MACS Monthly Newsletter - 2021 - SEP2

Fast forward to later models and there are two methods to
confirm the presence of a variable displacement compressor.
Looking at it, the modern VDC still has a tapered nose
and a boss on the rear
head for the control
valve. Using a 2014
GMC Pickup as an
example, from the top
the compressor type
can't be identified except
for observing the
clutch. If the system is
operational, the clutch
will not turn off under
low heat load.
If the compressor is
Figure 2: Look for the two-wire connector
for the control valve on the
rear head of an electronic version.
down low, raise the vehicle
and remove the
splash shield if necessary.
The compressor
is tucked away on the
lower right side ahead
of the engine mount
on Ford and GM pickup
trucks. From underneath,
we can see the compressor, the control valve, and its
two-wire connector. See Figure 2.
On some makes and models, you may even notice the lack
of a compressor clutch wiring connector and disengaging
spring clutch driver. A rule of thumb is, if the compressor can
not be " turned off " , it MUST be variable displacement.
Why Variable Displacement?
I suggested many reasons for variable displacement systems
in the April 2021 issue of MACS Service Reports. They
all still apply, though the quest to improve fuel economy
makes the most sense for later model vehicles. Engaging
and disengaging a compressor causes surges which require
adjustment to engine management strategies and refrigerant
cooling requirements.
On a cycling system, when the cabin temperature stabilizes
and the heat load on the evaporator decreases, the evaporator
temperature can drop to the point where condensate
on the evaporator may freeze. To avoid the evaporator and
therefore the water freezing, the compressor is cycled " off " .
Soon after, the evaporator begins to warm up and gets warm
enough that the system must again turn on the compressor
to provide cooling.
This UP/DOWN cycle increases fuel consumption, and the
refrigeration system cycles from high to low to high heat load.
High heat load = increased high side pressure.
High discharge pressure = increased fuel consumption.
It takes more energy to compress the refrigerant under
high heat load conditions.
When Is Output Reduced?
Rather than cycle the compressor, the variable output refrigeration
system allows the compressor to run all the time.
September 2021
2
Compressor Clutch/Drive Hub
* Do what you must to get a good view of the clutch.
MACS Service Reports
To avoid evaporator freezing, VDCs can reduce their displacement,
therefore output, down to 5% of their maximum.
The strategy, for a modern electronically controlled variable
displacement system leaves the compressor running
and varies the stroke/output of the compressor as cooling
demands fluctuate. The HVAC controller monitors evaporator
temperature via an Evaporator Temperature Sensor (ETS).
As the cabin temperature and humidity drop, the evaporator
temperature starts to drop.
Variable output refrigeration systems are typically programmed
to maintain evaporator surface temperature somewhere
around 33-35°F (0.6-1.7°C) under lower heat load conditions.
As
the cabin temperature stabilizes, the refrigerant in the
evaporator continues to absorb heat and vaporise, which
continues to cool the core surface. To maintain evaporator
temperature just above freezing, the compressor control
valve is commanded to reduce the stroke of the compressor.
Reducing the stroke (output) of the compressor reduces
the amount of refrigerant being condensed in the condenser,
lowering the liquid level in the receiver/dryer, and slowing
the refrigerant flow, therefore supplying less refrigerant to the
evaporator. The HVAC controller can fine-tune the volume of
refrigerant available to the evaporator, precisely maintaining
its surface temperature.
Precisely controlling the amount of heat absorbed by the
refrigerant in the evaporator means there is less heat to be
transferred out front to the condenser for removal once the
cabin stabilizes.
Reduced heat absorption = Reduced heat to be removed
by the condenser.
Reduced heat to be removed = Reduced discharge pressure.
Reduced
discharge pressure = Reduced load = Improved
fuel economy.
There is also the benefit of constant dehumidification of the
air passing over the evaporator core, efficiently stabilizing the
humidity level in the cabin.
Preliminary Testing
Always start with the basics. Gather data and then proceed
logically. Put the pieces together to form a diagnostic plan.
There are a few characteristics common to electronically
controlled variable displacement systems.
While driving the vehicle into the shop from the parking lot,
run through the HVAC controls to confirm the air distribution
system and blower are functioning properly. Engage the A/C
and note the duct temperature.
Once in your bay and turned off, as usual, perform a visual
inspection of the system. Now is also a great time to get the
UV light out and check for trace dye, which may indicate a
leak.
Inspect the cabin air filter. Reduced air flow will impact the
pressure and temperature readings during pressure testing
and/or dynamic tests.
Mike Bailey
MACS Monthly Newsletter - 2021

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