My volvo 240 1989 had no a/c.I have emigrated from the cold Netherlands to the Caribbean ( Aruba) and took the 240 with me. I have put a complete 1991 a/c combined unit in it myself (with all the wiring switches and relays) and had it filled by a local store . It is working alright but there is a problem with the fast idling with the airco on..
A signal from the micoswitch and the compressor should go to the ecu ( LH 2.4).
I found 2 wires attached to the old wiring of the old heater with the right colours ( green and grey/red) according to the wiring diagram so this must be the right wires.
I made the connection (with the ignition on (but without the motor running) with the airco switch on.After this the car was very hard to start.
The compressor signal between compressor ( ecu 14 ) and ground was 2.8 ohm = normal.The signal between microswitch (ecu15) and ground with ignition off and ac switch off was 10 ohm ( but should be 1000 ohm according to Bentley))
Can the switch be the problem? or is something else going wrong. Can it do any harm when I drive (and idle) the car without the connection between ecu and a/c?