MACS Service Reports - 2016 - APR2

Dave Hobbs

Dave Hobbs
Figure 1: The first step in ANY electronic system diagnostic approach
after verifying customer complaint is observation. Not only visual observations for loose or missing parts, aftermarket accessories and signs
of abuse or neglect, but also electronic observations. This begins with a
full vehicle, all module DTC check. If there are powertrain related DTCs,
observe and record (use your smart phone camera if necessary) information such as DTC Freeze Frames prior to clearing DTCs and advancing
your diagnostics. This vehicle has an ambient air temp sensor range /
performance DTC. This means it may not be shorted high or low. Ambient
temp sensors are being used for engine management as well as HVAC and
driver information systems on newer vehicles.

management 'dead zones' of > 4.8 and < 0.2 volts
that would definitely set a DTC. A software function called 'fail-soft' can produce an acceptable and
believable pressure PID of 3.6 volts, even though
the actual PID may have been 4.95 volts just prior
to a DTC setting. Remember step 1 is ALWAYS pull
DTCs FIRST. Note that the upper right hand corner
of the scan tool screen capture in Figure 2 shows
two DTCs present. High pressure can be due to air
in the system, too much oil, too much refrigerant, a
high side restriction, stuck wobble plate / displacement valve, engine / transmission temps that are
too high, or just a plain old lack of airflow across
the condenser. The latter could be caused by anything from a cooling fan issue to a mess of leaves on
the condenser to a lack of proper undercar air dam
components in place. In keeping with the big pic-

Dave Hobbs

and related relays / sensors) and under dash
(HVAC controls, blower motor, and blend/temp/
mode door control) systems. When diagnosing a
compressor that won't run, remember there's a lot
(thinking like a module) that can stop a compressor from running for a lot of good reasons. For
example, in Figure 2 notice the 3.6 volts on that
sensor PID (bottom number) isn't into the engine

Figure 3: More clues on why the compressor won't come on with a normal A/C
selection on the HVAC head; the PCM's disengagement history shows quite a
few times due to A/C pressure which appears to be moving to a fairly high level.

Figure 2: This GM pickup truck's A/C compressor clutch was not operational
when requested through the control head button but worked fine when jumped
at the clutch and when requested via this scan tool's bi-directional command.
What would you check next? Notice the "5 Volt Reference 1 Circuit Status".
It says "Fault" which would point to a sensor short / wiring problem with the
A/C high pressure sensor circuit. But what about the A/C High Side Pressure
Sensor PID itself?

April 2016

Figure 4: Road test / snapshot time. These PIDs on the Global OBD-II side
of any scan tool will not be as great in sheer PID volume, but any PID you
suspect of being a made up 'fail-soft' value (whether a DTC is present or not)
will be displayed as the real value in OBD-II Global. Notice the calculated load
of 100 percent along with the absolute TP being 85 percent. That just may be
enough to keep the compressor off for legitimate reasons.

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016