ac knock

[scottfc]

1999 Devile, 39,000 mi. The AC started blowing warm air at ide or slow speeds, at highway speed it is good. When the compressor runs there is a steady knocking. Would a new compressor be needed?

Thanks in advance,

Scott

[Cadillac Jim]

Usually compressor noises don't mean that there is a problem. You appear to have low Freon, based on having cold air at cruise but not at idle, or the cooling fans may not be working. These are things that are monitored by the car's on-board diagnostic (OBD) software and the trouble codes can be pulled by following the procedures here:

http://www.caddyinfo.com/readingcodes.html

Write down the codes and post them here.

[Ranger]

The compressor should not knock. If you are sure the knocking is coming from the compressor, I'd guess it is about to fail.

[KHE]

Can you elaborate on the noise from the compressor? Usually, if a bearing if going bad, it will growl and if a clutch plate is not set to the proper air gap, it may squeal or rattle.

With the small amount of refrigerant that is used (2.0 lbs.), it doesn't take much of a leakdown before a loss in performance is noted. Verify the cooling fans come on when the A/C is selected. If so, the system is probably just low on refrigerant and needs a top-off.