Here's a link to a modification instruction of the relay: ACRelay.pdf

Climate Control

Compressor Clutch Power Relay, Story #2

Symptoms: Compressor doesn't run when panel AC switch is pressed.

1) Checked system pressure with gauges to confirm that there is actually some Freon in the system. I had evacuated/recharged the system just a week before, so I was really looking for a catastrophic failure of a hose or compressor. Fortunately found Freon and no failure.

2) Started working back from the compressor to the controller using a DMM to check voltages. First check, according to the schematic in the manual, is at the pressure switch on the drier. With the wires disconnected, I found 12v at the supply side. I also did a quick check of the compressor clutch coil with the ohmmeter, and found about five ohms through the coil -- about what I had expected to see with a good coil.

3) Checked the pressure switch with the ohmmeter, and found it closed. At this point all the checks said the compressor should be running.

4) Wiring diagram shows the compressor wire going through a connector on the fender well on the right side of the car. Found the connector and confirmed that the diagram is correct. I'd already proven the circuit through the clutch from the switch, though. I took a minute to treat the connections to a coating of dielectric grease on my way through.

5) Hooked all the wires back up, and still no compressor. Using the voltmeter, I checked the coil voltage at the pressure switch. No voltage. Pulled the wire to the compressor from the switch, supply voltage is now 12v. Jumpered the switch with test lead, no voltage. Hmmmm--
There's a voltage drop somewhere in the circuit.

6) Wiring diagram shows the compressor fed from the controller, through the 'anti- freeze switch', the pressure switch, and that's about all. The freeze switch is under the plastic rain cover at the top of the hood, so that gets checked. No voltage on either side with the compressor attached, 12v with it disconnected. Test jumper around this switch, still no compressor.

7) Power to the compressor is fed through a relay in the controller. Coil on the relay is actuated by the AC button on the dash. I have the wire numbers on the connectors, so out comes the AC controller. Two connectors -- one for the blower switch, other for the temp and vacuum relay connections.

8) With the controller out, the covers come off with five little screws and a small screwdriver to pry open the little locking tabs. Nice piece of hardware, BTW, made by BEHR and definitely an industrial-strength piece. Follow the traces back to the little on-board relay that runs the compressor. Little blue relay, with the pins identified on the top cover. I double check the continuity from the pins to the external connectors, and also the contacts on the relay itself. Everything tests fine so far. Rating on the relay is 2.5a at 250vac, so that is one piece that isn't quite up to the service rating-- noted for later, though.

9) I reflowed the solder joints on the relay, and plug it all back in. Still no compressor. With the thing hanging out of the dash but still connected, I can prove the circuit from the switch on the dash, the power supply to the relay, but-- no output from the relay. Found the culprit!

10) Finding the problem is most of the solution, I think. Finding an exact-replacement relay is tougher. The final solution is an external ice-cube relay, rated at 10a contacts at 125vdc. I removed the original relay, and used the pin connections in the board as handy connection points for some wires to connect the external relay. Wires go out of the plastic case at an unused external connection port, to where the relay is mounted externally on the right side of the case. A little double-stick foam tape attaches the relay.

Total cost of repair was about $7 and four hours of diagnosis and shopping.

Here are steps for quick diagnosis:

With the engine running and the AC switch on, look to see if the compressor is actually turning. If it's not moving, pull the plastic tray at the top of the engine compartment/base of the windshield, and locate the anti-freezing switch that hangs in the center on that little bracket. Use your meter to measure the voltage at one of the switch terminals to ground without pulling the wire connectors off. Relay good will give you 12v, a controller relay weak/bad will give you much less. If you find low voltage, double-check your diagnosis by pulling the connector at the compressor and try again. If the voltage comes back up with the clutch coil disconnected, the problem is definitely that controller relay.

Good Luck!

dr bob


I've been working on the non-functioning AC in my '90 GT and found one of the problems to be failed relay contacts for the relay that powers the AC compressor clutch. It's located in the HVAC control unit. I found the above write up to be very helpful. One thing I would like to add is some detail on the relay that is used to fix this problem. I used a relay I found at Radio Shack and it seems well suited for the task. It is Radio Shack part number 275-218C. It is not the normal Radio Shack low quality part. It is a Siemens/Potter Brumfield brand, part no.
K10P-11D15-12. The coil resistance is 160 ohms, and the contact rating is 15 amps @ 30 VDC.
The original Gruner 202 relay has coil resistance of about 230 ohms and contact rating of 2.5 amps @ 250 VAC. The compressor clutch will draw about 3.5 amps @ 13.5 volts so it is no wonder that the contacts do not last. I mounted the relay as suggested in Dr Bob's tip and it works well. Clearance is tight in there so be sure there is enough room before finalizing the mounting location.

Louis Ott


You probably have one simple problem that you haven't fixed yet.

Checking voltage doesn't always tell the tale on the A/C clutch circuit. It is very possible for the little blue relay to pass enough voltage and amperage to light a small test light, or to show 12 vdc on a meter, but not to pass enough amperage to operate the clutch.

Go to the low pressure switch on the A/C receiver/dryer. With the plug still attached to the switch so that it will operate the compressor clutch, hook a voltmeter between one of the terminals and a good ground. Turn the ignition switch on, and turn the A/C on. If you get 12 vdc and the clutch clacks, your problem is elsewhere. If the voltage is less than 11 vdc, and the clutch doesn't clack, either the blue relay or the anti-freeze switch is probably bad.

If so, remove the plastic cover at the base of the windshield and find the anti-freeze switch. Square block, about 2" x 2", with two electrical connections and one small capillary tube, mounted near the center of the car near the A/C tubes.

With the clutch hooked up (no disconnected wires) check for 12 vdc between each of the electrical terminals and a good metal ground. You should get 12 vdc on both. If you get it on neither, the little blue relay may be bad, or the problem may be between the relay and the central power panel. If you get 12 vdc on one connector and not the other, the anti-freeze switch is bad. A simple series string - just find where you are losing the voltage.

Wally Plumley
928 Specialists