Rick's Free Auto Repair Advice

Clutchless AC Compressor — How it works

Clutchless AC Compressor

How a clutchless AC compressor works

As the name implies, a clutchless AC compressor doesn’t have a clutch to engage and disengage the compressor shaft from the drive belt pulley. Instead, the drive belt turns the compressor shaft all the time. That doesn’t mean the compressor is compressing refrigerant full-time though. These new variable displacement compressors use a swashplate that varies the stroke of the pistons based on how much refrigerant pressure is metered into the crankcase of the compressor. When the AC doesn’t require compressed refrigerant, the swashplate is in a neutral position where the piston stroke is zero. Here’s a youtube video that shows how a swashplate system compressor varies the amount of refrigerant it outputs.

To prevent AC drive belt disengagement in the event of a compressor seize, the compressor pulley contains a built-in torque limiter slip clutch. In the event the compressor seizes, the torque limiter slip clutch will shear off the engagement points, allowing the compressor pulley to continue to spin while no longer applying rotational force to the compressor input shaft.

clutchless AC compressor

How to check for a sheared torque limiter slip clutch

During normal operation, the center bolt at the front of the pulley will rotate at the same speed as the pulley. If the pulley spins but the center bolt does not, the torque limiter slip clutch has sheared due to internal binding and the compressor must be replaced.

If the center bolt is rotating, proceed with the diagnosis

What moves the swashplate?

That’s a simple question with a more complicated answer. It depends on the vehicle brand and compressor model. Some manufacturers use an external control valve (ECV) solenoid that pulses high-pressure refrigerant into one side of the crankcase to force the swashplate to an operating angle that results in varying stroke lengths. That way, the HVAC head can control how much refrigerant is pumped based on the cooling needs of the vehicle.

In other versions, a mechanical device moves the swashplate to the desired angle. In yet a third design, a sensor detects the flow rate of the refrigerant and the HVAC head uses that data to alter the swashplate angle.

What goes wrong with a variable displacement AC compressor?

The external control valve or swashplate solenoids are common failures on these newer compressors. They work in conjunction with the low and high-pressure switches, cabin temperature sensors, and the HVAC head to determine when to move the swashplate angle. To diagnose a “no cooling” or “low cooling” condition, you’ll need a shop manual.

Test ECV resistance

Since the ECV is nothing more than an electric solenoid, test using a multimeter. Refer to the shop manual for acceptable readings.

external control valve

External control valve

For example, The normal ECV resistance for this Hyundai variable displacement compressor should be 10.1 ~ 11.1Ω when the ambient temperature is 77°F. However, resistance may vary due to higher underhood temperatures. The acceptable range is (8 Ω ≤ ECV Resistance ≤ 14 Ω).

If the resistance is outside of the acceptable range, replace the ECV.


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Posted on by Rick Muscoplat

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