Checking the ground connection

GENERAL INFORMATION

The battery negative terminal is connected to vehicle ground – the engine/transmission metal and the vehicle body – and most electrical systems are designed so that only one supply wire goes to each electrical component, and the current returns through the vehicle body metal.

This means that the mounting of an electrical component and the vehicle body are part of the electrical circuit.

Therefore, a loose or corroded connection can cause a large number of faults in the electrical system, ranging from complete failure of the circuit to unreliable operation.

In particular, bulbs may appear dim (especially if another circuit using the same earth is switched on), motors (for example, wiper motors or radiator cooling fan) may run slowly, and switching on one circuit may affect the operation of another circuit.

Note that many vehicles use earth straps between various components of the vehicle, for example between the engine/transmission and the body, usually where there is no metallic contact between the components due to the use of rubber mountings, etc.

PROCEDURE

1. Disconnect the battery and connect one probe of an ohmmeter to vehicle earth (ground).

2. Connect the second probe to the wire or earth point to be checked. The resistance shown on the ohmmeter should be zero; if not, check the connection as follows.

3. If you suspect the connection is faulty, disassemble the connection and clean the contact surface and wire terminal, or the surface of the earthed component, down to bare metal.

4. Completely remove dirt and corrosion, then use a knife to remove the paint layer so that a reliable metal-to-metal contact is obtained.

5. On installation, secure the connection firmly; when fitting the wire terminal, use serrated washers between the terminal and the body.

6. After connecting, to prevent corrosion, apply a layer of petroleum jelly or silicone grease to the connection, or treat it with a sealant such as Holts Damp Start or a water-repellent lubricant such as Holts Wet Start.