Some problems take more than one crack to solve them.

This was one such.

An ongoing project; almost finished at Christmas and then re-finished by January 2nd was getting a Subaru Outback running again after replacing failed cylinder head gaskets. A fun job that involved removing the engine; twice. Replacing the engine; twice. Cursing copiously 23rd December when I realized on starting the engine that in attempt one I had put one of the gaskets in backwards; once. But it took a few minutes to get it all of the choice language out of the way before attempt two could commence. I digress…

As of last Sunday there were two remaining issues. The engine reported three codes – P1700, P0731 and another one lost to me right now. However P1700 and the P- that shall remain nameless both pointed to the Throttle Position Sensor (TPS) not doing its job. P0731 said the transmission was doing something weird. The transmission oil temperature light was also flashing and, employing the secret-handshake method to get the computer to flash out its code, it said – ’31’; also indicating the TPS was to blame.

The car drove fine – maybe idling a little high – and jarring teeth with its gear changes and inability to turn sharply at low speeds without the front and back tires fighting. But apart from that fine. Ish…

Having determined the TPS was a likely culprit, I dropped $135 on a new one and installed it and guessed at the adjustment position. And lo and behold – the ‘check engine’ light went out. Wahoo!

However the gear changes were rattling my fillings still, and the AT OIL TEMP light was still blinking in blunt accusation that we did not yet know what the problem actually was.

I flushed and refilled the transmission and applied a new filter. The gear changes were now jarring, but my teeth were safe for now. The light looked a bit happier (projection..), but it was still flashing on. Still code 31. TPS.

I took the vehicle over to see a friend with much experience with many vehicles, engines, trasmissions. He bought out his code reader. We did not get any further than seeing code P0731 come up again (transmission misbehaving) and code 31 on the transmission itself. Then we talked and theorised and socialized. Moved a little further. And the germ of a theory hit me. Perhaps the vehicle was of two minds. Literally and electronically…

The idea was that the ECU (Engine Control Unit) was now happy with the Throttle Position Sensor, but the TCU (Transmission Control Unit) remained in a mood. The discussion with my friend led to the conclusion that the transmission was working in a basic ‘limp home’ mode. The missing TPS input probably convincing it (via the mind of the software designer) that this was the best course of action under the circumstances.

Two minds…

Now in my mind (mind #3) – the mind of an electronic engineer amongst other long-term and evolving guests – the transmission control unit must be getting this signal somehow. There was only one TPS so the signal must somehow get to both TCU and ECU. Surely one of them digitizes it and then sends to the other? However this was getting into complex speculation. I needed wiring diagrams. These can be found if you are willing to pay. I paid. This was pissing me off. The next stop would be a transmission shop. In a word – NO.

I found the wiring diagram which split the signal wire inside the wiring harness to go to BOTH the ECU and TCU. Both units would digitize the signal before using it. Independently. Ah well, apparently the designers had not yet optimized how this stuff might be done.

As part of the data I had found there was a 25-step debugging guide for just this transmission trouble code – 31. It involved disconnecting the TCU, the ECU and testing connections from the TPS (remember that?) to the connectors for each of these. I worked diligently through it under the steering wheel, hood and in the passenger footwell…

Then came step 15…

‘Turn the ignition on, and ensure the throttle position signal on Pin2 is between 0.3 and 0.7V of the ground on pin10’. It was not. It was 0.12V. After about 10hrs on this issue…

I went under the hood, undid two screws and rotated the body of the TPS to adjust it. I tightened the screws and checked the signal again – 0.32V. Hmmm. In range but not enough margin. I adjusted it again – 0.44V. Ok; I will take that.

And that was it. I realized that this signal had been wrong. I corrected it. I put things back together enough to start the engine. The engine warning light and ATF OIL TEMP light were off and remained off. Time for a test drive.

I put it all back together and spent 20 minutes enjoying smooth-to-the-point-of-undetectable gear changes and effortless low speed turning. No warning lights. Problem solved.

The problem? Two egos in the same vehicle. One that said ‘0.12V? No problem! We can work with that!’ and the Nazi TCU saying ‘The specification says minimum 0.3V dimwit!’. Once both were satisfied, problem solved.

It turns out, your vehicle can be in two minds. Who would have thought it?