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2005 Mustang V6 PCM, Temperature, AC problems

5197 Views 2 Replies 3 Participants Last post by  TexasBlownV8
Searched forum but could not find similar problem as what I'm having. Symptoms: Hot days, engine warm, temp gauge will drop to Cold and AC will shut off. PCM thinks engine temp is 253F and seems to go into limp mode. Sometimes gauge goes back to normal and AC will engage again.

Independent shop put on their live scanner; had P117 code; temp sensor voltage works normally but PCM temp shows 253, even when cold. Engine does not overheat and radiator fan works normally on both speeds. Thermostat and temp sensor recently replaced. I don't think it's a stepper motor. They think it might be a bad PCM and suggested taking it to the dealer to have them scan it.

Took it to the Ford dealer; no codes and found no problems. Had to pay over $100 to get it out of hock with no additional information. Still has the same problem but has not thrown another code since it was cleared. I pulled all 3 connectors from the PCM and reconnected, hoping for a better connection but that didn't work. Thinking bad PCM. Any thoughts?

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Had a similar issue years ago in a Taurus. It was a bad PCM.
If the coolant temp as read by the pcm is always that high, even when cold, it's like the temp sensor circuit is shorted.
Also, hope you didnt disconnect/reconnect the pcm with the battery connected; that can cause issues sometimes (ie, can damage the pcm).

If you're able to read the coolant temp signal from the pcm, turn the key on, read it, then turn key off. Get a 33k ohm resistor (1/4 or 1/2 or 1/8 watt), disconnect the coolant sensor in the t-stat housing, place the resistor into the sensor connector, harness-side, using the resistor to act as the sensor. Leave the conenctor unplugged, turn the key on, see if the temp drops to around 80-100 degrees. If it drops, the temp sensor is bad or wrong; if it stays the same, you have a wiring issue or pcm problem.
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