Hello all. When defrost or ac is on the higher the rpms the harder and cooler it gets. A mechanic buddy of mine tells me it's related to a bad ground situation and to replace my two main grounds. Those being engine to chassis and battery to chassis. Just thought I would see what you guys think. Thanks, Stuart.

 

Truly I wonder how he could explain that.
If electromagnetic clutch engages the a/c compressor to the serpentine belt, it has nothing more to do with electricity, but a pure thermodynamics. A higher RPM may compensate a worn off a/c compressor or the improper quality or amount of gas in the system.

 

Seems like there's two things going on actually, one of which your friend's guess could have not been that far fetched and I can see where he could have been thinking a bad ground to be the cause.

The lower speed of the air blowing out of the vents at idle indicates a voltage drop at idle. The alternator might only be putting out 12-13 volts at idle. As soon as the RPM's are brought up it's jumping up to the full 14-14.5ish volts and making the AC blower motor spin that much faster and the AC blow a little harder. Easiest way to test this out is to put a volt meter on the alternator output and check the voltage output both at idle and at higher (maybe 2k) RPM's. It should NOT drop below 12.5-13 volts at the lowest at any given time. In fact even at idle, about 14 volts is ideal, but I've see/had vehicles operate for years with a weak alternator only putting out just over 12v at idle but as soon as the RPM's are up a hair jump up to 13.5-14 volts.

The getting cooler at a higher RPM, that is usually symptoms of a worn AC compressor. Not that you would need to go out and install a new compressor any time soon, it is just a quirk that you'd have to deal with for a while. The only other thing that comes to mind is a low freon charge and that's an easy check with a gauge on the low pressure port.

 

You may also put the voltmeter over the suspected ground wire to see if there is a voltage drop. The reading should not exceed 1V (unless You're cranking the engine, that time it could go far above).

 

Thanks for your responses. Hurricane Flo is dumping on me right now but I'll check out the volts when she moves out. Just maybe the alternator is the noisy pulley under the hood. I just got it running last week from a bad fuel pump so I've just been enjoying having a vehicle that runs. Thanks again,. Stuart.