2001 Mustang Cobra - Next Issue - A/C Clutch Not Engaging

[foofinator]

The next big thing on the list is the AC system.

The clutch does not engage. What I have done:

1. I checked all of the fuses. Notably, the 2001 owner's manual says that fuse 24 in the engine bay--which I do not have--is for "A/C Pressure." I might have a 1999 engine bay fuse box, because the 1999 manual says that fuse is only for 3.8L cars.

2. I replaced the CCRM because I noticed that it had suffered physical trauma. Got a reman unit from the autoparts store.

3. I get 12v at the low pressure switch. Jumping the switch does not turn on the compressor.

4. If I put 12v to the clutch, it operates flawlessly.

5. The ground wire to the compressor is good.

6. I have no DTC codes.

What do you guys think I should check next? I am wondering about the high pressure switch and the (if my car has one) WOT cut off relay.

 

[foofinator]

Just a thought. I replaced the engine bay harness, including the central junction box (which was destroyed). The old central junction box indeed has a place for fuse 24 - just checked.

Could that be the cause of my issue? If so, I wonder if using a 1999 CCRM would fix my problem, or, if I could run my own wire and fuse for that missing circuit.

 

[COramprat]

Is it wired? Most of the time the wiring is there...just not fused if not needed.

 

[wmburns]

Starting with the 2001 model year Ford changed how the AC clutch is controlled. Before then the AC clutch logic was "baked into" the wiring. After 2001+ the AC clutch control was turned over the the PCM. This was done in an effort to reduce compressor oil slog in cooler locations. Basically the PCM will keep track of how long the AC has been off and every so often turn on the AC compressor for a few seconds to move the oil about.

This has some "other" advantages in that the PCM can make "other" decisions about when to run (or NOT run) the AC. Some examples include (but not limited to):

• WOT
• during motor cranking
• motor over heating
• Other engine related DTC's.

The take away from this should be that your car's wring "might be" perfect and the PCM is locking out the AC for some reason. How to find out? The easiest way is to use an advanced ODB2 scanner looking for the AC related PID's. It's also possible to test for a ground/open at the CCRM AC clutch control signal wire. The PCM will ground the circuit IF the AC is enabled. Note it is possible for you to ground this control circuit as a test. Then you will know if it's the PCM locking out the AC.

This is very important if mixing and matching parts across this model year boundary. So in your case using a BJB=F1 fuse box from a 1999 model year is not a good idea as fuse F1.24 is important for the AC clutch to function.

Wiring diagrams are your friends.

 

[foofinator]

I do appear to have all the correct wires, but, as wmburns said, they are routed differently between 1999 and 2001. The primary difference seems to be that, in 2001 cars, the dark green/orange wire into the CCRM is hot key on and is fused. On the 1999 cars, the dark green/orange wire is downstream of the low pressure switch.

Looking at the diagrams, it seemed like I could swap in a 1999 CCRM. I tried and was rewarded with blown fuse(s) (I haven't checked which ones yet, had to leave before I set the car on fire) after running the AC (the clutch still did not engage) for awhile. Interestingly, every thing else on the car seems to run fine. The idle did not change with the AC on like it did for the 2001 CCRM though. This confirms that the CCRMs are at least very similar (I didn't see any differences other than the AC clutch circuit in the wiring diagrams).

One thing that bugs me is the red wire into the CCRM, whether 1999 or 2001, should always be hot. I'm at work now, but I think it's pin 12. I unplugged the CCRM and probed that pin, no juice whatsoever. I'm still not convinced the 2001 CCRM will not work; I'd like to get that red wire to be hot like it's supposed to be and see what happens then. If I could get it running, I'd just put an inline fuse right at the CCRM to mimic the 2001 wiring.

I'll replace the blown fuses, plug in the 2001 CCRM, and see if the PCM is commanding AC on.

This all sucks, because the harness I got was listed as for a 2001 car. I really don't want to spend the cash for yet another harness nor am I particularly interested in doing the work to replace it (again).

 

[foofinator]

Alright. I caved and bought a 2001 Cobra harness off E-bay. My OCD wouldn't let me go any other way. I'll sell this 99 harness (which is pretty nice now, after replacing the busted connectors, replacing peeling tape, etc).

Looks like I know what I'll be doing this weekend.