MACS Monthly Newsletter - 2021 - SEP3

* Observe the drive pulley alignment.
* Relax the belt tensioner to check the hub pulley bearing.
*
If the clutch is magnetic, note the condition of the wiring,
driver, and air gap.
* Try to turn the clutch driver hub by hand. Variable displacement
compressors tend to be very easy to turn by
hand. Don't be fooled into thinking the shaft is broken. As
vehicles come in for other service, get a feel for the force
needed to turn the compressor driver on functioning systems.
*
If there is no clutch, are the pins/insulator intact and
able to turn the compressor shaft? See Figure 3.
HVAC module commanding the compressor to MAXIMUM
STROKE. A properly operating, fully charged system should
achieve the readings like those shown in Figure 4. Duct temperature
should be cold AND what you would expect from a
properly functioning fixed displacement system.
* Low Charge: IF duct temperature is warmer than acFigure
4: Maximum heat load, gauge readings in the bay.
Figure 3: Subaru clutch coupler. Made of rubber and can
not disengage.
Pressure Analysis / Dynamic Testing
* Maximum Heat Load.
* Maximum Heat Load should command MAXIMUM
STROKE.
* Connect a gauge set or RRR machine and set the A/C
controls to " OFF " . We do this first as some service ports
are not easy to connect to when the engine is hot.
* Connect a scan tool to the vehicle and monitor the
evaporator temperature sensor reading. It should NOT
drop below freezing and should remain above freezing,
with the temperature close to what is represented on your
pressure/temperature relationship chart for that specific
refrigerant.
* Lower the hood while being careful of connectors and
hoses. Run the vehicle until the engine is up to operating
temperature and the engine bay is heat saturated.
* Open the hood and set the A/C controls to " ON " and
configure the controls for MAXIMUM HEAT LOAD. See
the April 2021 MACS Service Report for details on this important
step.
* Place a thermometer at the HVAC air inlet point to measure
incoming air.
* Allow the system to run for five minutes to stabilize.
* Record your readings.
*This is where problems may become apparent. A properly
functioning system should have responded with the
September 2021
3
ceptable, and High/Low gauge readings are suff iciently
separated however lower than normal (EXAMPLE...Low
side gauge 25 psi / High side gauge 80 psi), stop here.
* It is impossible to performance test a Variable Displacement
system with a low charge. At this point you must
recover, evacuate, recharge, and then leak test before
continuing with the performance testing. Follow local refrigerant
handling regulations using proper equipment.
* Run the MAXIMUM HEAT LOAD test again with the
correct charge.
* With a proper charge, under maximum heat load, your
pressure readings should be EXACTLY THE SAME as the
familiar fixed displacement, cycling clutch systems you are
familiar with. Duct temperature should also be EXACTLY
THE SAME as the cycling clutch system under MAX heat
load.
*IF the system is functioning properly under MAX heat load,
the next step is to remove the heat load from the system and
monitor the response. The system should respond accordingly
to the reduced heat load and the compressor should be
commanded toward minimum stroke.
* Minimum Heat Load.
* Minimum Heat Load should command REDUCED
STROKE.
* Leave the hood open, set the A/C controls to " ON " and
configure the controls for MINIMUM HEAT LOAD. See the
April 2021 MACS Service Report for details on this important
step.
* Use the scan tool to continue monitoring the evaporator
temperature sensor reading. It should NOT drop below
freezing and should remain above freezing, with the temperature
close to what is represented on your pressure/
temperature relationship chart for that specific refrigerant.
* Evaporator temperature should remain close to 3335°F
(0.6-1.7°C) or what is specified in the service manual.
It should NEVER drop below freezing.
* Place a thermometer at the HVAC air inlet (passenger
floor) to measure the temperature of the incoming air on
MACS Service Reports
Mike Bailey
Mike Bailey
MACS Monthly Newsletter - 2021

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