New to the forum.
Own a 1998 S70 GLT 50k miles.
Having some issues recently and I am told different things from different mechanics.

I have no ground at AC relay, so my compressor does not kick on. I was told I need a new ECM but am not sure,
I do not know how to test any of this and am spending money having the car checking and as stated I am getting different opinions.
AC system has been recharged and checked several times.
I also have a weird idle drop issue at times, while at idle, rpm drops slightly and jumps back up.
Any help will be greatly appreciated.
thanks.

 

On the diagram - 2/22 is the compressor relay, 8/3 is the compressor. Note that relay 2/22 is grounded through the blue/black wire at terminal 40 on 4/46 - that is the Engine control unit. So yes the ECU determines if and when the compressor comes on.

For your fluctuating idle - your idle motor/iac/auto air valve/whatever you want to call it may be sticking. I would clean the throttle housing and set the basic idle first and go from there. If you have EGR make sure that valve is opening and closing properly - if it sticks open the idle will suffer also.

 

Thanks, but how do I check to find out if my ECU is bad and needs replacing?

 

On later cars - the ECU turns off the compressor if the engine is too hot, or too cold outside, ac pressure is too low or too high, alternator not charging, and possibly at other times also. I would use VIDA on those cars. How would I check a 1998 ? I would use my Volvo scan tool for the 1988-1998 cars and see if I would address functions related to the compressor. I'd verify the two AC pressure sensors - High and low - are working properly. Those are 7/8 and 7/53 in the diagram. You shouldn't just ground the blue/black wire bypassing the ECU - the compressor might self-destruct if those safeties are not in place.