MACS Monthly Newsletter - 2021 - SEP6

Figure 13: 2018 F-150 REDUCED STROKE control valve
wave form.
Figure 12: 2018 F-150 MAX STROKE control valve wave
form.
very top lines represent battery voltage supplied by Fuse #
36. The bottom of the wave form represents the solenoid " on
time " , " duty cycle " or " pulse width " . In this example, the control
valve is spending almost all its time in the " ON " position because
of the amount of time the ground is applied to the solenoid.
The solenoid completely defeats the spool valve spring
and the spool slides to open the passage from compressor
crankcase to suction port. High pressure in the compressor
is evacuated and the compressor inlet pressure can push the
pistons deep into their bores on the intake stroke, generating
maximum stroke and output.
We already learned that the compressor defaults to minimum
stroke without a control circuit. Figure 12 represents
maximum stroke and maximum solenoid " on time " .
Figure 12, a " known good " waveform for this vehicle, represents
MAXIMUM HEAT LOAD and maximum compressor
stroke. With the vehicle fully warmed up, hood open and the
controls configured to apply maximum heat load to the evaporator,
allow the system to stabilize. You should see a similar
waveform on your scope. So far so good.
Next, configure the controls to drive the system to minimum
stroke. As before, 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 PCM reduces the amount of time the control circuit
for the solenoid spends at ground. See Figure 13.
The spring in the spool valve is now allowed to slightly
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.
Figure 13, a " known good " waveform, represents the control
valve being commanded to a reduced output and stroke.
Specialty tools are available to control the compressor
control valve and exercise the compressor through its range
September 2021
6
of operation. These tools have been covered in the February
2019 MACS Service Report. They are difficult to find in some
markets, like here in Canada, and cost a few bucks. The testing
methods described using a lab scope will provide the
same end results as well as diagnosing the control circuits.
Understanding the operation and control of the electronic
control valve allows diagnostics using the system itself.
Type 1 Diagnostic Results:
* IF supply voltage to the control valve is missing or not
at specification, follow power distribution diagrams to
determine the reason why voltage is not up to specification
at the solenoid. Use the power distribution diagrams
to locate other items that also may not have an adequate
power supply. This can aid in locating high resistance or
open circuits in the supply line.
* IF the solenoid resistance is out of specification, it
should be replaced. 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.
* IF voltage and control circuits and signals check out,
but stroke does not change, the control valve is stuck and
requires replacement.
* On this 2018 F-150, with the control valve disconnected,
the control wire will show the pattern shown in Figure 14.
This represents the transistor operating without carrying
current through the solenoid. For this Ford product, if you
have the connector plugged in and you see this signal on
Figure 14: 2018 F-150 PCM control circuit with the control
valve unplugged.
MACS Service Reports
Mike Bailey
Mike Bailey
Mike Bailey
MACS Monthly Newsletter - 2021

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