Tony wrote to the list:

With AC button pressed the "AC" light illuminates. I have LP switch jumpered, freeze switch jumpered, both cooling fans come on. a good 12V measured at LP switch, freeze switch and the connecter leading to the compressor on the front of the engine. When i cycle the AC switch the voltage changes accordingly...12V...appx  0...12v..etc etc

Question: I have good voltage but my guess at this point is not enough amps in the circuit to engaged the clutch. What amperage would/should i see?

I know quite often the "little blue relay" in the control head is the culprit but i have a good 12v all the way to the compressor. Could a bad little blue relay still cause bad amperage even though i have good voltage??

Hi Tony--

Nice to meet with you again at the Devek weekend.

I did a write-up on the relay diagnosis, and also the cure I used to fix my '89 when it was suffering from the same problem. I'll send a copy of that to you directly in a separate message.

For those reading along at home, the key to deciding whether the relay is the problem is how you do your troubleshooting with the meter. The problem we most often see is a high resistance in the relay contacts. With the compressor clutch coil disconnected, the current in the circuit is almost nothing, and the resistance causes virtually no drop in voltage on the clutch circuit. Connect the clutch coil and the current requirements go up to maybe six amps, and the voltage drops due to the contact resistance.

So the troubleshooting technique should include testing voltage at any of the test points. The freeze switch is easiest to get to with meter leads with wiring still attached, but you can do it at the pressure switch, the sandwich connector by the jump terminal, or at the compressor wire connector too. You already made jumpers for the switches, so either point will be OK. The key is to do it with the load (compressor clutch coil) still attached.

The little blue relay in the control head is not rated for six amps, is not rated at all for DC service, and is not protected from inductive field collapse when the clutch coil is disengaged. All these things work together to cause contact arcing whenever the relay is used. This arcing will soon cause the contacts to pit, and the excessive current can cause the arm in the relay to warp from all the heat as the contact resistance grows. After that, it's a death spiral for the relay until it no longer carries enough current to pull in the clutch coil.

The 'fix' that I came up with involves removal of the original underspec'd relay, replaced by a larger more capable relay that's commonly available. Unfortunately, I was not able to locate a little flat-pack power relay as a direct replacement, so I wired a larger relay outside the controller case and attached it to the case with double-stick tape. After four years it's still hanging on there, so the mechanical portion of the install is fine. The electrical function has also been flawless.

If you decide to go this route, the relay and the connecting wires are common Radio Shack parts. You'll need a soldering iron and rosin solder for the wiring, wire cutters, etc. To remove the controller, you'll need to remove the center vent, the plastic "H" trim on the console, the radio (needs a coat-hanger tool), and finally a small Phillips for the screws on the controller itself. A couple small screwdrivers will get the controller sandwich case apart, and after that it's the electrical repair.

The complete repair procedure is described is in the diagram which is available directly from me, and also is archived on the Greg Nichols website in the AC section. I'm on a dial-up connection in the wilds of Illinois right now, so if you can grab it from Greg's site it will make my day a little shorter. ;-)

It's important to note that both new and rebuilt controllers come with the same underspec'd relay inside. In my limited experience with such things, it's a matter of "when" rather than "if" the relay will fail. Mine was history after only about 28k miles, albeit soCal miles where the AC was used a lot. Living in Las Vegas, Tony probably only uses the AC once a year, turning it on in April and off in October. It might get a vacation for track days but that's about all.

Hope this helps!

dr bob