How to Diagnose Electrical Ground Issues

Slide Show

Chasing down electrical gremlins in a motor vehicle can be an exercise in frustration—the kind of frustration that makes young men old and old men talk to themselves.

But many times, an electrical problem can be traced to a single source: a bad ground connection. A bad ground can cause noise in an audio system, make electric fuel pumps run hot or produce low pressure, and make electronic engine controls do weird things.

Many think that as long as the accessory’s ground wire is touching some part of the vehicle, it is grounded. That is not the case. You must make sure the ground wire is attached at a point that is free of paint, rust, or plating. Paint on body panels and the engine acts as an insulator, resulting in a bad ground connection. If you are grounding an accessory to the engine, it’s good practice to run a ground wire directly to the alternator case and make sure there is no paint between the starter and the engine block mounting surface.

If your accessory still doesn’t work properly after redoing the grounds, you will need a voltmeter or multimeter to trace the wiring. Set the voltmeter to read ohms (resistance) and probe the battery’s negative stud and ground connection on the accessory (the ground terminal on an amp, for example). If you have a reading less than 5 ohms, the ground is OK.

If the resistance is OK but the accessory still isn’t working right, set the voltmeter to DC current (voltage). Turn the accessory on and retrace the grounding path as you did before. The voltage should be no greater than .05 volts under load. If you find a point where voltage is present, then you need to add a bonding strap or find a new grounding point so no voltage is present at any of the grounding points.

If the reading is higher, you need to probe the grounding path between the accessory and the battery. Starting at the battery, run the voltmeter probe from the battery to the first grounding point, usually a fender on musclecars and trucks. Continue to where the fender attaches to the main body, and from there to the accessory. If you find a point of high resistance (over 5 ohms), you will need to attach a bonding strap or wire between the panels or parts where resistance is highest.

One of the best things you can do to help ensure a properly grounded vehicle is to replace or add an engine-to-chassis grounding strap; Taylor makes a nice 4-gauge, braided stainless steel strap ideal for most vehicles.

If you are adding a number of accessories or ones that draw a large amount of current, you should also replace the battery-to-chassis ground with a larger gauge wire. That's because the factory ground wire is usually a less-than-adequate 10 or 12 gauge. A ground wire must be as large as the positive, or supply, wire to the battery.

Hopefully you will never have to experience the joy of tracing a bad ground. But if you find yourself in such a situation, these tips will help make the job go smoother--and get your ride back on ground level.