Rick's Free Auto Repair Advice

Compressor clutch not engaging

Car air conditioner compressor clutch not engaging

If you’re here, chances are you’ve tried to recharge your AC system and found a compressor clutch not engaging issue. If so, try the troubleshooting techniques described here. This is a full explanation of how the system works. After reading, go to this post to learn how to diagnose a compressor clutch problem.

What makes an AC compressor clutch engage?

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 towards the pulley. The clutch disc is splined to the  compressor shaft. As the clutch disc pulls in towards the pulley, the pulley provides the rotating power to operate the compressor.

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. That means the clutch has properly engaged with the pulley. If only the pulley is turning, but the clutch disc isn’t, there’s a problem somewhere in the system.

Why AC clutch won’t engage

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. AC compressor clutches receive power in a number of ways. They can activate through a relay that’s energized directly by a proper condition of the low and high pressure switches. Or, the relay can be operated by the PCM which has interpreted the condition of the low and high pressure sensors. In that case, the PCM would see that the refrigerant pressures are within bounds and the PCM would provide a ground connection to the control coil side of the AC compressor clutch relay. In both of these scenarios, you’d start the diagnosis by checking the compressor clutch fuse.

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.

If the fuse is good, disconnect the electrical connector from the compressor 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, 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.

Is the compressor clutch air gap set properly?

The compressor clutch air gap is critical to the AC compressor clutch engaging. To learn more about AC compressor clutch air gap, read this post

Should you jumper across the low pressure switch?

NEVER! Air conditioning compressors need a constant supply of oil to prevent catastrophic failure, and it’s the refrigerant that carries the oil through the system. 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 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 before the system reaches full charge. NEVER try to bypass the low pressure switch. Doing so will force the compressor to run with inadequate oil and can destroy the compressor.

Is the AC system too low to charge?

If an air conditioning system is so low on refrigerant that it can’t engage the compressor clutch, it’s really 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.

Can you check AC charge with a DIY kit?

Not really. DIY kits come with a low pressure side gauge and they’re designed to provide a static pressure (engine not running or AC off) and a running pressure. Static low side pressure won’t give you any idea of system charge. You could have a low side target range of 50-psi and see that static pressure on your gauge. All that tells you is that there is some refrigerant in the system. But 1 ounce of refrigerant will provide a 50-psi reading the same as a full system. The only thing a kit gauge can tell you is that your system is completely empty if you attach the gauge and get a reading of 0-psi. Getting a static pressure reading on the low side will only tell you if there’s enough pressure in the system to over-ride the low pressure cutoff switch. Period.

A static pressure taken on the high side is a slightly better indicator of system charge because the high side is much more responsive to ambient temperature than the low side. But even then, it will only tell you if there’s some refrigerant in the system. However, to read the high side pressure, you’ll need a manifold gauge set.

However, I’ve been around long enough to know you’ll try to recharge it against my advice. 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.


Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
65°F Low side pressure 25-35 psi High side pressure 135-155 psi
70°F  Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 psi
Ambient temp is the outside atmospheric temperature.

How to charge AC compressor even if compressor isn’t running

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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