JPTL

A nice day yesterday so I decided to take the GT out for a drive. I noticed a very subtle rattling sound coming from the engine bay. Not a crunch, groan or whirring sound - more of rattle as if something was loose.
To be on the safe side I turned around and headed home.
Today I popped the hood and have identified the sound as coming from a wobbly AC compressor pulley. There is enough runout(?) of the pulley to be visually noticeable. I didn't feel it necessary to turn on the AC/engage the compressor, because the wobble is when the compressor clutch is idle/disengaged.
The AC was working a few months ago. It does need a charge every few seasons, and it's never been ice-cold like new cars, but I wouldn't describe the compressor as being abnormally loud when using the AC. I did a 134a conversion many years ago (regretfully) but that's water under the bridge. I've never replaced the compressor but the PO records show that Porsche replaced it back in '95 when the car had 23k on it. So although the compressor only has 30k on it, it's 25 years old.
I read Nicole's thread about replacing the clutch bearing and even though the noise isn't nearly as profound she and many others describe, something's clearly wrong.
Step one is to pull the belt before driving the car.
Step two is to troubleshoot before I dig in. It's chilly out and my garage isn't heated, so I don't anticipate digging in until the Spring.
Questions:

• Has anyone experienced compressor issues starting with a faint sound of a wobbly pulley?
• Do the AC clutch pulleys simply become loose? I'd love to hear that it's just a matter of tightening bolts, but I'm pessimistic about that being the fix.
• Are there other things that could be causing the wobble - aside from a bad bearing(s)?

Any input would be greatly appreciated!

gazfish

First thing id try is checking the nut is tight on the front, if it is then go with your plan of removing the belt until warmer weather

dr bob

Wudja hit? The compressor clutch is vulnerable to casual impact when the belly pans are off and the car has sagged to "fashionaly low" in front.

The pulley lives on that clutch bearing, while the drive hub (the front part that's stationary with AC off) slides over the end of the compressor shaft. If the pulley isn't running true even with the AC off, that bearing is suspect. You can remove the drive plate after removing the nut in the middle, so the shaft is completely isolated from the pulley and bearing. Does the pulley still wobble when you spin it by hand?? Try rocking it with your hand to see if the bearing has failed that much. If it's loose then a bearing might solve the problem. If the pulley itself is bent, Mark may have one for you. If the drive plate is bent from impact, you can probably buy those too. You can use a dial indicator to help see what's actually bent of course.

Shark2626

I’m not sure that a 928 AC compressor is all that different from most others. But if you believe it’s a Porsche specific issue then why not google it? And if you prefer Rennlist answers then just add that. For instance: Porsche AC compressor making noise Rennlist.

Its one of three things. If you are on a budget take them step by step least expensive on up, if not source a replacement rebuilt or new compressor.

You are correct, that belt may give you grief (smoking+) until you make the needed repair(s).

JPTL

The belly pans are intact and I definitely didn’t hit anything. I suppose a bad bearing is inevitable.

worf928

Yes. Every factory-installed sealed bearing is now 30 years old.

A new compressor is $400. It comes with a new clutch, pulley, bearing, etc. And it doesn’t leak at the nose, seams, or manifold.

From your description, your GT could use the ‘full’ a/c service (i.e. o-rings front and rear, expansion valves, r/d, etc.)

928cs

Are you sure this sound comes from the AC compressor, and not the air pump? I ask because I know more air pumps failures than AC failures and they are close in the engine bay.