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Diagnosing intermittents
Posted to Technical Tips Forum on 5/19/2012 41 Replies

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 what.

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, ECM/Inputs/Outputs Photo]

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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car Vehicle Data

2007 Pontiac Grand Prix GT 3.8L

Engine3.8 L
Trans4-speed Automatic (Electronic)