Compressor Clutch Power Relay
My car is an 87 S4. I had the condition I hear described alot when the ac does not work due to a low current in the wire going to the ac clutch. I wrote regarding the the a/c fix when you are getting voltage to the compressor but low current (not enough to energize the clutch). Here is what I did. First I pulled the a/c controls, switch etc and made sure all connections were good and working. I veryified that they were. I made sure that nothing else was causing the loss of current ie low pressure sensor. After narrowing it down to the control head, I decided to try to fix it instead of spending $200+ to replace it. I made sure that the wire to the compressor energized when I pushed the ac switch to make sure that the system would work correctly when I "rewired it." My first thought was to use the low current wire to activate a relay which would relay battery voltage from the + terminal under the hood straight into the compressor whenever the a/c switch was pressed and the original wire to the compressor was energized. I tried this but it didn't work because the current in the wire was not even enough to activate a common 4 prong automotive relay. I decided to look for a relay that would activate under a lesser current. I measured the current in my wire and got about 5mA I think. The lowest rated automotive relay I could find required 15mA to activate. So I thought smaller.... I went to radio shack and bought a reed relay like something from an electronics component that works on like 2mA. There were 2 problems with this. One is that the connectors are tiny, phone size. The other was that I didn't know if this tiny relay could handle the current pulled by the compressor clutch. I decided not to find out. I bought some small telephone crimp connectors to solve the first problem. Then.. I decided to use the reed relay to relay battery voltage into the larger relay. That voltage then activated the larger relay and sent battery voltage to the compressor clutch. WHAT A RIG!!! It really wasn't hard to do. Here is a simplified version of what I just said. The wire from the a/c controls goes into the reed relay on one side of the normally closed switch. The other side of the normally closed switch goes to ground. One side of the normally open switch goes to battery voltage at the + terminal under the hood. I took it off and routed the wires behind it to make it neater. The other side of the normally open switch goes to one side of the normally closed switch of the larger relay. The other side of the normally closed switch on the larger relay goes to ground. One side of the normally open switch on the larger relay goes to battery voltage at the positive termainal. The other side of the normally open switch on the larger relay goes to the compressor clutch. Works like a charm!!! Its been functional on my car for about 6 months.
The wiring diagrams for the relays are printed on them to tell you which terminal does what. I could draw this diagram if someone needs. After I finished connecting everything, I made a little box for the relays and coverd the whole assembly in electrical tape to keep everything nice and tight and as waterproof as possible. I then hid everything under the little plastic cover on the + terminal and enjoyed my nice cool car. Any questions feel free to write Steve Lewis
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