Checking the bulb

Remove the bulb and inspect it visually. If the filament is burnt through or the glass envelope is loose in the base, replace the bulb.

To determine conclusively whether the bulb is intact, proceed as follows: connect a positive (+) wire and a negative (-) earth wire directly to the battery terminals and connect them to the bulb. It does not matter how the wires are connected to the bulb.

Connect one wire to the power terminal and the other to the bulb housing. If the bulb does not light, replace the bulb. Note: make sure the contacts on the bulb and in the bulb holder are not corroded. If necessary, sand off corroded or bent terminals and make proper contact.

If the bulb is intact, install the bulb and switch it on.

If the bulb does not light, check the power supply with a test lamp. To do this, connect the test lamp to earth. This means: one lead of the test lamp must be connected to a good earth point on the engine (bare metal) or directly to the negative battery terminal. Hold the other test lamp probe (+) either against the live connector or pierce the live wire with the probe. If the test lamp now lights up and the bulb still does not light, the earth supply to the bulb is interrupted. To check this, connect a temporary earth lead to the bulb holder.

The bulb must now light.

Note: some bulbs have only one live supply wire, for example interior vehicle lighting.

These bulbs are connected directly to vehicle earth through their housing.

If the live wire to the bulb carries no voltage, so the test lamp does not light, the switch is very likely defective. Check the switch for continuity.