical Support Specialist

Sorry, that was misleading how I wrote that. This was the 4th truck with a failed TCM. Also, Allison has an updated controller and there is no core on the old one.

Ð Awarded
Bob Diagnostician

Nice case study Matt. I also see you used the edit function to add the tcm clarification. That's a great feature for these forums. Take care

Ð Awarded