Professional_Tool_And_Equipment_News_March_2020 - 32

p This image from an AutoEnginuity scan tool shows no temperature
from the disconnected evaporator temperature sensor.

p This image shows the Autoenginuity scan tool reading the
evaporator sensor temperature once it was plugged in.

We then checked the vehicle information on Mitchell 1 to see if there were
any relevant service bulletins or TSB's;
there were none.
Before we removed the evaporator,
we decided to research this particular
issue a little more since neither we nor
the owner had any of the vehicle's history. First of all, we noticed the A/C
control panel looked like it had been
replaced as the normal fading of the
temperature indicator was not present. Since we suspected this had been
replaced, we decided we should use our
scan tool to calibrate the system, which
resets all of the door actuators to their
normal positions. Once we did this, we
drove the vehicle again. To our surprise
the system seemed to function normally.
It seemed as though someone had
replaced the controller but had not taken
the time to recalibrate the system, which
caused the blend doors to malfunction
and block air from coming across the

PTEN

MARCH 2020

evaporator, allowing the evaporator to
freeze and block air flow.
We drove the vehicle on a longer road
test to make sure that it continued to
work well. It did until we lowered the
temperature a few degrees, then the
system started blowing warm air again.
We returned to the shop where we
connected the scan tool to observe the
blend door actuator positions. Each door
operated correctly and when commanded, opened and closed fully. All of the
pressure and temperature sensors were
within normal operating ranges.
This is the point where we got a little
creative, much like the doctor that diagnosed my wife. If you have an issue on a
vehicle that has a production date near
the beginning or end of a model year,
take a look at the system in the year
earlier or later to determine if there are
any variations that may be affecting the
vehicle. In the case of this GMC, the
production date was August of 2010,

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making it a 2010 model year. But there
was a discrepancy in the available sensor
information between a 2010 and a 2011.
Starting in 2011, GMC used a temperature sensor in the evaporator to monitor
the temperature of the core itself, which
then allowed the computer to command
the A/C compressor on and off in order
to maintain the proper pressures and
temperatures within the system.
When we scanned the vehicle using
the correct VIN 2010 model year, the
sensor read 0 degrees. When we connected the scan tool but changed the
VIN to read as a 2011, the sensor data
showed a negative number, meaning
the sensor was unplugged. Now, keep
in mind we had no previous repair
records on this vehicle, so we had no
way of knowing what issues had been
addressed before or if any modules had
been reprogrammed.
Since we found an open circuit in the
evaporator sensor, we found and re-connected the harness. Once the harness
was connected, we scanned the vehicle
and found normal readings as shown in
the image. An extensive road test was
performed while setting the climate
controls at various temperatures. There
were no additional problems.
This A/C system was able to cause an
issue by having a sensor disconnected,
but it did not set a fault code. What we
determined was due to the merging of
two model year technologies. The sensors were installed in the 2010 GMC, but
the logic that would cause a fault code to
set was not present in the BCM which
controls the HVAC system.
In the 2011 model year, GMC switched
over to an HVAC module to control the
system rather than the BCM. Since this
system was a blend of different technologies, it did not follow the normal diagnostic test procedures which should have
easily shown where the fault was. But, just
like the doctor that finally thought outside
of the box to diagnose my wife, moving
away from pre-conceived notions we had,
we were able to diagnose this A/C problem by thinking outside the box.

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Professional_Tool_And_Equipment_News_March_2020

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