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.

Before you start, know the difference between a clutch type AC compressor and a clutch-less compressor

Late model vehicles use a clutch-less variable displacement AC compressor. These compressor don’t have a clutch. The pulley drives the compressor the entire time the engine is running. When the AC isn’t on, the wobble plate inside the compressor is in the 0° position, so the compressor isn’t compressing any refrigerant.

Older vehicles have a clutch type compressor. The pulley spins the entire time the engine is running, but the clutch plate is stationary when the AC is off. When the AC is turned on, power flows to the clutch coil, creating an elector-magnet that pulls the clutch plate against the pulley, which drives the compressor shaft.

What makes an AC compressor clutch engage?

compressor won't engage, compressor clutch

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

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, a low pressure switch lockout, or even a bad engine coolant temperature switch.

AC clutch not getting power

An AC compressor clutch relay can switch power in a number of ways.

• From the low pressure switch. When the low pressure switch sees refrigerant pressure above the low pressure limit, it closes a switch to complete ground to the AC compressor clutch control coil. The control coil energizes and closes the relay contacts to provide power to the AC compressor clutch coil.

• From the PCM. The PCM/ECM monitors the low and high pressure switches and if the pressures are withing spec, the PCM/ECM provides ground to the AC compressor clutch relay control coil .The control coil energizes and closes the relay contacts to provide power to the AC compressor clutch coil.

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. NOTE: Some vehicles only have a power wire going to the clutch coil. The clutch coil ground is internal to the compressor

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

If you see battery voltage on the compressor clutch connector, then the problem is in the clutch itself.

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

Never  jumper across the low pressure switch?

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 think they can just jumper the low pressure switch to force the compressor clutch to engage while they recharge.

THAT’s A BAD MOVE. If you jumper across the low pressure switch with little to no refrigerant in the system, you’re basically running the compressor with little with no lubrication. Would you run your engine with no oil? Well, the same thing applies here. You may get the compressor to accept a charge, but you’ll be damaging the internal components in the process. 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. If the pressure is below the low pressure switch limit that means you have a large leak and there’s air and moisture in the system. If you try to fill the system with the air an moisture still inside, it’ll never work properly. The moisture will cause the expansion valve or orifice tube to freeze. And the moisture will react with the refrigerant and oil to cause acid and sludge that can destroy the system. Air in the system will reduce cooling, even if the charge pressures look good. You’re not doing yourself any favors by forcing refrigerant into a leaking system.

Can you check an AC charge level with the gauge on a DIY kit?

Not really. DIY kits come with a low-pressure side gauge and they’re designed to provide a static pressure reading (engine not running or AC off) and a running pressure reading.

But the pressure readings alone can’t tell you the refrigerant charge. You must also know the ambient temperature. In fact, if you see a static pressure reading of 50-psi. all that tells you is that there is some refrigerant in the system. But unless you refer to a temperature pressure chart that reading can’t tell you if the system is low on charge.

What does a low-pressure gauge static reading tell you?

The only thing a static reading without correlating it to temperature can tell you is:

a) Your system is completely empty (0-psi.), or
b) There’s enough pressure in the system to over-ride the low-pressure cutoff switch. Period.

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

You can add more refrigerant to the system without hot wiring the low-pressure switch and running the compressor. It’ll just take a long time.

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 the refrigerant 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 hotwire 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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