Jumpering the 2 wires did not produce the desired results - still no A/C compressor. So, I took a longer look at the schematic and once I figured out (I think) what does what, I can say I've never seen such a convoluted circuit as the A/C system. <img src="/forums/images/graemlins/scared.gif" alt="" />

It appears that I was half right about the A/C OFF engine coolant temperature switch controlling the compressor shutdown on overheat..... and Charlie was half right about the ECU providing the shutdown. Although I have no schematic for the ECU internals, it appears that both components are involved.

Here's how I "think" the A/C circuit works:

1) Power from the main 100A fuse is always available through Fuse 6 to the Heater Relay AND through C-24 20A fuse (behind the glove box) to the Condenser Blower Motor Relay AND to A/C Relay A.

2) When the Blower Switch is in any position other than OFF, power is made available to the A/C Switch on the dash.

3) When the A/C Switch is closed, power flows through it and through the Thermistor (behind the dash) and makes power available to A/C Relay B.

4) A/C Relay B is triggered by the Ignition Switch through the Cigarette Lighter circuit and is closed full-time whenever the key switch is ON.

5) Power from A/C Relay B is used to provide coil voltage on A/C Relay A and feeds the Dual Pressure Switch.

6) If the Dual Pressure Switch passes voltage, it triggers the Condenser Blower Motor Relay (which in turn activates the Condenser Blower Motor) AND passes power to the A/C Engine Coolant Temperature Switch.

7) If the A/C Engine Coolant Temperature Switch is closed, it passes voltage to the ECU at C-39 on the GR#1 wire. NOTE this is 12v(+).

8) The signal coming out of the ECU at C-40 on the BW wire is the coil trigger to close A/C Relay A. NOTE this is a 12v(-) trigger to complete the relay coil's ground path through the ECU.

9) Once A/C Relay A is triggered through the ECU, it passes the voltage supplied to it by the Key Switch (see Step #1 above) to the A/C Compressor magnetic clutch.

Now, the question remains - how does the ECU take a (+) input and use it to trigger the ground path of another component? The only answer I can fathom is that there is another relay inside the ECU that is triggered by the incoming signal on C-39... which when closed, completes the ground path trigger for A/C Relay A.

Simple, eh? <img src="/forums/images/graemlins/lol.gif" alt="" />

It appears I need to add a relay behind the glove box.... trigger it with the C-39 GR wire and when triggered, have it complete a ground path for the C-40 BW wire.

If anyone wants to review this and suggest a misread or an alternative method, I'm all ears. <img src="/forums/images/graemlins/lol.gif" alt="" />

Frank