P38A Strange Transmission Fault

The other day on 180 mile round trip to Bournemouth I got "gearbox overheat" message for a few seconds, and then it went away. Hasn't happened since.

Today : . . . . .Put Nanocom on it and checked for faults - no faults reported, but ECU voltage reports as 10.7V. All other ECU's report 16V as per usual in Nanocom.

• Checked voltages at Alternator, Battery, BECM & Gearbox ECU.
  • Alternator = 14.0
  • Battery = 13.8V
  • BECM Input = 13.8V
  • BECM output to Gearbox ECU = 13.6V
  • Gearbox ECU = 13.4V before earth fix, and 13.6V after earth fix.

Checked earth point under carpet behind BECM. Resistance to gearbox ECU was 2ohms, so I soldered the crimp terminals, and got the resistance down to 0.1ohms (or as near as I can measure).

Finally I tried using the ECU from my 4.0 project, but this also reports 10.7V in Nanocom.
Note: Both ECU's are GS2.38, although one is for 4.6 and the other 4.0

1. So did I have a real overheat, and need to check the fluid & cooler ? Trans fluid was done about 3000 miles ago last summer.
2. Or could the original message, simply be bad earthing, and my soldering will fix it ? Guess driving for a few days might answer this one.
3. Anybody know if the 10.7V reading in Nanocom is typical, or maybe I have two faulty ECU's ?

Pete

Sounds like a @Saint.V8 question!

Is there a connector on the side of the gearbox that might be a little corroded?

Not been underneath yet, but planning for the weekend. I recently bought an IR Thermometer, so will drive a few miles, and then get underneath to check the actual temp. ECU doesn't provide temp, but given the known fault messages with low voltage I started there.

Gearbox overheat isn't the one that's a sign of low voltage... if you get just GEARBOX FAULT, then that's usually the first sign of low voltage/bad connections etc.

I doubt the gearbox is overheating (worth checking with IR thermometer just to be sure though!) but is more likely to either be the temp switch on the side of the gearbox cooler, or the connection to the sensor.

I'm doing a diagnostic hookup for another owner in about half an hour, so I'll have a look on the Nanocom at the gearbox ECU on his one and see what voltage it reports, but something niggles in the back of my mind that it is normal for it to read that... Out of interest, did you try measuring the voltage at the gearbox ECU with a multimeter?

Nanocom doesn't seem to give gearbox temp details at all, but will look for the sensor & wiring. ECU voltages were as listed in first post.

The fault hasn't appeared since, but have only driven short journeys this week, & not had time to do the IR Thermometer check either.

So they are... my bad... I read the post, saw the voltages all looked about right and moved on!
I've just read my one (I couldn't get it to connect to the gearbox ECU on the other one I looked at, but I think one of his diagnostic wires might have issues) and I read 11.5V in the autobox ECU on Nanocom. I do have the later GS 87.1 ECU though.

I'll have a quick look in the Nanocom forums in case it's been brought up before...