Belts flapping when AC is on?

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ESP47

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This is on a 75 Dodge D100 with a 318.

The belts are all tight and run fine when the engine is idling. As soon as I turn the AC on, the two compressor belts start flapping as if they're loose and it makes an annoying racket. Turn the AC off and they're back to running smooth and tight. They're all tight with the engine off as well. Anyone know whats going on here?
 
This could be caused by something like resonance. It may get worse or better with changes in RPM. In the early 70's I had adapted an aftermarket setup on my Road Runner, came off the 440, onto the 340. Everything was fitted "square" and lined up, but the belt would make racket at some RPMs. This setup had one of those off-center adjustable idler setups like Ford used on so many cars, and one day I realized, I COULD TURN IT AROUND and change the length of two sections of belt on which it rode. ......I got lucky. FIXED!!

Here's one I stole off the internet, on a Ford of all things. The idler is at the top. There's a bolt coming into it from behind. You loosen the bolt, twist the hex to adjust, and then tighten the bolt. If you can see, it's eccentric, sort of "leaning this way." If you have the room, and belt length, you can flop it over "the other way" which changes the lengths of the belt sections "before" and "after" the idler.

idler.jpg
 
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I had the same problem with a 73 Valiant that had been parked for about 2 years. When I removed the belts I found they had hardened to a triangle like shape. A tire will ( in most cases ) get hot enough to round out its formed flat spot. These belts wont. A new matched set of belts fixed it.
 
AJ/FormS said:
OP
Did you catch the matched set part?
Click to expand...


"Matched set" is nearly a joke. I'm not even sure you can actually GET belts "in matched sets" anymore. Worse, ANY wear or misalignment in the pulleys will cause them to "fight" and accelerate wear between the two.

I used to maintain a big air handler involving a direct fired gas makeup air unit. Had two big "C" belts on it. (Nominally 7/8" wide) I finally pulled one belt off and just ran it as a single. Guess what? Belt life went UP
 
AJ/FormS said:
Does that machine run at a semi-constant rpm?
Click to expand...


The one I mentioned? Yes. AC 3 phase 440 I forget about a 10 hp motor driving a big blower. It was part of a make up air system to 'make up' for the range hood and other exhaust.
 
Thanks guys. The compressor used to be a bit annoying at idle and low speeds and relatively smooth at freeway speeds. Now it's getting pretty bad to the point where I don't even use it anymore for fear of something going wrong. Doesn't sound right at freeway speeds anymore either.

How do I even go about getting a matched set?

Also would low oil in the compressor have anything to do with it?
 
Kinda need to know what pressures you are seeing too. If you have a stuck expansion valve you may have high head pressure casing the jerking belts.
 
of course the condition of the pump is a player in all this. Daughters Durango is example. Early spring of 2015 she had expansion valve replaced and system recharged. It worked OK until Mid May this year. We know that expansion valve is full of crap again and we know where the crap is coming from. The pump is shot. Today when the clutch is energized the pump runs for a short time before that serpentine belt begins to slide and smoke.
New pump, new dryer, new valve, complete system flush, and hope the flush gets 99% of the crap out, very expensive gamble. Meanwhile, no a/c .
 
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The belts are ALWAYS going to loosen a bit on the slack side when a load is applied. In this case, the 'load applied' is the compressor being turned on. The slack side will be the side between the compressor pulley and the crank pulley. If the belts are not hardened up, then I'd tighten them up again.
 
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