One of the first things you should check when your car's air conditioner stops working is the compressor. The compressor can stop working when the clutch goes bad. To check the clutch, take a voltmeter and attach it to the compressor. If there is electricity getting to the compressor, but the clutch isn't engaging, then the clutch needs to be replaced. Here is how a semi-experienced do-it-yourselfer car repair person can replace the clutch on the compressor of a car's air conditioner.

Removing the Compressor

You will have to remove the compressor so you have space to work on it.

1. Start by removing the serpentine belt from the pulley on the front side of the compressor.
2. Next, remove the manifold connected to the backside of the compressor. You'll need to use a ratchet with an extension to reach the 10mm bolt holding the manifold in place. Remove the bolt and pull the manifold away from the compressor.
3. Remove the O-rings from the ports where the high and low pressure lines connect to the compressor. If the O-rings are in good shape, you can use them again when you reconnect the manifold to the compressor.
4. Remove the cold air intake duct, and disconnect the electrical connector to the compressor.
5. There are usually four 10mm bolts that connect the compressor to the top of the engine. Remove the bolts and lift the compressor out of the engine compartment.

Removing the Old Clutch

1. Place the compressor on a work table with the pulley end facing up. You need to remove the top hub covering the top of the pulley. If there is a center bolt holding the hub in place, simply remove the bolt and lift out the hub. Some hubs are pressed onto the compressor. In this case, you should use a clutch remover tool to loosen the hub and remove it. Insert the tool into the compressor and turn the nut on the tool until the hub disengages from the compressor.
2. The next step is to remove the pulley. The pulley is held in place by a snap ring. Remove the snap ring with a pair of snap ring pliers. List off the pulley and set it aside. If the pulley doesn't lift up, you might have to tap the pulley with a rubber mallet to loosen it (in some cases, you'll need to use a puller tool to remove the hub). The clutch will be exposed once the pulley has been removed.
3. Gently pry the clutch out with a long screwdriver.

Installing New Clutch

1. Set the clutch in the place of the old one. You put the new clutch into position by putting a screwdriver into the center ring area of the clutch and gently tapping the clutch down until it slides into its proper position. Put the pulley back into position and reinsert the snap ring.
2. Replace the hub. There needs to be an air gap between the hub and the pulley for the compressor to work well, so don't tighten the hub down all the way. Check with the manufacturer of your compressor to find out how much of an air gap is needed, and use a gap gauge tool to determine how far down you should tighten the hub.
3. Put the compressor back onto the top of the engine and put everything back together in the reverse order in which you took them out.