Does anyone have any ideas about my A/C mystery?

Ever since I got the car about a year ago, the A/C compressor has always come on when the blower in the cabin is turned on. The "ac off" switch does nothing. Even with the heat at full power the compressor runs. If you forget to turn the blower switch off before starting the car, the compressor engages while I'm cranking the engine, causing rough startup and lots of wear on the starter - there's no timer delay function. I just got the system recharged today, and the mechanic said that this was bad for the compressor, since it's supposed to cycle on and off periodically when in use. Is this really true? I've already tried a couple of replacement control units from junkyards which I was (fortunately!) able to return when they all did the same thing. So I know it's not the control unit.

Does anyone have some good knowledge of the electrical system for this car, or any similar experiences? I'm thinking it must be some kind of electrical short which is bypassing the control unit in some way. Something must be getting through, since when you turn the blower off the compressor disengages. The mechanic (a certified Volvo specialist) couldn't think of anything apart from the control unit, which I know to be good. He also said there were lots of different sensors in the system which could be to blame, but didn't want to get involved with tearing everything out and doing extensive testing.

Sorry for the long post, but I'm a little worried about burning up my compressor!

Many thanks,

Ben

Check the switch, not the blower one, the one to the right of the recirculation switch. To turn off mine, I have to push it in sort of hard, even sort of punch it, before it will stay in and turn off the compressor.
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Leon Ramseur, 1991 940, 1990 760, 1990 740, 1990 745T & 2 1989 780s

Power for the compressor clutch comes from either the control unit or the ECU port 14 (at least in the wiring diagram for the 1995 940. Test both when a/c switch is off to determine where the power is coming from. I assume you do not have the defrost on, since that will automatically turn on the compressor.

Thanks very much to both of you for the suggestions. I don't have the defrost on, and I'm sure it's not my a/c switch, since I've tried 3 separate control units. If the ECU is keeping it on all the time, what's the fix? A new ECU (yikes!) Or could there be some kind of other fault?

Many thanks,

Ben

Note your problems on 1996 960 - I have reverse which if I took short cut would end up with your problem.

My compressor stoped pulling in few days ago but comes on for the odd minutes on a couple of times. Also Face vents do not blow in AUTO position.

Have tried 3 ECC units at junk yard but no help -- However according to engineer in aircon shop who checked usual gas and electrics says to operate normally needs 12v to Purple switch on radiator.

Have connected 12v to this and works -- maybe in your case previous owner wired somehow to this directly as well. (Most likely because no "ON" signal coming from Engine management box??

My next step is to have volvo shop check electronic analyser unless someon ecan tell me how to find and check for this signal (12V?)

Do you have any info for me???

One thought. Check the wiring going to the compressor. I had one wire become frayed once and it gave me all kinds of trouble. Check the ground wire too.
--
Leon Ramseur, 1991 940, 1990 760, 1990 740, 1990 745T & 2 1989 780s