All.
What is the purpose of the Diode going across the A/C Compressor plug.

Thanks
Brad

 

I believe GM uses it to absorb the electrical spike caused when disengaging the A/C clutch. The clutch draws a lot of amperage and the spark would generate electrical interference as well as wear out the relay's contacts sooner.
Just my 2c, Randy

 

ANY switched inductive load needs a fast diode across its leads. It's called a flyback. The diode needs to be oriented "against the flow", meaning with the cathode pointed towards positive polarity.

If an inductive load is disconnected from the source of voltage/current, its natural behavior dictates it to keep the current running and it self-generates voltage, which happens to be of opposite polarity than what used to be connected to it. The diode must short out that unwanted voltage becaues if it didn't, the thousands of volts generated on the inductor would travel through the wires to whatever they can reach and would fry other parts of the circuit.

So if the ECM engages the A/C clutch, it does so with a low-voltage transistor of some kind that can hold let's say up to 50V. But if there was no diode on the clutch, disengaging it would generate thousands of volts and send them down to the ECM. Guess what happens next?

Hope this helps.
Lou