I've posted before about a noise audible from within the car ('94 940 wagon, non-turbo). Got answers back about tranny fluid, exhaust manifold bolts, all has checked out.

It sounds like a straining power steering pump but constant. Only happens when hot, or so I thought. I've failed to pinpoint the origin, even when listening to various areas of the engine bay through a tube while the engine is running.

Today (another hot one in NJ) I made a discovery: [sm=yikesomg.gif]

Engine whines with A/C on, RPM sensitive
Turn off A/C switch: whine stops immediately!
Turn A/C switch on again: whine starts within 5 seconds or so.

Hmmm....

The A/C system works OK...recharged it ~2 months ago with R134 w/PAG oil.
Any ideas?

 

Atwell, a couple of things come to mind. One is the ac compressor clutch is not happy. Either it is not getting full voltage ( low system voltage /bad connections) or it is getting old and tired. I sympathize, so am I. The other possibility is the alternator complaining because of the increased load. You can find out which one by waiting until it is hot again ( won't be long of a wait), run the engine and then turn on every electrical device at hand, except the ac compressor, high beams, blower fan on high, rear defrost, seat heaters, stereo etc, and see if it makes the noise. To confirm which one, you could also remove the alternator belt, run the car ( I assume a battery with a full charge ,in good condition) and then turn on the ac. Checking for correct voltages and grounds at both would also be a good idea.

 

Well, it IS old, the compressor is a SS 121DS5 dated 5/93 so I'll assume it's OE. Seems to work OK, crisp engagement and freewheels with out drag when off, though there was a lot of rust on the clutch when I got the car in April.

I cleaned the rust off the clutch with alcohol (poured the alcohol over it to flush the clutch facings). Had to tighten the belt once, it would squeal on engagement. (I sanded the glaze from the belt and pulleys).

What else could go wrong with the clutch? I would think that the bearings would fail audibly, or that wobble would be evident when it was disengaged with the belt off.
As for the voltage at the A/C clutch, I couldn't find a safe place to probe with the meter.

OK checked voltages again. The charging system is maintaining 13.52V full load, 13.81 no load. No increase in whine when loaded.

No voltage drops/resistance between the alternator or A/C compressor, to the block.

I listened again with the hose to my ear, neither the alternator nor the A/C compressor was the source of the whine. ((Until I got too close to the coil, then I started whining when I got shocked. Note to self: use the longer hose next time, you idiot!))

In fact it seemed to be coming from father back, as if inside the bell housing, which is why I flushed the tranny even though the P.O.'s records indicated it had been done 18 months ago. (That had absolutely no effect)

You can hardly hear the whine with the engine running and the hood open, too much backround noise. From the drivers seat with the windows rolled up it's a different story.


Hopefully I'm not just rambling...

 

Sounds like the compressor is going bad.Does it sound like a ticking noise?

 

Hi Tech,



No, no ticking noise. A pump noise, like:

Whaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

From inside the car, varies in pitch with RPM.

If you were in your Mustang, not moving, turning the steering wheel... it sounds like the Ford power steering pump straining.

But it's in the A/C system. Apparently when the refrigerant builds up pressure. Stops immediately when the A/C switch is turned off, even at a constant engine RPM, but it takes a few seconds to become audible again with the A/C switch on...as if the pressure has to build up...

 

Still sounds like the compressor.
If it only makes the noise when the compressor is on that should be it.

 

If I find a compressor at a junkyard, does it have to be exactly the same model/type as originally fitted, or do I have a choice of any 740/940 compressor model?

 

You will need to find one that looks like yours.They had a couple of different ones between the different models.