Buddy,
The two switches on the back are pressure switches and they both close to ground. The High Pressure cut-off switch is normally closed and when pressures exceed a certain level (over 400 psi sticks in my mind), the circuit will open and cause the compressor to not kick in. The Low Pressure cut-off switch is just the opposite, if a certain, minimum pressure isn't there then it will be open. It closes under some pressure allowing the compressor to run.
To test them, if you have a can or so of freon in the system, a DVOM should read a closed circuit between the switch contact and a good ground.
You often have to jump the pressure cycling switch on the accumulator to charge the sytem. Take the connector off and either make you a short jumper wire or use something around the shop to jump the two terminals in the connector. With the key on and the a/c set to max, you should have one terminal hot and jumping the two terminals sends power back to the relay.
With the key on and a/c set to on, if you have a hot wire at that connector you don't need to look at the fuse or a/c control head, that part of your system is working.
If everything is hooked up properly, one of the two contacts on the compressor clutch should show a ground. The ground terminal on the connector goes directly to the High Pressure cut-out switch and, with just a can or so of freon in the system, the switch should be closed to ground all the time. The only time that switch is supposed to be open is if you have an extremely high pressure in the system which it doesn't sound possible for you to have right now. Now this part is up to you, but before I'd change the high pressure cut-out switch I'd take that wire, put a ring terminal on it, and ground it. My reasoning here is that if pressure does get that high, the belt will slip long before it can cause any damage to your system and that's also been my experience in the past as well.
On the low pressure switch on the compressor, for testing purposes you can also ground that. I don't recommend disabling that switch though, as it's the protection from running your compressor with no freon in it if you should develop a leak. I have seen people that have disabled it, though, and let the pressure cycling switch handle that job. That works, but it's not very fail-safe and if you run your compressor for any length of time with no refrigerant you WILL be buying a new compressor.
On the relay at the firewall, you should have four pins that are used on the connector. One group of three will be filled and the other group of two will use just one connector. The group of two may have two or more wires going to that connector, that should be hot all the time. If it is, you can rule that out as a cause. On the group of three connectors, the middle one is the connector that goes to the clutch. If you jumper that with the hot wire, you should hear your clutch engage if all your wiring is intact.
If your switches on the compressor check out, you have hot wires at the pressure cycling switch and the relay, and you can make the compressor clutch "click" from the relay connector, that's good news because your wiring is intact and about the only thing it can be is the relay on the firewall.
Good luck, read my earlier post as I went through the same thing you are last week. 
John Stricker
| | | quote | | Originally posted by buddycraigg: We (KCFOG) do not know if it worked before. I tested with a pos/neg test light and didnot get a ground or a 12+vdc signal at either connection on the clutch. The only fuse that I see labeled for AC is “Heater/AC” 20A and is good. I’m thinking relay or one of the two sensors in the back of the compressor, but since I don’t know what they do, I can’t test them. |
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