1997 850 glt me4.4 ac compressor won't come on

[Ozark Lee]

The thermal switch is like the flame trap on a non turbo, it is best simply removed. It causes more problems than it will prevent.

...Lee

[Ozark Lee]

You might want to take a peek at the 850 service manual:

https://www.matthewsvolvosite.com/downlo ... vo_850.zip

See the file named ac heater system auto.pdf after you unzip it and use the 2.3l guide. There is a snubber diode in the relay so be careful about polarity.

Positive battery comes from the controller to Pin 86 (Yellow/Gray wire) and ground is sourced from the ECU to Pin 85 (Blue/Black wire) The High pressure switch also feeds the ECU and my guess is that the ECU makes a decision about turning on the relay based on the signal from the high pressure switch. Any schematic you will find has the ECU as a black box and even if you had a schematic of the ECU you really don't have the source code to figure out what conditions cause what actions. The high pressure switch also, via firmware in the ECU, should trigger the engine cooling fan after the pressures come up to normal operating pressures.

My guess at this point, based on your observations, is that you have a bad high pressure switch and it is falsely telling the ECU that the pressure is too high even when the compressor isn't running but once the pressure comes up to normal operating pressure the switch toggles to the pressure OK state and thus holds the relay closed.

Volvo legal got their panties in a wad when we hosted schematics so that is now verboten for me, as a moderator, to post them for you.

...Lee

[misha]

There is no need for a dealer.
Just take the car to a good ac service shop.

[Ozark Lee]

I think I would try to jumper the Blue/Black wire at the A/C relay (Pin 85) to ground and see what happens. Perhaps a harness issue between the ECU and the A/C relay is causing the negative side of the relay to not pull all the way to ground potential.

...Lee

[erikv11]

Is there r134a pressure in the system now, or did it all leak out?