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Ground wire

EFI Wiring Fundamentals

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I have a relatively basic question regarding vehicle ground. I understand potential difference is the difference in voltage between 2 points. When a circuit such as a headlight bulb is complete 12v power is applied and current flows from the battery to the bulb and then back through to the battery through the ground wire. So if there is current flowing through the ground wire how is there no voltage present? Perhaps it's not there is not voltage but, that there is just a difference in voltage between the power and ground?

Thank You

There will be a voltage drop at every link in circuit (sometimes this is very small). If you measured the voltage between the battery positive, and the positive lead on the bulb, you would find that the voltage drop was perhaps 0.1 - 0.3 volts. If you measured the voltage across the bulb you would find that it is less than the battery voltage (most most of it since that is the primary load in the circuit). Lastly if you measured the voltage from the negative lead on the bulb to the battery ground, you would again find a small voltage (0.1 - 0.3 volts).

Knowing this, you can diagnose bad connections. If there were corrosion on your ground connection to the chassis (or the car was repainted, and the paint was not removed at the ground connection), then you might find the voltage drop would be higher (say 1 volt). Finding the voltage drop where you don't expect it isolates the problem.

If there is a fair amount of current flowing through a wire, and you don't have sufficient wire diameter, there will be a larger voltage drop and the wire may actually heat up. Again finding a voltage drop where you don't expect it, says something is wrong...

Very good information thank you. So does current actually flow through the ground since a good ground will read 0 volts?

Yes it does. You can confirm this by putting a current clamp on the ground wire.

Awesome, thank you