Vibration in A/C Belt at Idle

[Bob Chadwick]

I am getting close to getting my a/c working perfectly. The thing blows at 38 degree while moving and once I get the heat from the floorboards insulated, I should be in good shape.

The problem I am having is that at idle and as the RPM's come down from revving the engine, there is a noticeable shake in the a/c belt. It can actually hit a metal tab below the belt and scare the s**t out of you if you don't know what it is. If you accelerate, the vibration goes away.

It has a new R-134 compressor. We checked with GM today and it is the correct part. The belt has been changed out twice.

77-4Speed had what appears to be the same problem, http://corvetteactioncenter.com/forums/showthread.php?s=&threadid=12581&highlight=compressor, though I can't tell whether or not he ever resolved it.

The a/c shop seems to think that I need to replace the plastic washers and spacers on the mounting bolts as there is the possibility that they have worn and are allowing the vibration.

Anybody else have any other ideas or any ideas on where I can get these things or the descriptions so that I can attempt to locate them. I have the GM part numbers, 120394, 492346 3880353 and 3820389.

Bob

 

[c4ever]

Bob,
Join the club. From all the sources I've asked, this is a problem without a fix. I get the same slack in my belt at idle. I would inquire furthur if this information you got is really a fix. If you do come up with something please let me know. As of now I just live with it on the 82. By the way, all my spacers, washers,etc are in fine condition and I still have the same problem.
Carlo

 

[Bob Chadwick]

Carlo

77-4speed had mentioned the possibility of replacing stock with a March Performance Pulleys' compressor bracket. I wonder if anyone has done this and had any better luck?

I went to their site and while I couldn't tell which bracket to use they are all nice to look at.

Bob

 

[c4ever]

That would probably work. I know keeping it stock doesn't seem to provide the answer. March makes some sweet product. Keep me posted if you go this route.

Carlo

 

[77-4speed]

Bob,

I never did get it resolved! My A/C worked fine until 2 things happened:

1: I installed a new waterpump and flexfan
2: I had the A/C system worked on

You see, I had the A/C repaired initially and all went well. No shaking, shimmying, or any strange noise. Then the A/C was not used for 4-5 months(Sept-Dec). Well, in Dec. the water pump went out so I decided to "upgrade" to a flex fan for better cooling. I drove the car with the flex fan, never using the A/C until about March of 02. The first time I turned on the A/C, I got no cold air. So I took it to the shop to repair what wasn't fixed the first time (out of refrigerant because of leak). Got the car back and fired up the A/C and walla: shake, rattle, and roll.

I am going to try to put the stock fan back on to see if it makes a difference. I did not buy the March bracket because I'm cheap and it is not!!!!! I'm hoping that the shimmy has something to do with the flex fan.

Bob and C4ever, do you guys run a flex fan or the original?

I do hope you can find the problem. If so PLEASE PLEASE PLEASE let everyone know.

Russ

 

[Bob Chadwick]

Russ,

Glad to hear from you. I think your problem is the same as mine. The load from the a/c compressor, there would have been none with no refrigerant, creates the belt vibration. If I give it a little gas the vibrations go away, though they do come back. Same thing if I turn the a/c off.

I have the original fan.

I'm going to replace my water pump over the Winter. Perhaps there is enough play in the existing unit to create this problem with the additional load of the compressor.

What was the part number and cost of the March bracket? I'm cheap as well but have a compulsion to make things work right.

Bob

 

[77-4speed]

Bob,

Unfortunately I did not write down the part number for the March bracket since it was something like $200. Probably not that high but really up there. I was thinking though, since my A/C didn't vibrate when I had it initially repaired, perhaps there is too much refrigerant in there now. I realize that it is probably "technically" the right amount, but perhaps if I released some of the refrigerant there wouldn't be as much of a load and the vibration would go away.

I think I have the cheap refrigerant in there now so it wouldn't be that big of a deal to release some. Maybe I can ask around and find out how to do that.

I have zero knowledge of A/C systems and am always leary of any high pressure fluids.

Just a thought.

Russ

 

[Bob Chadwick]

I don't know much about a/c either but I would suspect that there was not as much of a load on the unit for two reasons; low refrigerant and cooler temperatures. The a/c guy that does my house tells me that the load is lower in cooler temperatures and increases as it gets hotter.

When the temp and the level of refrigerant went up, the load increased and the vibration arises.

The only reason I even knew about mine was because it on occasion hits the "tab" below the belt making a sound like a bell being rang under the hood.

If March would guarantee that it would take care of the problem I might consider the bracket as a last resort, which it appears I am at.

Bob

 

[c4ever]

Russ,
I'm running the stock fan.

Bob,
As Russ says please let us know if there's a reasonable fix. I get pretty tired of that bell myselfad

Carlo

 

[Bob Chadwick]

Carlo, I didn't realize that you had the bell as well.

Russ, are you still running R12? I have the R134 though that shouldn't make any difference.

Bob

 

[77-4speed]

Bob,

I think I'm running R134 but I'm not sure. I *think* they switched over when they did the work last time. I'll have to check to be sure.

I really don't know what else to do about it. I do need to change my belts because the alternator and A/C are almost to the end of their adjustments.

It seems like it's just that the belt goes through the natural harmonic frequency of the fan/pump assembly and voila, instant vibration.

I don't have a bell sound though, instead I get a shudder when I decelerate through about 1100-900 rpms. It's almost like the belt is shaking and then it catches a smooth turn again at 900. It's definitely annoying but I don't know what to do.

Keep us informed,

Russ

 

[Bob Chadwick]

I think your right about the harmonic balance thing. Perhaps changing the water pump to one of a different material, aluminum v. cast, would make this go away. When I change my water pump this Winter I'm going to go with one of the aluminum ones and see if this makes a difference.

I know the solution has been to tighten the belts and I'm concerned that this will wear component bearings causing premature failure.

Bob

 

[77-4speed]

I'm with you. I already feel like the belts are too tight, yet I still get more than 1/2" deflection on the A/C belt. I'd just hate to tighten them up too much and wear out the water pump bearings or somehow damage the harmonic balancer.

I do have an aftermarket cast waterpump. (flowkooler)

Russ