Example of an Automatic transmision diagnostic and basic fix

on Wednesday 14 November 2007
in A-Z Index - Everthing is listed in here > Transmission

The Overview:

My car is a 1990 Japanese spec Nissan 300ZX twin turbo, auto, 2 seater. I bought it in June, 1999 with an indicated 75K on the odometer. The condition of the car suggested to me that the reading was probably correct - overall it was in excellent condition. It is well maintained and in the last 18 months has had a new PTU, TPS and a fully rebuilt transmission with external cooler fitted. The 100K service was carried out on schedule and included replacement of variable timing cam gears.

The Terminology:

  • ECU: Engine Control Unit
  • ECCS: Nissan terminology for ECU
  • ACU: Automatic Transmission Control Unit
  • TPS: Throttle Position Switch/Sensor
  • AT: Automatic Transmission

The Symptoms:

There has been a long running problem with the transmission incorrectly changing gears (even before the transmission rebuild, the problem existed). The problem was intermittent and manifested itself it the following ways:

  • Would not change up through the gears at the correct revs. Would rev higher than normal and them slam into the next gear
  • When cruising, even slightly touching the accelerator would cause the AT to drop down a gear
  • When backing off accelerator, AT would change into a lower gear

Overall a very annoying problem that took most of the fun out of driving the car.

The Attempted Fix:


I took the car to the local Nissan dealer and they diagnosed a faulty TPS - hence the replacement. This coincided with cooler weather so the problem seemed to have been fixed. When the weather got hotter again the problem reappeared but never when the Nissan dealer had the car. There was a fall out with the dealer (over many other things) and the problem was not taken any further with them.



The transmission failed and so was reconditioned. This coincided with a spell of hotter weather. The problem came back but again never when the transmission specialist had the car. Through out winter there was no problem. This summer the problem came back on a daily basis and I decided that if it was going to be fixed then I would have to do it myself.


The Diagnosis:

The first thing that I did was an AT diagnostic check. This returned an error code of 3 - "TPS shorted or disconnected". Suspecting that the TPS was again faulty (if it ever was) I measured the voltages while changing the throttle position. The TPS seemed fine. I did an ECU diagnostic and got no errors regarding the TPS. This was confusing to me because I believed then that the diagnostics where on the same controller. Finally, some email from Reebe Mayer focused my attention on the possible problem. After consulting the schematic (484K) for the ECCS (ECU) I discovered that the signal from the TPS goes into one pin of the ECU, out another pin, and then back into the harness. Unfortunately the signal never made it back out of the harness. The diagnostic message was spot on - disconnected. Finally, I had found my problem.

The Fix:

The thorough way to fix the problem would have been to trace the harness (and open if neccessary) to try to find the exact location of the problem. The circuit diagram showed 2 connectors between the ACU and the ECU. Working in the passenger footwell I could see neither of these connectors and decided to take a short cut to rectify the problem. I neatly ran a jumper wire from the output of the ECU to the input of the ACU. I connected pin 34 of the ACU to pin 56 of the ECU. (Please double check these pin numbers if you feel that this fix may be a solution to a similar AT problem.)


The Result:

The car is a different beast. There are no problems with gear changes and the car cruises much more smoothly than before. I have a large grin between each ear, partly from having a smoothly running car again and partly from knowing that I fixed a problem that professional mechanics were either unable or unwilling to spend enough time to do.

- EvZ