Hello,

The ac compressor is only powered when the engine is running with cabin fan and ac switched on.

I think what most likely happened is that the magnetic clutch - where the wire goes to the compressor - is draining too much current from the battery when the ac is running. So the previous owner left it disconnected.

High current drain could be due to bad magnetic clutch with electrical shorts within its coil windings. Using a volt-meter you can measure the coil's resistance. It should be between 3.5 to 6 ohm (depends on compressor model). An almost zero ohm (no resistance) would indicate electrical short.

To measure using the volt-meter you have to calibrate the zero-ohm reading first (otherwise it would display incorrect value). If you're using a digital meter the zero-ohm reading is automatically set. If you're using analog meter (with moving needle as indicator) set the dial to the omega sign (ohm). Choose the X1 setting (means whatever the value read is multiplied by one - for low ohm reading). Touch the two meter probes together. The needle should jump to exactly zero ohm. If its not there's a smaller dial on the meter fascia which you can turn clockwise/anticlockwise to set the needle exactly to read zero ohm (while touching the two probes together). Now you can start using the meter.

Engine should not be running. Better to work on cold engine to avoid any scalding. Disconnect the ac compressor wire. Pick the wire that goes straight to the compressor (not the wire that comes from engine wiring loom). Touch one of the test probe to the compressor wire and the other test probe to the engine metal for ground (anywhere would do). Read the ohm value indicated on meter.

To replace the magnetic clutch requires special tools to work on the ac compressor. Better let the ac shop handle this. But at least you know where the fault is. Also depends on the compressor's overall condition a replacement compressor might be needed. New compressor comes with new clutch already fitted.

Idle surge especially when ac is running could be due to bad magnetic clutch too. The way the ac circuit is wired is that part of the compressor wiring is routed back to the engine computer (ECU) to monitor the load on the engine. Bad magnetic clutch gives the impression of heavier engine load. This causes the ECU to adjust the engine idle repeatedly causing idle surges. You have to correct the clutch problem first or refrain from using ac till then.

Hope this helps.

Amarin.