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 fenderwell
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.

Here is a downloadable diagram of the modification, with
instructions. You can view it and print it out using Adobe's Acrobat Reader. If you do not
have this common program installed on your computer you can get a free copy directly from
Adobe

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