A/C Clutch voltage drop

[shostang]

I have been having problems with the AC clutch on my '93 SHO. The clutch engages intermittently; when it is engaged the AC works great, but I am losing voltage to the clutch. I made up a jumper harness to the clutch using a spare connector; when I connect battery voltage and ground, the clutch stays engaged; when the factory harness is reconnected, back to the intermittent engagement.

I have been following diagnostic procedures from a Ford service manual, but have hit a stumbling block. The system pressures are fine, the clutch cycling switch has the proper voltage and is doing it's job. According to the manual, the clutch is supplied a ground by pin 16 of the CCRM and battery voltage by pin 23 of the CCRM. When testing this circuit, the voltage on pin 23 will start at approx. 13.4 volts but then drop to 7 or 8 which causes the clutch to disengage. It will then sometimes drop further, sometimes climb and the clutch will re-engage. In a period of a few minutes, this may happen 10-15 times. There is no pattern; it is very intermittent.

According to the diagnostic procedures, the voltage dropping at pin 23 of the CCRM could be caused by an improper TPS signal, improper coolant temp signal, or an improper signal from the PCM to the CCRM. This is where I hit the stumbling block; how can I diagnose the PCM without a breakout box? I borrowed a friend's OTC Genisys scan tool; there are no trouble codes.

To further complicate things, the Ford manual says to disconnect the RED wire from pin 22 of the ICRM (ICRM, not CCRM; aren't they the same thing?) connector to see if the AC clutch signal voltage changes. If the circuit then works properly, it says to replace the CCRM (I had already replaced this after a Ford technician said it should fix my problem, but it didn't). The manual says that if the condition stays the same, there is a problem in the PCM to CCRM circuit. Problem is, my CCRM does not have a red wire for pin 22, it is a pink/yellow wire. According to my Ford electrical manual, this is the way it is supposed to be; the red wire is on pin 24. Am I looking at the wrong component; is the CCRM not the same as the ICRM? If not, where is the ICRM?

Any help would be greatly appreciated; I have spent many hours trying to find this pesky problem

 

[Racer X]

I think you should check for a bad or intermittant ground. That's usually what the case is when you get a voltage between 0 and 12 in a situation like this.

 

[projectSHO89]

Which engine. 3.0 or 3.2?

R-12 or R134a from factory? If R12, has it been converted? If converted, was a high pressure cutout switch added?

Manual or EATC?

93 was a transition year for the terminology used for various components in Fords. I've also noticed that mneumonic usage was inconsistent in the 93 & 94 service docs. I can also see that both the 93 and 94 service instructions have the same error in the color code of the pin 22 wire. Corrected in the 95 docs. The wire s, as you've noticed,

AC clutch operation is controlled within the relay module based on two inputs: 1) ACCS feed from the cycling switch on pin 21 and 2) WAC (WOT A/C Cutout) input from the PCM on pin 22.

I don't believe this is a ground issue based on the symptoms.

Measure the voltage on pin 22 with your meter when the clutch is both working and not working. If the input voltage did not change, I's suspect a faulty circuit inside the IRCM/CCRM. If the voltage DID change, I'd suspect either a harness problem either going to PCM pin 10 or from PCM pin 54 to the IRCM/CCRM pin 22 or possibly the PCM itself or one of the aforementione PCM inputs.

Steve

 

[shostang]

questions

Thank you for responding.

I have the 3.0 engine with R12 refrigerant and EATC.

The voltage on pin 22 when the clutch is working measures from 8 to 13.4 volts (it is constantly fluctuating). When the voltage drops below 8, the clutch disengages.

I have checked the other PCM inputs (TPS and coolant temp); there are no problems in these circuits.

Thanks again.

 

[projectSHO89]

The voltage on pin 22 when the clutch is working measures from 8 to 13.4 volts (it is constantly fluctuating). When the voltage drops below 8, the clutch disengages.

That's your problem.

Either the PCM output driver is bad or you have a wiring fault on that line.

If you disconnect the red wire from pin 22, the relay box's logic should allow otherwise normal A/C clutch operation with the only drawback being loss of WAC function.

Steve

 

[Howdy_Doody]

You should also check the clutch air gap. I was running into the same problem on my 1992 and thought it was electrical. However, I checked the air gap and found it to be outside of specification. Resetting the air gap completely cured the intermittent problem. Do some searcing n the forum, it's a 20 minute procedure to adjust the air gap.

Before doing this I had replaced the IRCM (over $100 from Ford at the time). That was before we had shoforum.com and all these great tools to help us own the SHO.

 

[projectSHO89]

Heya Howdy,

His symptoms and diagnostics point specifically to an electrical/electronic fault, not an air gap issue. If it were an air gap issue, there would have been no fluctuation of the coil drive voltage nor of the WAC input signal.

I'll stick by my last post in the absence of any evidence to the contrary.

Steve

 

[shostang]

Thank you very much!

 

[Howdy_Doody]

Heya Howdy,

His symptoms and diagnostics point specifically to an electrical/electronic fault, not an air gap issue. If it were an air gap issue, there would have been no fluctuation of the coil drive voltage nor of the WAC input signal.

I'll stick by my last post in the absence of any evidence to the contrary.

Steve

And I'd check that air gap before spending any more time on the electrical diagnosis...I measured a voltage drop on mine too, but after resetting the air gap I no longer had any intermittent operation.