MACS Service Reports - 2015 - AUG3

From CCOT to VDC
Dave Hobbs

Cycling clutch orifice tube systems under moderate heat loads have saved fuel by allowing the compressor to turn off as the low pressure side drops to
pressures low enough for the evaporator to freeze
up. Naturally cycling the clutch was an effective
means of lowering parasitic drive belt load. This
can be a noticeable drivability complaint on lower
displacement engines however, as the full load of
the compressor with a fixed maximum displacement is applied and removed over and over again
via a clutch command.
Out of this issue came VDCs (Variable Displacement Compressors). The first ones showed up on
the smaller displacement engines, then spread to
larger ones. Designs of the 1990's and early 2000's
vintage were mechanically regulated. The change
in displacement was more gradual than an on/off
clutch strategy, which made diagnostics more difficult. With the fixed displacement cycling clutch,
we know approximately what the high and low side
pressures are supposed to be on a fully charged system (without air in it) when the clutch is engaged.
Now with variable displacement compressors, the
clutch does not cycle nearly as much. The angle of
the swash plate, however, does change to modify
displacement via piston stroke variations, which
are dependent on the Displacement Control Valve
(DCV) in the back of the compressor. See Figures 5
& 6. ■

Dave Hobbs

Figure 5: The Pneumatic DCV (Displacement Control Valve) controls
pressure internal to the compressor to move the swash plate into the position needed for high performance A/C without evaporator core freeze up.

Figure 6: Cutaway showing DCV installed in a compressor.

UNDERSTANDING AND DIAGNOSING THE DCV
The Displacement Control Valve is still misunderstood by most techs, even though it has been out
there controlling variable displacement compressors for many years. There are two types of DCVs:
internally controlled and externally (electronically) controlled. The compressors that these control valves are designed for are very similar in operation. Internally controlled variable displacement
compressors (iVDC) were the first type to enter the
market and are still in use today. The second type
is the electrically controlled variable displacement
compressor (eVDC). The basic theory of using pressures on the swash plate (sometimes referred to as
the wobble plate) to control the angle it maintains
as it creates a platform for the pistons to move back
and forth on is the same with both designs. The key
to controlling displacement is through the regulation of compressor internal case pressure.

August 2015

Low Internal Compressor Case Pressure =
High Displacement
If the valve moves to decrease the case pressure,
you'll see higher compressor output (displacement).
The normal start up position of the valve should
go to low internal case pressure / high compressor
output (displacement) in a short time. This means
the high pressure gauge goes up after compressor
start up and the low pressure gauge goes down as
compared to their former equalized readings with
the engine off. In other words, on a 75°F day you
should see readings of about 75 PSI on both the
high and low side with the engine off. Start the engine (and run the compressor) and you should see
a pressure differential; high side going higher and
low side going lower. How much depends on the
heat load and amount of charge in the system, along
with other variables such as condenser airflow.

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015