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For a 2002 SPI engine and 4F27E automatic transaxle with 80,000 miles, my tranny problems began with shuddering under seemingly random circumstances in 3rd and 4th gear. The only prerequisite being that the problem only appeared after the car had driven at least a couple miles and warmed up a bit.

The shuddering graduated to something worse in those ranges: the engine racing and then slamming into engagement. A cycle that repeated e.g. it would slip again with engine racing, then slam back into engagement.

At the same time, I also noticed a finer point -- that once the car had warmed up and the problem begun, 1-2 and 2-3 up-shifts were also harder.

I found DTC's P0750 and P0755, so I replaced the A and B solenoids along with fluid and filter.

This gave me some improvement. I found that I could drive in 3rd with O/D switched off at up to 50 mph, at which point the race/slam behavior would appear. If I drove with O/D on, the bad behavior would appear anywhere in the 3rd or 4th range.

Because of the 50 mph trigger, I suspected that I had a problem with torque converter control operations.

Working through some of the FSM's pinpoint tests such as I was able with my tools, in test A8 I found an open in the transaxle vehicle harness in the wire to pin 8, which serves solenoid B and supports TCC operations. I found and fixed a broken pin 8 wire between C439 and C96, which is easily accessible. (I also found and fixed another wire there that was damaged but not broken.)

I thought I would be good to go at that point. And indeed, fixing that wire had given me back TCC operations and I no longer had any of the shuddering or racing/slamming behavior.

But I had lost 1st gear (under D or manually selected), reverse engagement was now harsh, and there was also a harsh downshift to 2nd at no throttle. All these problems were entirely new. It may be that all up-shifts are a bit harder, though I'm not sure. And 4 codes were then reported: P0750, P0755, P0760 and P0765, which is "solenoid circuit failure" for A, B, C and D.

Here is one of my only clues now: I was able to run pinpoint tests A5-A8, A10-A14, and B3-B10. And I show that everything is OK/in range, except that in test B5 which looks for a short to ground through the vehicle harness or PCM, pin 5 is at 11.5 Kohms, but pin 4 is 0 ohms. Test B6 follows up by removing the PCM and testing again to see if the problem is isolated in the harness. But I now showed pin 5 at infinite ohms and pin 4 at 100 Kohms (anything over 10 Kohms is OK). This is supposed to be an indicator that there is a short to ground in the PCM.

In fact, I had gotten the same result during my tests before finding the broken/damaged wires. But I rationalized that since pin 4's function is "sensor signal return (reference/ground)," maybe it was OK if it connected to ground.

I pulled the PCM and confirmed that the same circuit (pin 91 at the PCM) connects to ground (I tested it against pin 103: un-switched ground).

But there are other pins on the PCM with the same general function of "sensor signal return (reference/ground)." So I tested those to see if any of them connected to ground. None of them did except pin 76. Even though pin 76 seems to share a "sensor signal" function, which I didn't understand, I gained some confidence that pin 91 really should not be connected/shorted to ground.

I opened up the PCM but did not see any obvious problems.

At this point, I have a very tentative diagnosis of a bad PCM, but since the circuit in question (pin 4 at the vehicle harness connector, pin 91 at the PCM) is supposed to be working with the TFT sensor, I don't know why I'm getting all these solenoid codes, and also why I'm not getting any codes that more directly implicate the PCM.

In short, it doesn't quite add up for me. Anyone see clearly through the haze?
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