2000 Volvo S40 Dropping Idle When A/C Is On At Idle In Driv

Check the idle air control valve and clean it out. Let me know if that works.

When you cleaned it out did you try and see if the valve was able to slide back and forth by moving it manually? If that was ok and you do not visaually see any vacuum leaks try scanning for codes. If no codes then you will need to replace the idle air control valve. Before you replace the valve try cleaning the throttle body also. Sorry forgot to mention that on my last reply?

What were the fault codes scanned?

Can you get it rescanned?

Get a volt meter and back probe the mass air flow sensor signal wire. At idle you should see aproximate 0.6 volts. The voltage should increase when you accelerate. If that is ok then I would suspect a bad map sensor under the fuel rail cover. Back probe the signal wire and with key on only you should see 1.8 volts and with engine idle you should see close to 0.7 volts. If you are not close those numbers replace the map sensor. They cause a lot of driveability problems and setting off Mass Air Flow sensor codes when they are giving faulty information. The mass air flow sensor uses the map sensors signal so then there becomes conflict when one or the other is wrong. Keep me posted and let me know what you find and hope the info helps? Also the signal for MAF is pin#2 and pin #3 for MAP sensor. Also you posted another definition? Did you have more then one code? If you do have other codes posted or pending let me know? If you do and the fault is for sensors that are in the same circuit then most likely fault will be a bad A/C high pressure sensor. I believe the A/C pressure sensor is some how connected to the MAP sensor, Throttle Position Sensor, Altitude Sensor? Maybe why it only acts up when the A/C is on now that I am giving it some more thought? Check the first 2 things and let me know what you find?