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May 23, 2017 01:00 AM

Diagnostic solutions usually rooted in basics

Dan Marinucci
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    Marinucci

    The tougher an automotive diagnosis seems to be, the more likely its solution is to be something basic.

    Unfortunately, these challenges easily sway some service personnel toward the other extreme — the unusual, weird possibilities.

    In my last column, I urged tire dealers and service shop operators to coach all service personnel to pursue automotive basics. Perform those basic but essential tests before leaping to conclusions about a vehicle's problems.

    Some technicians rely too heavily on gut instincts and a selective memory of what fixed previous repair jobs.

    Often, it seems, the gut feeling is that tough symptoms equate to tough solutions rather than basic ones. Time and again, situations I've witnessed in the service bays support this conclusion.

    I'll cite a classic example: A loose ground cable that should have been identified within the first hour of diagnosis.

    A 2000 Honda Civic came into a large, independent service shop. An old pal of mine is the shop foreman.

    Oodles of these Civics are in service. Overall, the marque has earned a solid reputation for reliability and simplicity. But this particular Civic suffered from a combination of strange symptoms.

    First, the charging system performed normally right after starting the engine.

    After five minutes or so, however, the charging voltage began dropping and continued dropping — as if the alternator was gradually failing. Replacing the alternator didn't fix the car.

    Second, the Civic's engine surged during light-throttle cruising conditions. Typically the surging occurred on level highway within the range of 40 to 45 mph.

    Both the service writer and technician who road-tested this Civic insisted that this surging sensation felt like a torque-converter clutch failure inside the automatic transmission. (Usually, torque-converter failure prompts transmission replacement.)

    Third, a winking instrument panel symbol suggested that transmission trouble codes were stored. However, no codes for either the transmission controls or engine controls were stored. (On this range of Civic, a flashing D4 on the instrument panel means that transmission-related trouble codes are present.)

    My buddy took over this Civic's diagnosis after it had frustrated his technicians' efforts. It didn't take long for him to suspect a faulty main engine ground.

    Our collective experience has been that a loose or broken ground connection may create myriad weird symptoms. He performed a very basic procedure: A cranking voltage drop test between the engine block and the negative battery terminal.

    The preferred measurement on a healthy ground system is approximately 0.5 volt or less.

    Typically this measurement on a healthy Honda is less than 0.25 volt.

    Instead of one-quarter to one-half volt, his measurement was several volts. Instinctively, he traced the car's main ground circuit.

    A bolt that was supposed to secure a ground cable to the engine was so loose it was about to fall out. Perhaps the last person to work on this car had forgotten to tighten this bolt.

    After my buddy tightened the bolt, the car's symptoms disappeared. Then he reinstalled the original alternator, and the car performed fine — and still runs fine to this day.

    Current, which is measured in amperes, is electrical volume. Any current that flows out of the vehicle's battery ultimately must return to the battery. The ground circuit is the path that carries the current back to the battery.

    Experience has taught us that when the normal ground "path" fails, the current tries to find an alternative route back to the battery.

    In this case of this Civic — and many others — the vehicle continues running but doesn't run well.

    What's more, a poor main ground brews a variety of odd symptoms.

    Once again, our collective experience has been to verify basics such as good grounding before leaping to other conclusions.

    Indeed, basics may sound boring, but they do matter, and they do fix cars every day.

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    Do you have an opinion about this story? Do you have some thoughts you'd like to share with our readers? Tire Business would love to hear from you. Email your letter to Editor Don Detore at [email protected].

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