That is a good question. When you are testing with an ohm meter, you are not testing under actual operating condition where an ammeter is (almost). A glow plug gets hot, if not perfect, internal connections can open up (like a thermostat) as it heats up and not glow anymore. This is rare but could happen, especially to ones where the center electrode has spun. Such a glow plug could test good per an ohmmeter but still be bad.
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Originally Posted by solostyle
Also how does the ampmeter/ammeter help? If you know the resistance going into the plug by doing the probe-in-the-relay-plug check, and if you know there's exactly battery voltage going into that plug by doing the probe-on-the-pin check, then isn't it clear that you're getting V/R amperes into the plug? How is this check any different?
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