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Discussion Starter #1
My A/C compressor clutch seems to be lazy these days. It's always worked great, but the last time I drove the car, it didn't engage.

If I attempt to operate it by the switch controls, nothing happens (ignition on, engine running or not). However, if I bypass the circuit and connect direct to the battery, the magnetic coil kicks in.

There's voltage at the clutch, and when energized by the switch circuit, it will engage with light finger pressure (ignition on, engine not running of course).

Any ideas?

Someone suggested an "air gap" in the clutch that can be shimmed, but I've never heard of it.
 

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Todd,

You might measure the voltage when connected to the clutch to see if the drop is excessive (indicating too much drop in what precedes the clutch). As I recall the voltage on my car is about 10.xxV with the clutch connected. If yours is lower, I would check the drop at the microswitch on the AC/Heater controls (less likely but easier to get at), followed by the thermostatic switch on the top of the heater box (dash pad must be out to do this). The TS can be disassembled and the contacts cleaned.

There is a shim on the clutch that sets the gap but all things being equal the gap should not change over time.

Hope this helps.

Regards,

Bob
 

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Todd, Been there, fixed this in my 70. It was the "micro switch" in the hvac controls in the dash. I put in a different microswitch. The switch is a small rectangular box that a small button on it is depressed when you select a/c.

John
 

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Discussion Starter #4
You might measure the voltage when connected to the clutch to see if the drop is excessive

Bob
Below are measured voltages:

Car sitting for 1 month, engine not running:
Battery = 12.06V
AC Clutch = 10.79V

High Idle after startup
Battery = 14.43V
AC Clutch = 13.59V

Idle after warm-up
Battery = 13.00V
AC Clutch = 12.7V

I spoke with a rep at Classic Auto Air who said the GM clutches can be shimmed for an airgap, but not Ford. He strongly cautioned against operating AC if there's low voltage to the clutch and marginal engagement since marginal engagement can cause friction heating and ruin the input shaft seal. Based on the voltages above, I think I'm OK, though he recommended checking the alternator.

Now the clutch is working, so the limit switch may be on its way out.

Todd.
 

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Okay, I am sure that you tried it, but...did you just move the lever...to air conditioning....mine does nothing until I turn the fan switch on too. Then it roars to life. Mine is a 70 XR7
 

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Discussion Starter #6
Finally dug into this problem and determined both the A/C proximity switch (small limit switch on the temperature control unit) and fan switch were fluky.

As the diagnosis, I measured the resistance from positive battery cable to the clutch with all switches on (fan, A/C and ignition) and read 163ohms. Not knowing whether that was good or bad, I then measured from fuse block to clutch, bypassing much of the conductor path, and got about the same resistance - until I wiggled the fan switch and saw random numbers on the DMM that settled back to ~160ohms.

I replaced the A/C proximity switch and jumpered the fan switch to then read a steady 0.4ohms from fuse block to clutch. When I reconnected the fan switch, the erratic resistance from wiggling returned. I was able to open up the fan switch, clean all contacts, relubricate, and even soldered the spade wire terminals to their lugs. After reassembly and reconnection, resistance is now a steady 0.4ohms, regardless of wiggling the fan switch.

Solving the circuits problem before and after eliminating 160 ohms resistance says circuit current for a 12V supply has increased from a meager 70mA to 3.4A thru the clutch. Naturally, the A/C clutch engages repeatably now.
 

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Hi Todd,

LTNS! Is the AC proximity switch the same as the thermostatic switch which has a probe into the evaporator coil? Or, do you mean the microswitch on the AC/heat control that is actuated by a cam? I'm guessing the latter? Reason I ask is I had to take the thermostatic switch apart and refurb it for the same reasons you have described.

Regards,

Robert
 

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Discussion Starter #8
The 'proximity switch' is the microswitch on the AC/heat control unit.
 

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...I spoke with a rep at Classic Auto Air who said the GM clutches can be shimmed for an airgap, but not Ford...
I thought (think) they are adjustable. You just need a different thickness ring that goes between the end plate (with the leaf springs) and the bearing it presses into. Maybe they have no such thing but I would have thought so. This would accomplish setting the airgap.
 

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Those Proximity switches (micro switch and or cherry switch) are available in many voltage and current choices. I used to find beefy ones in microwave ovens. After I upgraded the "proximity" switch I started having fan switch problems. Last time I checked they were no longer available. I rebuilt the switch contacts using brass #4 machine screws and nuts and now those contacts operate high current relays. (The load is off of the fan switch) It may be a little more cluttered up in the dash but I should never have another fan switch fail.

John
 
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