MACS Monthly Newsletter - 2021 - SEP7

the control wire during testing, the solenoid winding has
open-circuited.
* IF the control (ground) circuit is not functioning, check
circuit continuity from the valve connector to the controlling
module. Then, load test the circuit using a load.
* SOME manufacturers may provide bi-directional control
in the HVAC module allowing control of the stroke via
your scan tool. If present, this can help to determine if the
compressor can be controlled and will respond to said
controls.
Control Valve Circuit Electronic Diagnostics
* Type #2
Figure 15 shows the compressor control circuit for the
compressor control valve on a 2019 Subaru Impreza. Many
the solenoid is then sent from the HVAC Control Head to the
control valve.
CAUTION: applying straight power and ground to the
valve can burn out the solenoid winding. DO NOT test by applying
continuous 12v and ground.
Hint: As with the Ford example above, the control valve
can become " stuck " if a piece of debris enters the sliding
spool valve. See the April 2021 MACS Service Report for a
picture of the ports and filter incorporated into its design. A
quick test for a stuck valve is to use a fuel injector pulse tester.
These will safely apply power and ground to the solenoid.
When activated, the solenoid will make a " ticking " noise. If
stuck by debris, the solenoid will remain silent when activated.
On
this Subaru, the HVAC Control Head will be applying
the pulse-width modulated signal to the POSITIVE terminal
of the solenoid. This is exactly the reverse of the previous example.
Figure 16 shows the compressor being commanded
by the HVAC Control Head.
Figure 16 is a " known good " waveform for this Subaru and
represents MAXIMUM HEAT LOAD and maximum compressor
stroke. With the vehicle fully warmed up and running,
hood open and the controls configured to apply maximum
heat load to the evaporator, allow the system to stabilize. Your
waveform should resemble the one shown in Figure 16.
Next, configure the controls to drive the system to miniFigure
15: 2019 Subaru Impreza control valve wiring diagram.
import
vehicles use this circuit design to control many diff erent
solenoids such as fuel injectors, vent valves, purge valves,
and transmission solenoids.
First step, measure the control valve resistance. This Subaru
control valve should measure around 13Ω.
The black wire coming from the control valve attaches to
chassis ground. Confirm the circuit is properly grounded.
On this model, the Blue/Orange wire attached to the valve
is the control signal wire. Based on input from the Evaporator
Temperature Sensor, In Vehicle Temperature Sensor, Sun
Load Sensor, High Side Pressure sensor, Ambient Temperature
Sensor and others, the HVAC Control Head calculates
the desired compressor stroke. The desired pulse width for
mum stroke. Always perform the maximum stroke test to cool
the cabin down in preparation for the minimum stroke test.
Allow the system to stabilize and inspect the waveform
on your scope. The compressor is being commanded by the
PCM toward its " FAILSAFE " or REDUCED STROKE. It will
not be commanded all the way to minimum stroke, only to
the stroke required to provide cooling and maintain evaporator
temperature just above the freezing mark. To accomplish
this, the HVAC Module reduces the amount of time the control
circuit for the solenoid spends powered up.
The spring in the spool valve is now allowed to slightly
Figure 16: 2019 Subaru Impreza control signal wave form
maximum stroke.
September 2021
7
move the valve, exposing the discharge port to the crankcase
which pressurizes the chamber behind the pistons, forcing
the piston intake stroke to reduce, decreasing refrigerant output
to maintain evaporator temperature just above freezing.
It should be apparent now that the control head signal will
spend less time at 12v when a reduced stroke is being commanded.
Type
2 Diagnostic Results:
* IF the modulated signal is NOT available to the control
valve, the compressor will remain in minimum stroke by
default. Verify the ground to cover your bases and then
check the circuit from the control valve plug into the cab
at the HVAC module. Do not just rely on a DVOM and
resistance measurement. Load the circuit and measure
voltage drops as per the Ford example.
* IF current is out of spec (due to resistance issues) a
module may shut down control circuits for the solenoid to
protect itself.
* IF the solenoid does not " click " when activated using
an appropriate tester, the control valve is likely stuck and
requires replacement.
MACS Service Reports
Mike Bailey
Mitchell Pro Demand
MACS Monthly Newsletter - 2021

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