Just got A/C topped up. The Compresser engages and turns when A/C on. So while the clutch engages there is no air?? Mechanic says the compressor is cooked?? The only other problem I know with this 98 Deville is there is a loud whistle sound at idle which is a problem vacuum hose that is in an akward place and needs repair. Could that be related?

There are a couple of things that could cause this.

First, a worn belt can result n slippage and squealing when the compressor load is applied.

Second, a worn out compressor clutch can result in a metal-to-metal contact when the clutch engages. This will cause slipping and squealing.

Third, the compressor shaft could be locked up, so that when the clutch engages, the belt slips over the pulley and squeals.

see this steps and think with clearly and logically NOT feared. God bless you
Start with the compressor. Does it engage when you turn on the A/C?
If so, the compressor is working and the A/C system probably contains enough refrigerant to make cold air, so the problem is inside the HVAC unit. Replace the motor that controls the blend air door (this is a difficult job and best left to a professional since it involves tearing apart the HVAC unit -- about an 8 to 10 hour job!).
If the compressor does not engage when you turn on the A/C, see if it will run by jumping the compressor clutch wire directly to the battery (use a fused jumper wire). If the compressor works when you jump it, and the A/C blows cold air, the system contains refrigerant and the fault is likely a bad A/C compressor clutch relay or a bad clutch cycling switch or pressure switch.
If the compressor does not engage when you jump it, the problem is a bad compressor clutch.
If the clutch engages but the compressor does not turn (the belt will start to slip and squeal), the compressor is locked up and you need a new compressor.
If the compressor clutch engages and turns the compressor, but the A/C still does not blow cold air, the system is probably low on refrigerant and needs to be recharged.

Yes, it could be the ac clutch or the compressor itself.

are you getting power to the clutch compressor? if no power check the "wide open throttle" relay. the relay can go bad and disrupt power to the clutch. also check the voltage on the pressure transducer to make sure it is reading line pressure correctly, will need a scan tool to access this info.

Is the pressure detection switch engaged? It detects pressure in the cooling system. This switch shuts down the clutch to prevent the compressor to run when there's a leak.

Sometimes there's a looking glass on the expansion bottle showing tiny bubles. A large blob of air indicates too much air in the system. Pressure might not be enough to engage the switch when there's too much air left.

I would take it back to whoever replaced the module however, if you want to check for the problem yourself, Look for the AC compressor. with the AC turned off have someone start the vehicle, while watching the AC compressor have the person in the vehicle turn the AC on. If the clutch engages then you have lost your freon charge and need to have the AC unit serviced. If it does not start turn the engine off and check all the wiring connections going to the AC compressor. If all the connections are good start the vehicle, and have the person in the vehicle turn the control to cold and turn on the defroster. If the AC clutch engages then you have a bad ECM module or mis-programmed ECM take it back to be Properly programmed and repaired. If the clutch does not engage it is possible the AC compressor is bad. Take the vehicle to the Mechanic to have teh AC system checked to see why it is not engaging.

Chek compresser fuse. Feel the front of the compressor while car is off and see if the compressor clutch moves by hand. If it does not move by hand, You will need a new compressor, if it does move freely by hand it is a compresser fuse blown

Most likley needs to be recharged. Shouldnt be low, but if it has ever been in an accident it may have a leak or your mechanic stole the refridgerant when it was cold outside and you didnt notice untill you turned the ac on.

Is the A/C 134A or R12? There is safety switch in your canister on the passengers side, if it is below the safe level, it will not engage the compressor clutch. Make sure your refill charge is the proper brand, either 134A or R12. One sure way to check if your level is low, remove the connector wire to the canister and place a jump wire across to two, if the compressor clutch engages, your level IS low.

First thing to ask your self is, When you ingauge the clutch does cold air come out the vents? If so Charge is likely sufficent, If air is cool but not cold likely charge is low & is the cause of compresser not kickin on the clutch. place a proper charge in the system but make sure you know how much to add in, what type of refridgerant it takes ( 134-a ), (R-12). Besure that you use no sealers & that no sealers or Propane was used potentel system damage & injury or Death could result as improper proceeding at this point.
If you do have cold air check the lowside pressure switch function. You do this by first locating the switch disconecting the pigtail & useing a proper jumper wire across the 2 pigtail (female) ends of the harness connector not the switch its self.
If compresser clutch ingauges & Cold air blows from the vents replace low side pressure switch if this does not work grab your wallet,the yellow pages & the phone. I strongly recommend takeing your vehical to a qualified repair facility or Tech.
I Truely hope that I have helped your problem & Knowlege I have given you was useful!

Sincerly
Chris Rader
Mechanic @ Large Mobile Automotive Service Tucson,AZ
www.mechanic-at-large.com