i know that the ecu energizes the compressor relay by providing a ground source. but what does it provide the ground based on?

 

If you look at the circuit diagram, you'll notice that two different A/C wires pin to the ECU at A17 and C5. This allows the ECU to be notified whenever the A/C is turned on so that the ECU can delay activation of the compressor clutch for a couple of seconds until the idle speed is raised high enough to compensate for the increased load. It works like this:

1) Reference voltage (5V) is produced by the ECU at pin C5 whenever the key is on.

2) Pushing the A/C button to turn the A/C on grounds the reference voltage at C5, signaling the ECU to raise the idle speed.

3) After the idle speed is ramped up, the ECU then supplies ground to ECU pin A15 to activate the compressor clutch relay, which causes the compressor clutch to engage.

 

Ok i understand now thanks for clearing that up.