A/C compressors look different than technicians remember

Jan. 1, 2020
Each time you face a vehicle in your bay this spring with a reported A/C problem, you'll initially check the airflow through the ducts and the A/C controls, maybe static pressure in the system. Once under the hood, you will look at the compressor to

Today's units sometimes don't have a magnetic clutch assembly.

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Each time you face a vehicle in your bay this spring with a reported A/C problem, you'll initially check the airflow through the ducts and the A/C controls, maybe static pressure in the system. Once under the hood, you most certainly will look at the compressor to see if it's engaged.

Surprise! On some of the compressors in use today, there is no magnetic clutch assembly on the compressor for you to see. Here's a short rundown of the different compressors that may make their way to your shop.

Constant Run Variable Displacement Compressors

It takes a lot of energy to spin a compressor. On some vehicles, it can take more energy to run the compressor than it does to run the car at a steady cruise, and that affects fuel mileage and emissions. One answer to reducing that effect is the variable displacement compressor. After all, less refrigerant volume is needed to keep a car cool than is needed to get it cool, and less energy is needed to pump that reduced volume.
Constant run means the compressor is always engaged, driven by belt. This helps promote longer compressor service life by keeping oil flowing and seals lubricated. That also is why you should never run the engine with no refrigerant in the system, or worse, under any kind of vacuum. This could cause lack of lubrication in the running compressor, and result in compressor damage. Constant run compressors typically are equipped with a breakaway drive that will disengage the compressor shaft from the drive pulley in the event of compressor seizure.
Variable displacement is controlled by altering the angle of the drive plate (whether swash or wobble plate), resulting in a variance in piston stroke. A shorter stroke results in less volume. Some earlier designs control plate angle internally via a pressure differential control valve that responds to differences in low side pressure caused by refrigerant temperature changes in the evaporator. As the heat load reduces, evap temps go down and so does pressure. This results in a reduction in piston stroke and refrigerant volume. High heat loads will cause higher evap temperatures and pressures, resulting in a longer piston stroke and more volume. (For more, see Variable Displacement A/C Compressors, Motor Age, April 2005).

The displacement also can be externally controlled, typically by the Engine Control Module (ECM) in response to information it gets from refrigerant pressure and temperature sensors. This provides for more precise control over the displacement needed for optimum cooling while reducing the power demand on the engine.

Be aware that your gauge pressures may read just fine. Pressure readings are not dependent on volume. One indicator of a problem related to displacement is a system that is cooling, but not as well as it should. Gauge readings may be near normal or the low side readings may be slightly higher than normal. Both are symptoms of insufficient refrigerant volume through the evaporator, similar to what you might see with a clogged orifice tube.

Include temperature readings at the ducts in your diagnostic process. If you suspect a problem, you will need a scan tool to test externally controlled compressors and, on some, a special tool to substitute the ECM's control to vary the displacement yourself. On most, any failure associated with the compressor requires replacement of the assembly.

Electric Compressors

Hybrids are becoming more and more common, and most rely — either directly or indirectly — on the A/C system to keep the high voltage (HV) battery packs cool. Considering the age of the hybrid fleet, this is an A/C market you should become a part of. While some hybrids use conventional fixed displacement, belt driven compressors, many are using electric compressors.
These compressors are driven by either a brushless, three-phase AC voltage (ACV) motor or brushless DC voltage (DCV) motor using high voltage from the HV system for power. The first thing to look for is either a bright orange or light blue cable covering the harness leading to the compressor. Be sure you use all safety precautions when attempting to service any high voltage component! As with most A/C compressors, the control is provided by a control module based on a variety of input sensors. Totally electric compressors allow the control module to vary compressor speed independent of engine speed, and will require a scan tool for proper diagnosis.
Got a hybrid with what looks like a belt driven compressor? There are combination belt drive/electric compressors. The displacement on these compressors is split, with a small portion run by the same motor as the pure electric compressors, and the larger portion driven conventionally. Both of these designs allow the compressor to continue to provide refrigerant flow during "idle stops" when the internal combustion engine (ICE) is shut off.

Because both of these designs use high voltage electric motors internal to the compressor, they require special, non-conductive (designated POE) oil for lubrication. Even a slight contamination (less than 1 percent) with PAG oil can result in voltage leaking through the compressor case. To service these systems, you'll need a dedicated RRR (recovery/recycling/recharging) machine just for hybrids or a new machine that is certified to SAE standard J2788H.

This applies even if all you do is recover the charge to fix a leaking seal. There is enough oil remaining in your current machine's lines to cause sufficient contamination. Failing to follow this guideline can result in a comeback where at least one manufacturer requires EVERY component in the system be replaced.

On the Horizon?

No doubt about it, new refrigerants are on the horizon. Mandated by the European Union, R-134a will no longer be allowed in new platforms offered for sale there in the 2011 model year. Leading contenders for the replacement of R-134a are CO2 and HFO-1234yf. And the question is still open as to whether these systems will show up on our shores, or if our own legislature will see fit to follow the same ban.

CO2 systems will use much higher pressures than we are used to today. HFO-1234yf is almost a "drop-in" for R-134a with very little changes to the systems we are used to required. Either choice will require training, new servicing equipment and likely new certifications.

Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier at www.autoservicetech.com.