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Approach intermittent faults with logic, diagnostic curiosity

Tuesday, December 27, 2016 - 08:00
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This article could be aptly named “Compounded Human Error, part 2” (see”, August 2014). In this case though, I was one of the people that erred while diagnosing an intermittent charging system warning indicator on a 2009 Chevy Cobalt LT.  Yes, as hard as it is to believe, I make mistakes (ahem). I am indeed human after all!

What’s typical in a repair shop is not necessarily what a mobile technician sees. In fact, don’t you “stationary” techs and shop owners handle the typical situations? I’m usually not called to help until the shop’s exhausted most — if not all — of the ways to resolve their customer’s vehicle complaints. In some cases though, a shop will call me in because they are so busy with “gravy” jobs — and they also have a job that can potentially tie up a technician’s time in ways that aren’t as productive. This car’s problem happened “every so often” and the shop was very busy already, so I got the call.

2009 Chevy Cobalt LT; customer complaint: battery indicator illuminates intermittently

The nature of intermittents

The very nature of an intermittent fault implies that a considerable amount of time may be spent waiting for — or trying to recreate — the conditions in which the fault is present (and can be diagnosed). To an inexperienced or an impatient technician, they can bring a type of anxiety that may cause that person to jump to conclusions or to misdiagnose. I know this from personal experience. I don’t get scared anymore when the word “intermittent” is included in the problem description. I approach these situations as logically as possible.

I can’t recite an exact number, but I can say a majority of intermittent faults I’ve dealt with were related to a previous repair. If the customer is in your shop for the first time, do you ask questions about what may have been done previously? Do you strive to have repeat customers? Isn’t it great when you can look up the repair history on a vehicle when that customer returns for more work? Don’t you just love when you can verify which services and previous repairs had been done using your service history? Believe it or not, this is one of the most powerful tools a shop has when it comes to diagnosing problems.

This car had no history. That is, it had been sold at an auto auction a few months prior to being sold again by the used car dealer who called me in to look at the charging system problem it had.  The Carfax the dealer provided the customer was spotless. The dealer’s technician notes about the pre-delivery inspection performed prior to the most recent sale were minimal. Basically, I was without one diagnostic tool already.

When the repair/maintenance history isn’t available, I try to put all my senses to work. In addition to all the other diagnostic tools I may need, I have to be very observant. I have to listen like I’m in a quiet room, smell like I’m near a bakery, see things (not just look at them), etc., etc. I must be The Car Whisperer (also my company name, for those of you not in the know)!

AC pressure sensor location

In addition to getting what little history was available, I asked all the questions I could think of about the problem before approaching the vehicle. The tech had verified the alternator continued to charge when the “battery” indicator was illuminated. The battery had passed tests — both a conductance test and a load test. It was unknown whether the indicator would illuminate only after the engine warmed up or if it was lit when first starting as well. I found out it didn’t matter if the car was moving or not and it sometimes happened on turns, other times not. The tech did little more than this because he was eager to tackle the many jobs the shop had taken in. I was on my own from this point going forward.

Where would you begin?

You now have the information I had. Where would you begin your diagnosis? Do you approach all your diagnostic dilemmas with a logical, systematic strategy? I believe we must use a logical approach to properly perform diagnostics. My first step was to make sure I understood exactly how and why the car battery indicator would be illuminated.

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