The variables of variable displacement A/C compressors

In full stroke mode, the bellows compress allowing the ball valve to seat.  The suction orifice opens allowing compressor crankcase gas to flow to suction resulting in the compressor going to full stroke.

Why vary compressor displacement?

When I was a much younger technician, I recall an A/C component called a POA (Pilot Operated Absolute) valve that prevented the evaporator’s outlet pressure from getting too low.  We all know that when evaporator pressures get too low, the evaporator’s temperature then becomes too low causing condensation on the evaporator fins to ice up.  An iced-up evaporator won’t allow air to flow across its fins and that causes the passenger compartment to warm up.  The POA valve was mainly a GM thing with a few Fords using them as well.  Other OEMs did (back then) what they still do today – combine an expansion valve with an evaporator temp switch that shuts the compressor clutch relay off when the evaporator becomes too cold.  Engineers at GM amid the fuel economy crisis days (1970s)  concluded if cycling the compressor worked to keep the evaporator from freezing up, then why not go for a simpler (and cheaper) version of the expansion valve (the fixed orifice tube) and cycle the compressor more often to maintain evaporator pressures within the perfect range of cold but not too cold. This increased fuel economy on all but the hottest days when the compressor would run nearly full time.

The rise and fall of CCOT

The old GM CCOT (Cycling Clutch Orifice Tube) systems born out of increased fuel economy requirements did a good job in saving fuel while keeping evaporators from icing up on less than sweltering days.  There were less than desirable side effects, however.  You may recall that distinct feel of engine load changes when the clutches for those old R6 and R4 GM compressors cycled.  As engine displacement (and power) continued to decrease to address increased fuel economy requirements that “feel” of the compressor cycling between either 100 % displacement (clutch on) and 0 % displacement (clutch off) became more noticeable.  This led to the idea of a gradual compressor displacement change via a movable wobble plate that provided for the compressor's piston strokes to gradually increase and decrease to be less noticeable to the driver.  More importantly, the compressor's variable displacement output combined with a varying expansion valve opening allowing for evaporator pressures to be maintained in the sweet spot between not too warm (passenger discomfort) and not too cold (evaporator freeze up).

VDC (Variable Displacement Compressor) designs

Over the years both reciprocating piston designs and non-piston compressors designs (i.e. the scroll)  were first adapted to variable displacement operation utilizing a manual self-balancing pressure control valve (Figures 1A, 1B, 1C). The control valve changed the sump (internal compressor pressure) as it reacted to low side pressure (Figure 2) to move the compressor from full displacement (full stroke) to low or no displacement (Figures 3, 4).  The moving parts (besides the pistons) are the flex in the “wobble plate” attached to the compressor’s shaft (Figure 1A) or the swash plate (Figures 3, 4) and the shaft itself. 

Oil maladies – the compressor’s No. 1 enemy

As with any mechanical component failure, insufficient oil quantity and / or quality are the main enemies.  Adding too much refrigerant oil to the system can be almost as bad as having too little oil in the system.  Too little oil is obvious – just like in an engine.  Too much oil can be subtle.  If several technicians have worked on the vehicle before you, they may have added too much oil.  For example, just because a compressor comes shipped with X # of ounces of oil inside it doesn't mean all of that oil should remain in the compressor before installation.  ALWAYS perform the OEM recommended oil "balancing" procedure on any new compressor you install.  Basically, you drain the old compressor's oil from it and measure it.  Unless the amount you drain is under 2-3 ounces (see OEM specs) you put that amount of the correct type of new refrigerant oil into the replacement compressor.  Always spin the compressor shaft by hand before running it with the engine. Too much oil, too much leak detection dye/sealants along with too high of a refrigerant charge will cause the system pressure to increase.  Basic gas laws state the higher the pressure, the higher the temperature.  This is even true of that cold stuff we call refrigerant. On the flip side of the excessive refrigerant and/or excessive oil charge coin is too low of refrigerant charge.  Oil only moves with the refrigerant it was designed to work with. An insufficient refrigerant charge (or incorrect oil) will result in oil that’s present in the system but not moving through the compressor as intended. This will, in turn, lead to premature compressor failure.   

Electrically controlled pressure control valves

Similar in idea to those electric solenoids which control the flow of oil in an engine with variable valve timing, newer A/C compressors are using solenoids (Figure 5) to control compressor output.  The solenoid takes the place of the older style self-regulating pressure control valve. As the evaporator temperature (and pressure) begin to drop (as the vehicle's interior cools down) the HVAC controls (monitoring A/C pressures) command the compressor's solenoid to move the swash plate to reduce the compressor's stroke (displacement).  As with the self-regulating valve, the gradual displacement change is easier on the load changes applied to the engine. You probably began noticing this type of compressor appearing first on vehicles with smaller / lower powered engines. In more recent model years the electronically controlled variable displacement compressor is being used in all sizes vehicles and engines.  Many German and Asian imports have even eliminated the compressor clutch, using a full movement to zero displacement via the computer-controlled VDC's electric solenoid to discontinue compressor operation when not required.  In case of a compressor failure/lock up, the front of the compressor has a plate (Figure 6) with sheer pins that are designed to break away allowing the compressor's drive pulley to spin with a locked-up compressor. The idea is to prevent the serpentine belt from breaking/flying off. 

VDC solenoid diagnostics

While diagnostics on variable displacement compressors using the self-regulating pressure control valves can be challenging, electric solenoids can be monitored with a scan tool. Always consult the vehicle’s wiring diagram (Figure 7) to make sure you fully understand what component in the system controls the compressor’s clutch (if used) and displacement control solenoid.  There are differing opinions on what to do when you diagnose a faulty A/C VDC solenoid.  Most will agree if the solenoid seems to be inoperative (full stroke / evaporator freeze up or low / no stroke and no cooling performance / low pressures) and the compressor has a LOT of miles on it, replace the compressor. No sense in trying to replace an old and ailing compressor’s control solenoid only to see the compressor fail 6 months later.

On the other hand, if the compressor is not an aged high mileage specimen, a replacement solenoid is available AND the solenoid is easily accessible (Figure 8) many techs are replacing only the solenoid and reporting profitable and successful A/C repairs that are quicker and less expensive for the customer.  Always check the solenoid for a good connection and proper resistance (Figure 9) early on in your diagnostic process. If you’re not experienced in monitoring high and low side pressures on variable displacement compressors you might misdiagnose a low system or other problem when the system is performing properly. 

For example, on a 72-degree F. day with 50 percent humidity your RRR machine (or manifold gauge set) may read 60 psi on the low side and 100 psi on the high side.  With a conventional fixed displacement compressor, you might rightfully assume a low charge or underperforming compressor.  On a late model Ford Escape with electronically-controlled VDC this reading is normal.  The proof is almost always flushed out in a performance test indicating whether or not the system is cooling but sometimes we're tempted to ignore the obvious "if it's not broke – don't fix it" rule and be led strictly by our gauge readings (Figures 10, 11). For this reason, it is imperative to combine scan tool DPID info (Figures 12, 13, 14) along with scan tool bi-directional control of the VDC's solenoid to make the proper diagnosis.  Some OEMs don't even allow their factory scan tool to control their VDC solenoids so you might consider purchasing a specialized tool (Figures 15, 16) to ramp up the solenoid's output to determine exactly what the root problem is. Knowing the variables with variable displacement A/C compressors will be the key to the proper diagnosis for this spring’s A/C services.