MACS Service Reports - 2015 - AUG4

High Internal Compressor Case Pressure =
Low Displacement

on a warm day is mediocre at best. A lower than
specified charge (slight leak or undercharge) would
be a very likely culprit. So could that funny contaminate called air. Air can be pumped, pressures
can change some and of course it is not a good refrigerant so it won't cool well at all. Always connect your identifier first to check for air or other
contaminants. The key is ALWAYS check the basics
first before jumping into the exotic / high tech side
of the vehicle. That said, leveraging the accuracy of
an SAE standard J2788 RRR machine, you can confidently recover and measure the system charge in
the vehicle. If the system charge was pure and fully
charged to capacity, a recharge won't fix it. If you
didn't have signs of a restriction or any other classic problem, the variable displacement compressor
/ control valve could then be considered. ■

If the valve moves to increase the internal case
pressure, you'll see lower compressor displacement. This means when the engine is running and
compressor is engaged, you'll see little or no difference between high side and low side pressures because the compressor isn't pumping at all or much.

Full Charge / No Contaminants - Back to the Basics!
Another possibility is the swash plate or control
valve might not be stuck in the LOWEST displacement condition. A not so uncommon condition
would be for the pressures on high and low sides
to be a little bit different from each other, but not
as much as expected while the cooling performance

Stuck Variable Displacement Compressor Components Can
Result In One of Two Complaints
1. No A/C cooling performance after a period of great A/C. This may be from a control valve
or swash plate that's stuck in a high compressor output condition. This can cause evaporator
freeze up, or pressure switch / evaporator temp switch to turn off the clutch.
Diagnostic Strategy - Connect gauges, record and clear DTCs (in any module, and if PCM
related, store freeze frame for DTC too) then jumper compressor clutch if it won't run / continue
to run. If the compressor's control valve is stuck in the full output mode, you'll see the high side
go up to normal readings (the compressor can indeed pump) and the low side go into lower and
lower pressures that make the evaporator freeze up under certain conditions.
2. No A/C cooling performance at initial startup or after running for several minutes. This
may be from a compressor control valve / swash plate stuck in a position that allows the swash
plate to move to a nearly horizontal position (little or no stroke) and therefore, no cooling.
Diagnostic Strategy - Connect gauges, record and clear DTCs (in any module, and if PCM
related, store freeze frame for DTC too) then jumper compressor clutch if it won't run / continue
to run. If the compressor's internally controlled control valve is stuck in the high internal case
pressure / low displacement mode, you may notice an initial gauge pressure differential movement (high side goes higher and low side goes lower) until the A/C pressure internal to the case
can build and send the swash plate back to the low displacement angle. Then the low and high
pressure gauge readings should go closer to equal. On the other hand, if the high and low side
pressure gauges don't even try to initially part ways upon compressor start up, the DCV may be
fine, but the compressor swash plate is stuck in the low displacement position. Of course, the
compressor could just be worn out and can't pump. Keep in mind that the swash plate on internally controlled, DCV equipped compressors doesn't go to a perfect zero displacement. You'll
likely see a slight pressure difference between high and low side gauge readings, whether the
swash plate or valve is stuck.

August 2015

MACS Service Reports

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