How to Tell if the Clutch or the AC Compressor Is Bad

Shift the vehicle into neutral or park. Engage the emergency brake. Raise the hood. Touch the positive lead of a voltmeter to the positive battery terminal. Touch the negative lead of the voltmeter to the negative battery terminal. Turn the voltmeter gauge scale to volts and read the number. If the battery voltage reads below 12.5 volts, the compressor will not turn on. Charge the battery. If you still get a low reading, the battery has a shorted cell and must be replaced.

Start the engine and let it warm up. Stand in front of the AC compressor at the front of the engine. Have an assistant turn on the AC controls to maximum. Wait for the AC clutch in front of the compressor body to engage and spin. If it does not, shut the engine off and disconnect the negative battery cable with a socket.

Locate the main fuse box. Pull out the AC fuse and examine the filament inside the fuse. Replace the fuse if the filament has melted or blown apart. Look for the AC relay in the fuse box. Swap it with another relay that has the same amperage and pin configuration. Connect the negative battery cable with the socket. Start the engine and have your assistant turn on the AC controls to maximum. If the clutch engages, the fuse or relay was at fault.

Determine if your vehicle uses R12 or R134a refrigerant. For the R12, hook up the low-side hose of an AC gauge to the low-side nipple on the compressor. The low-side will have a 7/16-inch screw-on fitting. Connect the high-side hose of the gauge to the high-side compressor nipple, which will be a 3/8-inch screw-on fitting. For the R134a type refrigerant, push the high-side gauge hose onto the quick release 16 mm nipple on the compressor. Push the low-side gauge hose onto the quick release 13 mm nipple on the compressor.

Start the engine and turn the AC controls on maximum. Open the high- and low-side knobs on the gauge. Look at the reading. The low-side reading should indicate between 25 and 40 pounds per square inch (PSI). The high-side reading should fall between 200 and 225 PSI. If it reads outside of these limits or shows no charge at all, the cutout relay inside the compressor is preventing voltage from activating the clutch. Charge the AC fully with the correct refrigerant.

Inspect the clutch pulley belt with the engine turned off. The belt should have no cracks or oil contamination that would cause it to slip. If it slips, the pulley will not attain sufficient revolutions to engage the clutch mechanism. Check the belt for the proper tension. No more than 1/4-inch of free-play should be allowed. Loosen the bottom AC compressor mount bolts and the top adjusting bolt with a socket. Pull the compressor against the belt to remove excess slack, then tighten the bottom and top compressor bolts.

Place a mechanic's stethoscope probe on the top of the non-moving part of the clutch while the engine idles. Have your assistant turn on the AC controls to maximum. Listen for any internal buzzing or clicking from the clutch. If the clutch fails to engage and spin, but you hear electrical buzzing or clicks inside, it indicates an internal clutch short. Listen for excessive bearing noise coming from the clutch or compressor. Worn shaft bearings will build up resistance in the wiring and fail to turn on the clutch. Clutch pulley wobble also indicates worn shaft bearings.

Shut the engine off. Loosen and remove the three bolts that hold the circular faceplate to the outside of the clutch. Pull the plate off and look at the end of the shaft bearing seal on the clutch. If you see any oil residue, oil film or obvious leakage, it means the compressor shaft seal has failed and discharged refrigerant from the system. You must replace the shaft seal and recharge the system.

Disconnect the wire connector on the clutch with the engine off. Clean the contacts on both wire connector sides with electrical contact spray and a cotton swab. Start the engine and turn the AC controls on maximum. Have your assistant hold a jumper wire on the positive battery terminal while you touch the wire to the "hot" wire inside the wire connector (on the clutch side). If the clutch fails to engage, it means the clutch has an internal short.