FMTV transmission code 71

[justin.donze]

Just got done replacing an engine in a M1088A1 engine runs great no leaks all fluids are at proper level I have a check Trans light on dash board, hook up code reader and I have a CODE 71 mainly saying no communication with TCM but took it out for a test drive drives great goes thru all gears. It's a GENIV trans any suggestion would greatly be appreciated.

 

[DeMilitarized]

You probably lost either J1939 or J1708 canbus communication. Truck will operate "ok" but will be all sorts of angry. Check for 60ohms of resistance across the j1939 lines at the 9 pin diagnostic port off and running. If you get a different reading you have a module or resistors bad causing interference or grounding.

 

[Ronmar]

You meant to say "not running" right @DeMilitarized ? testing buss resistance should be everything off?

The biggest operational interaction is that the ECU shares throttle position sensor info with the TCU. This plays a BIG role in how the trans behaves and shifts. without it the trans has no idea what the drivers intentions are (steady, accel or decel) so it shifts based on it's minimum and maximum RPM points(makes for harsh, poorly timed shifting)...

 

[DeMilitarized]

You meant to say "not running" right @DeMilitarized ? testing buss resistance should be everything off?

The biggest operational interaction is that the ECU shares throttle position sensor info with the TCU. This plays a BIG role in how the trans behaves and shifts. without it the trans has no idea what the drivers intentions are (steady, accel or decel) so it shifts based on it's minimum and maximum RPM points(makes for harsh, poorly timed shifting)...

Yes my bad. Truck needs to be ignition off.

 

[justin.donze]

not get getting 60 ohms at the J1939 that's with ignition off, no harsh/poor shifting truck runs and drives great and idles good as well. Any other suggestion would be greatly appreciated.

 

[DeMilitarized]

Well.... What was the reading? if it was open loop (OL) then check both terminating resistors on the j1939 line and pull them out and check for 120 ohms across each resistor. One will likely be by the j1939 9 pin port and the other probably near the ecm, abs module, or tcm.

 

[Ronmar]

Yea one is back by the ECM, the other is under the passenger dash. triangular, probably grey, Deutsch connector with no wires coming out of one end. 120ohm each, in parallel measures ~60ohms thru the buss wiring...

 

[justin.donze]

well.. checked all of those am I looking at swapping ECMs?

 

[Ronmar]

Do you have CTIS? If so unplug it and then power up/start the truck and see if the TCU comms fault clears…

 

[DeMilitarized]

well.. checked all of those am I looking at swapping ECMs?

Probably not. What are your readings of all of those resistance values?

 

[Ronmar]

yea lost the plot on this one. What Demil said...

I am getting old and get confused easily...
1. You have a TCU comms fault and you don't have 60ohm at the service port?
2. Both 120 ohm end resistors are present and @120 ohm?
3. You still do not have 60 ohm at the service port and a comms fault?

 

[justin.donze]

Turns out bad diode in transmission wiring up above the starter was bad and throwing codes thanks for the help

 

[GeneralDisorder]

Turns out bad diode in transmission wiring up above the starter was bad and throwing codes thanks for the help

That's not a diode - that's the J1939 termination resistor I'm assuming you are talking about. Just for clarity incase someone runs across this same issue. That's a communication bus and not specific to the transmission - it is used by all the computer modules in the truck. The TCM and ABS are just the loudest at complaining about it. It's a 120 Ohm resistor in a Deutsch DT package.

There are diodes in the wiring but they are either in the PDP or in the Intermittent wiper module on the A1R

 

[Ronmar]

yea not aware of any diodes IN the transmission wiring... Did it look like this?