MACS Service Reports - 2016 - FEB2

end of the series of steps. And if this agrees with the percentages, the technician figures it's his best shot.
If a problem isn't common enough to have been logged,
or if the issue seems so specific that a technician can't really
tie it to any of the items on the fault list, that's when the technician needs an approach that narrows down the problem
area. For example, if the compressor clutch doesn't engage,
the technician begins with a check for A/C pressures, and
then perhaps follows up with various levels of hotwiring.

Although P0530 is a powertrain code, it does not turn on the
check engine light (MIL) because A/C operation is not involved
in emission control, even though system features are part of
greenhouse gas regulations.
Obviously, with a full refrigerant charge, the refrigerant pressure sensor is immediately suspect, but a circuit or systemic issue also may be involved. However, as we often see, replacing
that sensor was the first thing the shop did. And although it may
be # 1 on some pattern failure list, we can't say we were surprised when it did nothing.
The GM computer system does account for intermittents; to a
degree. The P0530 code goes into a history file if the PCM does
not detect the failure the next time the system is turned on. And
history codes clear after 40 ignition cycles without the failure.
Like active codes, they also can be cleared with a scan tool.

Happy days = a trouble code! (Or maybe not so happy)
If the problem that won't easily be narrowed down requires a
way to sharpen the focus, perhaps a diagnostic trouble code has
the opposite effect: "Hey, I know where to start." Except that in
a number of cases, the technician really doesn't. Here's a good
example: The A/C clutch doesn't engage on a 2004 Chevy Trailblazer with the 4.2-liter six cylinder, and the technician has connected his scan tool and found a P0530 code. That's a code that
occurs frequently, triggered by the refrigerant pressure sensor
circuit, which makes it a useful topic for discussion. The system
has a known full charge of refrigerant, so this possible cause is
set aside.
This still should be a piece of cake from here, right? Or at least
there's a road map to solving the problem? How good that road
map is depends on the particular carmaker's service information system development, including the skill of whoever developed the diagnostic routine, with some much better or worse
than others. Not every road map anticipates all the possible
problems, although the typical routine should get you into the
specific area for the one you're encountering. However, as we're
sure you're thinking, many of these road maps have at least one
of the following issues:
1. The OE sequence, as we noted at the start, may seem
to be too exhaustive, by which we mean it takes every possible step so it doesn't miss a single millimeter of the circuit.
That can make it not just time-consuming, but effectively
impossible to execute because many of the connections and
splices are nearly impossible to access without expending
enormous time and effort.
2. It includes "replace with known good part" at some
point(s) in the sequence, which may work for a dealer who
can pull one off the shelf, but not for the independent technician.
3. It is based on use of the factory scan tool. This may be
where sequential diagnosis is headed, and something you
can do in a few years when everything is based on PC applications you can download and use of short-term subscriptions you will have to buy. Then combine with an SAE
J2534 Pass-Thru device you'll have to acquire to plug into
the OBD-II connector.
The optimal approach will vary, and with a trouble code we
recommend that you start by finding out how the on board diagnostics log the code. In the case of P0530, the Trailblazer OE
service information lists the following:
1. A/C request is made.
2. The A/C refrigerant pressure sensor transmits a signal
to the Powertrain Control Module that is under 0.1 volt or
more than 4.9 volts, for at least five seconds.

February 2016

Starting the diagnosis

Paul Weissler

Our preference is not to throw parts at a problem, as we've
made clear in MACS Service Reports over the years. In this case
the technician did check pressures (they were normal) and he
hot-wired the clutch circuit. The clutch engaged and the system
cooled. He also had bypassed the ambient/low pressure switch
to no effect, which was no surprise based on the trouble code
and the effect of jumpering across the relay to get the clutch to
engage.
His scan tool delivers an A/C command to turn on the compressor, but the pressure sensor status reading still says it's off.
He checks grounds and power feeds, all of which are good.
Then he gets a used HVAC module (trip to the wrecking yard)
and plugs it in; still no fix. He tries clearing the code and the system works, but in a day the car is back with the same problem.
Obviously the short term operation indicates the issue is intermittent.
At this point we'd say the shop likely has invested enough
time in these activities to have gone through the OE test sequence, but instead he finally took a step that, certainly with
the intermittent operation clue, we'd have approved right at the
start: Although it's a GM vehicle, he performed the Ford longrecommended wiggle test (actually, as you'll read later, a wiggle
test is also part of some GM-recommended procedures). He
started by clearing the code and had the system working again,

Figure 2: Wiggle test is more than just a wiggle. You should test along the
harness and tug on the wiring to try to determine if there's an internal wiring break.

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