MACS Monthly Newsletter - 2021 - SEP4

RECIRCULATE. The MAX heat load test cooled the cabin.
This test takes advantage of the cool cabin so we can observe
the transition.
* 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
HVAC module commanding the compressor to REDUCED
STROKE. A properly operating, fully charged system should
achieve the readings like those shown in Figure 5. Duct temperature
should be cold and what you would expect from a
properly functioning fixed displacement system, HOWEVER
THE COMPRESSOR DOES NOT CYCLE, and the duct temperature
should not fluctuate as is common with the cycling
clutch system.
Figure 6: No electronic control to the control valve.
Control Valve Diagnosis
Following along so far, we have addressed undercharge
and cycled the heat load on the system to test system reaction
and performance.
Scan tool diagnosis is helpful. Most electronic compressor
Figure 5: Minimum heat load, gauge readings in the bay.
* Maximum Heat Load.
* This should command the compressor back to MAXIMUM
STROKE.
* Return the system to a MAXIMUM heat load and watch
the results. Allow the system to stabilize. Pressure readings
should mirror those obtained during the first MAXIMUM
HEAT LOAD step.
*IF the system passes the performance tests, the refrigeration
system is functioning properly.
Solenoid Control System Failure
This is important. Consult service information to confirm
which module makes the decisions for the operation of the
compressor's control valve, and which module provides the
electronic signal directly to the control valve. Sometimes one
module will make the decision, transmit orders via CAN, and
another module will send the actual electrical signal to the
solenoid valve.
*In the case of complete signal failure to the control
valve, MOST variable displacement compressors will
FAILSAFE TO MINIMUM STROKE...5% output. See Figure
6.
If solenoid control completely fails, MINIMUM STROKE, at
first glance, will give pressure readings very similar to those
observed when a compressor clutch does not activate. High/
Low pressures will look very close to identical, and it is easy
to be fooled into thinking the compressor shaft has broken.
The fact that very little effort is required to turn the compresSeptember
2021
4
control valves will generate trouble codes. PIDs to observe (if
available) while testing electronic VDCs are not limited to just
these. See Figure 7.
sor driver by hand may reinforce a broken compressor shaft
diagnosis, however the solenoid control system should be
checked first. Manually activating the control valve can be
helpful to determine if the compressor is damaged. Keep
reading.
Figure 7: Scan tool data PIDs.
* Evaporator Temperature
* Ambient Temperature
* Humidity
* Duct Temperature
* Sun Load
* Discharge Pressure/Flow
* Module Supply Voltage
Note the source of the HVAC data PIDs in Figure 7. This denotes
the FCIM (front controls interface module) as the logic
module for the control valve. The FCIM makes the decisions,
then sends the commands to the PCM, which performs the
actual adjustment of the control valve.
Trouble codes will also aid in diagnostics. Figure 8 shows
the trouble codes stored in the PCM. Hard trouble codes
describing the failure of the control circuit for the PCM are
stored.
If trouble codes are not available, responses to the performance
test can be used to help determine the cause of the
problem.
MACS Service Reports
Mike Bailey
Mike Bailey
Mike Bailey
MACS Monthly Newsletter - 2021

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