WEIRD A/C-related noise.. can you identify it?

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bstevens@rock.com - 22 May 2006 02:23 GMT

I have a 2002 Dodge RAM 1500 truck, a regular cab LWB with the 4.7 V8.
Here's what's happening:

If the truck's not moving, e.g. sitting at a red light or in a parking
space, and I turn on the A/C, then when I start moving or put it in
gear, it makes this REALLY loud sound. It sounds like air blowing out
of tire... sort of a very loud "SHHHHHHHHHHHHHHHHHHH' sound. You might
also describe it as a high-pitched metallic scraping sound, but the
"air blast" is really more accurate. If you've ever used the pressure
nozzle on the end of a compressed air hose, you have a good idea of
what it sounds like.

It lasts for anywhere from 5 to 20 seconds, then stops and doesn't do
it any more. It NEVER does it if the truck's moving and I turn on the
A/C: it'll wait until I stop, e.g. at a red light, then do it when the
light turns green and I start off. It ONLY does it when the truck
starts moving from a standstill - forward or reverse, it doesn't
matter.

But once it does it, it doesn't do it any more until I stop and turn
off the engine and let the truck sit a while.

It's definitely related to the A/C. If the A/C's off, it doesn't do it.
But even if I drive for an hour, then turn the A/C on, it'll do it the
next time I stop and take off. Again, though, after it does it once,
it's done until the engines turned off for a while.

If I reach down and click off the A/C, the sound stops almost
instantly.

It's not the blower fan, because it doesn't do it with the heater, only
the A/C.

The A/C works perfectly. Refrigerant pressure is perfect, it's nice and
cold.

I'm basically a self-fixer and I'm trying to save some $$$ here. Any
ideas? I've poked and prodded and peeped at this thing until I'm blue
in the face, and can't find a thing.

Thanks,
R. S.

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Kevin - 22 May 2006 04:11 GMT

> I have a 2002 Dodge RAM 1500 truck, a regular cab LWB with the 4.7 V8.
> Here's what's happening:
[quoted text clipped - 38 lines]
> Thanks,
> R. S.

There may be some other possible causes for the noise you hear, but it is
difficult to diagnose unheard sounds over the internet. The compressor
clutch slipping is the most likely culprit from your description of the
noise and the conditions under which it occurs..

Measure the compressor clutch air gap by inserting a feeler gauge between
the clutch plate and the pulley. It should be no more that .015". Too large
an air gap might cause the clutch to slip during engagement and make a sound
like you described.

Measure the compressor clutch field coil current with an amp meter. High
resistance at some point in the circuit might cause low current and
contribute to the clutch slipping during engagement. Low voltage supply due
to a charging system problem might also have the same effect.

Check the operation of your cooling fan clutch. Make sure the engine is
fully warmed up and have an assistant turn off the engine while you observe
the radiator fan. If it continues to spin for more than 1 second after the
engine stops, then the fan clutch is not engaging properly. That could cause
excessive head pressure on the compressor and contribute to the slipping
compressor clutch that you might be hearing.

Check the A/C compressor belt tension. A loose or worn belt might make the
kind of noise you described..

You said the refrigerant pressure is perfect. How do you know this? What are
the High and Low side pressures
during operation. Knowing these pressures is the only way to tell you if the
refrigerant is appropriate. Those pressures can also be used to diagnose
other possible malfunctions such as system restrictions or even insufficient
condenser air flow. Can you post back with the refrigerant operating
pressures?

Hope this helps.

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

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bstevens@rock.com - 23 May 2006 20:28 GMT

Thanks, Kevin. As far as refrigerant pressure, it's just the low-side
measurement, taken with a $12 gauge from Pep Boys, the kind with one
end screwed into a refrigerant can.

I'm curious about that clutch plate/pulley gap you mentioned. Is that
something I can check from outside the compressor, without
disassembling anything?

If that clutch plate/pulley gap is too big, is there some way I can
adjust it?

Many thanks,
R.S.

Reply to this Message

Kevin - 23 May 2006 21:58 GMT

> Thanks, Kevin. As far as refrigerant pressure, it's just the low-side
> measurement, taken with a $12 gauge from Pep Boys, the kind with one
[quoted text clipped - 9 lines]
> Many thanks,
> R.S.

A low side operating pressure reading by itself is basically useless for
diagnosis other than to see if you are nearly out of refrigerant. You need
to compare the High and Low Sides to get a picture of what is happening in
the refrigerant system.

If the compressor is mounted in an accessible position you should be able to
measure the air gap with out removing it. The air gap on some compressors is
adjustable by removing the clutch pulley and using a thinner shim behind the
shaft hub. That does usually require removing the compressor and special
tools to remove the clutch. If the gap is too wide it usually means the
clutch has been slipping and is worn. In most cases a replacement clutch is
needed to correct the problem. Checking the air gap is just a simple
diagnostic measure to determine if that is the problem.

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

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Telstar Electronics - 24 May 2006 12:59 GMT

The thing that doesn't make sense to me is that this noise just occurs
once per engine-start cycle. With that said... my only two ideas for
the noise he descibes are what Kevin was talking about... the
compressor clutch... or the sound refrigerant makes when escaping from
the emergency over-pressure valve.

www.telstar-electronics.com

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