Rick's Free Auto Repair Advice

Compressor clutch not engaging

Car air conditioner compressor clutch not engaging

If you have a situation where your car’s air conditioner compressor clutch doesn’t engage, try the troubleshooting techniques described here.

Car makers use an electro-magnetic clutch to engage and disengage the air conditioning compressor.

compressor won't engage, compressor clutch

The three components of a car’s air conditioning compressor clutch

When power energizes the clutch coil, the magnetic field pulls the clutch in to engage the compressor shaft. If the clutch doesn’t engage, the air conditioning compressor can’t pump refrigerant. So start your diagnosis by determining if the compressor clutch works.

Turn the air conditioning controls to MAX AC and start the engine. Then look at the front of the compressor clutch. Both the pulley and the clutch should be spinning. If only the pulley is turning, there’s a problem somewhere in the system.

If the clutch doesn’t engage, the problem could be a blown fuse, an open in the wire to the clutch coil, a bad clutch coil, a poor ground, or a low pressure lockout.

compressor clutch wiring diagram

In some vehicles the compressor clutch relay is turned on and off by the low pressure switch and the evaporator temperature sensor.

 

compressor clutch wiring diagram

In a computer controlled system, the compressor clutch relay is grounded by the computer.

Disconnect the electrical connector from the clutch coil. Move it away from moving parts so you can safely check for power and ground. With engine running and the AC set to MAX, use a digital voltmeter to check for battery voltage and good ground at the connector.

If you’re not getting battery voltage, check the fuse. If the fuse is good, pull the compressor clutch relay and check for power and ground on the relay control coil terminals in the socket.

Wiring varies by make, model, and year. Some car makers trigger the relay control coil ground through the PCM or ECM. In order for the computer to provide ground, it must receive a good signal from the low pressure switch, high pressure switch, and evaporator temperature sensor.

In other vehicles the power flows through the air conditioning switch to the fuse and from the fuse through the low pressure and evaporator temperature sensor to the clutch coil.

It’s important to note that air conditioning compressors need a constant supply of oil to prevent catastrophic failure. That’s why car makers install a low pressure switch–to prevent the compressor from running if the system is low on refrigerant. DIY recharging kits tell you to recharge through the low pressure port while the compressor is engaged. But if the system is too low on refrigerant, the clutch will never engage. Some DIYers then hotwire the low pressure switch to force the compressor clutch to engage while they recharge. BAD MOVE. At that point you’re running the compressor with little to no oil. You may get it to accept a charge, but you may have already damaged the compressor. NEVER try to bypass the low pressure switch. Doing so will force the compressor to run with inadequate oil and can destroy the compressor.

Theoretically, if an air conditioning system is low enough on refrigerant to disengage the compressor clutch, it’s too low to be recharged with a DIY kit. The leak is large and should be repaired, because there’s probably air and moisture in the system. Moisture reacts with the refrigerant and oil to cause acid and sludge that can destroy the system. Then the system should be fully evacuated to remove all air before recharging.

However, I’m smart enough to know you probably won’t do that. There is a way to add more refrigerant without hot wiring the low pressure switch and running the compressor. Be warned that I take no responsibility for any damage you cause by using this technique.

Connect the recharging kit to the low pressure (suction) port. The low pressure tubing is always a larger diameter than the high pressure line. Plus, the DIY kit fitting will only attach to the low pressure port. Turn the key to the RUN position and turn the blower fan to HI. Turn the AC to MAX to start the radiator fans. Then start adding refrigerant.

If your vehicle is an orifice tube system, the refrigerant gas will flow through one of the open compressor reed valves and into the condenser coil. The radiator fans will cause the refrigerant to condense and turn back into a liquid. It won’t be under pressure since the compressor isn’t running. But chances are some of it will still reach the orifice tube and flow into the evaporator coil. With the blower fan running, any liquid that enters the evaporator will boil and turn back into a gas. After a few minutes, you may be able to build up enough pressure to close the low pressure switch. So try starting the engine and checking the compressor.

If you have an expansion valve system, the blower fan will prevent the sensing tube from shutting down the expansion valve while you try to add refrigerant.

If this technique doesn’t work, take it to a shop. Do NOT hot wire the low pressure switch.

compresor clutch, wiring diagram, recharge air conditioning system.

Air conditioning compressor and clutch assembly

© 2014, Rick Muscoplat

Posted on by Rick Muscoplat


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