Diagnostic Dilemmas: 8 Simple Rules … – UnderhoodService

Diagnostic Dilemmas: 8 Simple Rules …

Many of the "Diagnostic Dilemmas" Gary Goms is called upon to solve aren't really all that complicated - but were caused by basic oversights and incorrect assumptions on the part of the technician. This situation has largely been caused by a lack of training and, in some cases, poor training.

Since I began doing part-time mobile diagnostic work for local shops, I’ve noticed that most “Diagnostic Dilemmas” I’m called upon to solve aren’t really all that complicated.

Most of these Diagnostic Dilemmas were caused by basic oversights and incorrect assumptions on the part of the technician. This situation has largely been caused by a lack of training and, in some cases, poor training.

For example, when on-board diagnostics were popularly introduced in the early 1980s, many early trainers had to fill voids in OEM information by gathering information from anecdotal sources. But, in hindsight, much of the information provided 20 years ago has been proven incorrect.

Unfortunately, the incorrect information and incorrect assumptions developed 30 years ago is still part of our diagnostic dialogue. In most cases, they fall into one of eight categories of common diagnostic mistakes.

photo 1: to solve for amps, cover the ampere portion with your finger and divide voltage by resistance. a circuit operating at 14.2 volts against a resistance of 2 ohms will, for example, carry 7.1 amperes of current.1. Skipping the Customer Interview
The most common mistake of service writers and technicians is assuming that any problem can be diagnosed by simply retrieving a trouble code. For that reason, many service writers fail to determine the actual customer complaint or gather the background information needed to quickly and efficiently address the problem.

To illustrate, last spring I was called to solve a cold-stalling problem on a 2005 Jeep Wrangler. The PCM had stored at least one throttle sensor-related DTC and, following a service bulletin on that particular failure, the shop had removed the driver’s air bag to test the cruise control circuits.

I won’t explore this issue since it became apparent that it wasn’t directly related to the stalling complaint. But it did momentarily divert my attention away from the immediate problem of the engine cold-stalling until it had warmed up for about one minute. After a short warm-up, the engine started and idled perfectly for the rest of the day. My first impression was a fuel quality problem, but I set that theory aside for the moment because fuel quality hasn’t been a problem in my community during the past eight years.

photo 2: assuming the resistance (ohms) remains the same, volts and amperes are inseparable components of any 
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