MACS Service Reports - 2015 - AUG7

EXTERNALLY CONTROLLED VARIABLE DISPLACEMENT COMPRESSORS (eVDC)
Clutch and Clutchless

We can cycle off the compressor by looking at
EOAT to prevent freeze up while also complimenting the amount of cooling the customer wants (Series Reheat Reduction). But why not vary the displacement electronically to do the same job instead
of cycling the clutch? That's what the eVDC's do.
They don't only change the compressor 's internal
pressure to vary the swash plate for displacement
/ evaporator pressure control to prevent freeze up.
Since they are electronically controlled with variable duty cycles, they can control evaporator temperature to compliment the HVAC temperature settings. This prevents the on/off clutch feel drivers
sometimes notice / complain of. In fact, the clutch
on some models with eVDC systems has been removed entirely. See Figure 7.

When checking for a no A/C complaint, remember these
electronically controlled variable displacement compressors are
made in both clutched and non-clutched models. I've only ran
across those still using the electromechanical clutch on GM, but
there may be more models still using a clutch. On the clutchless
models, consult your OEM repair information to determine the
construction of the breakaway feature required to prevent accessory drive belt breakage when a compressor locks up. The telltale signs this has occurred will vary by compressor manufacturer, and of course the pulley will still turn when this has occurred.

Electronically Controlled Variable Displacement
Compressor Diagnostics
Regarding scan tool PIDS, GM is one notable OEM using a
PID called Compressor Control Valve Duty Cycle. It's an easy to
understand PID, basically going hand in hand with compressor
displacement. The higher the duty cycle, the higher the displacement command to the compressor. You can run your HVAC
temp setting to the coldest and watch duty cycle go to 100%.
High side pressures should go up as well as you feel or measure
cooler duct temps and watch the low side pressure gauge drop
down. Other OEMs such as Toyota use a PID for the current in
the pressure control solenoids. Current draw on these compressors' electronic solenoid control valves can range from near zero
(little to no displacement) to around 0.65 amps which would be
the highest displacement (and lowest temperature set point). See
Figures 11 & 12.

An eVDC can be used to tailor the
compressor's output from high to low
for a multitude of conditions.

Dave Hobbs

1. Overly High Pressure > 464 psi
2. Non A/C Mode Selected
3. Low Evap Pressure < 29 PSI
4. Faulty Sensor Signal / DTC
5. Green / Econ Mode
6. Blower Not Running At All
7. OAT Very Low < 37°F
8. Engine Temperature Hot > 245°F
9. Pressure Sensor Fault
10. Evaporator Temperature Near Freezing
11. Low or High Vehicle Battery Voltage
12. Software Configuring / Programming Issues
13. Engine Just Started < 4 Seconds Ago
14. Engine Load High
15. RPM > 3000 for > 5 Sec
16. Vehicle Speed Very Low < 10mph
17. Very Low Blower Voltage < 3 Volts
18. Power Consumption of Compressor Too High
19. Temperature of EOAT and HVAC Head Setting Require Less A/C Cooling
20. Anti-Compressor Slugging Strategy

Figure 11: These scan tool PIDs were from a 2013 Malibu E-Assist
equipped with a belt driven eVDC. When the temp control knob was
moved to 60°F, the Desired Left Duct Temp PID dropped into the 40's
and the solenoid received a 100% duty cycle. The result was over 150 PSI.

Diagnostics: FRED Controls the Displacement!

August 2015

Dave Hobbs

You may recall this tech writer 's pet name for
electronic modules of any nature as Frustrating Ridiculous Electronic Devices, or F.R.E.D. Well now that
FRED controls the very physical internal displacement of the compressor, the typical plus and negative side of this changes. The negative side is the
pressures you are used to seeing in the bay just may
vary a bit all over the place as some of the conditions above occur to lower or even bottom out compressor displacement.

Figure 12: Toyotas like this Corolla will have a PID averaging 0.5 amps
when their belt driven (but clutchless) electronically controlled variable
displacement compressors are running at around 50% duty cycle for
middle of the road compressor output. Note the sensor PID descriptively
titled "Flow Sensor."

MACS Service Reports

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