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The Flyin' Miata Techline

Troubleshooting ground problems.

Ground issues can be some of the toughest electrical problems to find. Nearly everybody Iíve seen trying to test ground connections in the field, plus plenty of texts that should know better, tell you the wrong way to test grounds.

Here is how NOT to do it. Do NOT use an ohmmeter from ground to your suspect ground. The little batteries in your ohmmeter donít flow enough current to do a valid test. Do NOT hook your test light to +12 volts and touch it to the suspect ground. This will indicate a total lack of ground, but will not indicate the quality of the connection.

The proper way is to use a voltmeter (or test light) with the suspect ground supplied with its normal power. Voltmeter should go from known good ground connection to the ground side of whatever youíre testing. An example on a Miata might be the ground strap that goes from the left side rear of the cylinder head to the chassis (on í90-í93) or to the sheet metal divider between trans bellhousing and engine block on a later model car. Connect voltmeter to chassis (must be free of paint, corrosion, etc) and to cylinder head. Watch the meter while you crank the starter. If the voltmeter reads anything, it means that it is easier for the voltage to go through your meter or test light than it is to go through its intended ground strap.

You can also test hot side losses the same way. An example would be looking for voltage losses in the starter circuit. You could put your test light or voltmeter from positive post of the battery to the positive post on the starter. Crank the engine, watching the light or meter. If you get any significant voltage, you can work your way back to the battery, eliminating/repairing connections one at a time.

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