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Your cars ground wire, also known as a "ground cable" or "ground strap," is perhaps the single most important wire in its entire electrical system. Think of the ground wire or cable as your electrical systems foundation, the bridge over which all electrical flow must tread. A bad ground connection will ruin your electrical systems day, which makes establishing a good one an extremely high-priority project.
No-Start Condition
This is one of the more obvious signs of a bad ground, manifesting in much the same way as a loose battery cable or a dead battery. When you attempt to start your car, you may hear a single click or a rapid tapping; this is the sound of the starters solenoid opening or closing, or the sound of the starters Bendix drive moving. The solenoid requires a certain amount of voltage to operate; if the ground is bad, then the solenoid will function, but the starter motor will absorb all of the current flow and shut the solenoid off.
Dim or Flickering Lights
You headlights will do the same thing as the starter, but will likely dim instead of dying outright. A constant bad ground -- resulting from a frayed or damaged cable -- will create resistance in the circuit, which will deprive the headlights of power and cause them to dim. This may or may not be the case with xenon arc HID headlights, where a drop in input voltage may fail to initiate the lighting arc altogether. A cable that is simply loose may cause the lights to flicker as the circuit gains and loses the ground.
Dead Battery
A battery that refuses to take a charge is one sign of a bad ground. The ground is a major part of the batterys charging system, so assuming that youre getting proper voltage output from the alternator wire, and the battery isnt hashed, then you may be looking at a bad ground wire. If the ground wire is loose, then the alternator wont deliver its full power to the battery, particularly at idle.
Testing the Ground
The simplest way to check for a bad ground is to run a continuity test between the battery and the chassis. Disconnect the negative battery cable and connect the probe ends of a digital multimeter -- set to read volts DC -- to the negative and positive battery terminals. Record the reading; you should get something in the neighborhood of 12.6 volts. Next, remove the DMM lead from the positive battery terminal and touch it to the terminal on the disconnected negative battery cable. Your DMM should read within about 0.5 volts of your battery with the key in the "Off" position. If you get a voltage reading of anything below 11.5 volts, start looking for a bad ground.
