Where is the temp sensor?

Ok, some new information. For the 1999-2004 MY, when the AC is on, the PCM calls for FAN. The speed of the fan is determined by two things. The temperature of the engine OR the pressure in the AC system.

There are several PCM PID's that control this process. AC-pressure (low/high), LS fan (on/off), HS fan (on/off), AC requested (on/off), and finally engine-coolant temperature.

With AC off the selection of fan (off, LS, HS) is determined by coolant temperature. With the AC on, things get just a little more complicated.

Note, the car's speed also comes into play as the fans do not run above 45 MPH.

There is an extra AC pressure switch. Normally the value of AC pressure is "low". The PCM responds by making sure the fan is running (at any speed). Should the PCM get a signal that AC pressure is "high", this forces the HS fan to run until 20 seconds after the AC pressure returns to "low".

OK, we know that the LS fan works. However, there some things we do not know. For example, what does the PCM believe the engine coolant temperature is. Does the PCM know that AC has been requested?

Regarding the AC pressure switch. It is actually a two position switch. At 325 PSI it tells the PCM to activate the HS fan. At 425 PSI, it shuts power off to the AC clutch.

I really don't have a good theory to work on. But consider if the PCM is not getting the AC pressure signal. The PCM would not know to activate the HS fan. The LS fan would then run until the AC pressure reached 425 PSI. This would make the AC clutch dis-engage.

What could cause this? A bad AC pressure switch. Also a bad switch ground could also keep the switch from sending the correct signal. Ground G103 is located in the right front engine compartment.

What else could casue this? An incorrect engine coolant sensor value. Air in the cross over could also do this.

A while back, I told you this problem would be much harder without an ODB2 scanner.

Does the car overheat if driven normally with the AC off? Please restate the symptoms after the CCRM repair. Just how hot is it where you live? Please review diagram-54-4.

Signed.
<unsure, dazed, and confused>

Diagram-54-4
http://www.stangnet.com/mustang-forums/817829-c-not-working.html

joshwfl said:

With the ac off at operating temp the fan kicks on for about a min, then shuts off for about 3min. Repeats cycle.

Click to expand...

The fan cycling on/off with the AC off sounds normal to me.

joshwfl said:

With the ac on max at operating temp, the fan and compressor shut off with throttle.

Click to expand...

This is the most purplexing symptom. What if the AC system is OVERCHARGED? When the high pressure switch cuts off power to the AC clutch (to protect the AC compressor), this also cuts the AC signal to the PCM. The PCM will respond as if the AC were turned off. IE, revert back to normal engine fan cycling.

Another possibility is the AC high pressure switch is bad (False high pressure signal) or a kink in the AC hard lines.

It honestly sounds like the original CCRM repair was effective. The fan is working as it should with the AC off.

Consider taking it back to where the AC work was done and have the AC pressures tested. We really need to confirm the AC pressures before working on any new theories.