MACS Monthly Newsletter - 2021 - SEP8

* IF control circuit signal and ground circuits check out,
but stroke does not change, the control valve is stuck and
requires replacement.
* 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 Test Using an Injector Pulse Tool
As mentioned, a variable displacement solenoid test tool
may be used to move the compressor all through its stroke
range.
If a manufacturer provides bi-directional control for the
control valve through a scan tool, exercise the valve with your
scan tool and observe the readings while setting the vehicle
heat loads to correspond with the compressor output you
have chosen with your scan tool. Check both extremes and
all through the compressor range provided by your scan tool.
As a last resort, you can get an idea about the control valve
using a fuel injector pulse tester.
I have used this method successfully to determine if a conThe
reaction is very subtle but gives a good indication as
to whether the control valve can control the stroke. Remember,
if the compressor is stuck in minimum stroke, the control
head should be attempting to command the control valve to
maximum stroke when the vehicle is configured for MAXIMUM
HEAT LOAD.
Observe the low side gauge and let the readings stabilize
as shown in Figure 17.
Press the test button and watch the reaction on the low
side gauge. Repeat a few times to confirm your results.
The example shown in Figure 17 vs Figure 18 shows a drop
of 10-12 psi. Observing this pressure drop suggests that the
control valve should be able to control the compressor. It also
shows that the compressor should respond.
Proceed to diagnosing the control valve signal.
Figure 18: Pressure drops from 82 psi down to 78 psi when
activating the injector pulse tool. The valve works!
Compressor stuck in maximum stroke
A compressor stuck in minimum stroke is one thing. A
Figure 17: Low side pressure with control valve disconnected.
Pressure at 82 psi before using an injector pulse tool to
exercise the control valve.
trol valve is stuck in minimum stroke. High and low gauge
readings will be very close to each other as the compressor is
in minimum stroke. This is usually an early step to help guide
the diagnostic routine and not miss a control valve that is also
faulty.
Disconnect the control valve and use the necessary jumper
wires to connect the injector tester to the solenoid. Connect
the leads for the injector tester to the battery. Fire the injector
tester. If the valve can move freely, you should hear it click
as the tester cycles the signal. Be suspicious of a valve that
doesn't make any noise.
Start the vehicle.
MACS Service Reports is published monthly by the Mobile Air Climate
Systems Association. It is distributed to members of MACS and
is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will
be considered for publication, however, MACS reserves the right to
choose and edit all submissions.
Unless otherwise noted, all photos/art by author.
September 2021
8
compressor stuck in MAXIMUM STROKE is an entirely different
thing.
A compressor stuck in maximum stroke will not be able to
reduce the amount of refrigerant in the evaporator. Stuck in
maximum stroke, the variable displacement compressor will
mimic a defective cycling clutch system that can not turn the
compressor off to warm the evaporator.
Therefore, running a full performance test is important. If
you only test maximum output, when the vehicle stabilizes on
a humid day, the evaporator can freeze, turning into a block
of ice. The customer complaints will be a lack of cooling and
a lack of airflow that gets progressively worse. If the vehicle
is parked for a while, the ice melts and the cycle will repeat
when the vehicle is driven again.
This is becoming more common as variable displacement
compressors become more popular. 
Editors:
Production Designer:
Elvis Hoff pauir, Steve Schaeber
Laina Forcey
Manager of Service Training: Steve Schaeber
Mobile Air Climate Systems Association
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsmobileairclimate.org
Website: www.macsmobileairclimate.org
MACS Service Reports
Mike Bailey
Mike Bailey
http://www.macsmobileairclimate.org
MACS Monthly Newsletter - 2021

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