Posted to Technical Tips Forum on 5/19/2012
I just wanted to share this story to allow others to see
the importance of thorough diagnosis of electrical problems
and to point out that sometimes the problem will be obvious
if you simply take the time to look.
This 2007 Pontiac Grand Prix was on its third trip to the
repair shop I work in, with the same problem that it came in
for originally. The customer complaint was that the A/C did
not work sometimes.
On the first trip, the tech simply operated the A/C for
about 1/2 hour and wrote "No Problem Found" on the repair
order after scanning and finding "No Codes Present" in the
HVAC control module.
On the second trip, a second tech decided that it would be
best to scan ALL modules, NOT just the HVAC module. This was
an excellent idea, because what was found was a code "P0530
A/C Refrigerant Pressure Sensor "A" Circuit" which was
located in the PCM. The HVAC module STILL said "No Codes
Present" Exactly why that is...I do not know yet, but this
proves what I have learned in diagnostic classes where I
have been told that any diagnosis should begin by scanning
ALL available modules.
The problem is that even though the second tech began
correctly, he still did not complete the diagnosis. When he
saw what code he had, he simply replaced the A/C high
pressure switch and ran the vehicle out the door.
The third trip is where I got the vehicle. I scanned all
modules and found that the same code was present that was
found on the second trip. At this time, I did not even know
that the vehicle had been there before or that any parts had
been replaced. I was simply told "FIND the problem, whatever
it takes..." I knew something was up, but I was not sure
So, anyway, the first thing I did after seeing the code was
to look at the A/C high pressure sensor PID. I am looking at
the thing with Key-On-Engine-Off and the sensor voltage is
totally erratic. See the screen shot [2007 Pontiac Grand
Prix GT, ECM/Inputs/Outputs Scan Data]
I could wiggle the wiring harness where it runs from the PCM
and loops around by the cooling fan then goes towards the
firewall near the transmission side pan and make the voltage
remain steady or make it drop to zero and stay there. So I
removed the air intake snorkel and the air cleaner assembly
to get a better look at the harness. The FIRST thing I
noticed was that the harness was rubbing against the high
pressure A/C line and the pressure sensor itself. When I
pulled the harness back, the problem was obvious. There were
holes rubbed through the harness conduit!
So I disassembled the conduit, repaired 4 wires that had
been damaged (one was the VREF for the high pressure sensor)
and I fabricated a bracket from a piece that was laying
around the shop. The bracket was fastened to the stud-bolt
on the side of the transmission housing and the harness was
zip-tied to the bracket to prevent future problems of the
same nature. [2007 Pontiac Grand Prix GT,
What blows my mind about this whole thing is that the first
tech failed to scan all the modules and simply blew the
customer off, and the second tech was RIGHT THERE and never
LOOKED at the harness! He just threw a part at the code
description! (which happens WAY too often)!
With times as tough as they are, customers cannot afford
several trips to the shop - especially if it can be
prevented by proper diagnosis the first time.
David from Missouri
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