The headlight switch controls most of the vehicle's lighting systems. Many complaints concerning the lighting systems can be the result of a faulty headlight switch. For example, dim or no instrument panel lights, dim or no headlights, dim or no parking lights, and improperly operating dome lights can all be caused by the headlight switch.
A headlight switch.
You'll need to remove the switch to test it. With the switch removed it can be tested for continuity and the connector plug will serve as a test point for the lighting circuits. First test at the connector.
In this procedure the terminals are identified as follows:
- Terminal B = battery
- Terminal A = fuse
- Terminal H = headlights
- Terminal R = rear park and side marker lights
- Terminal I = instrument panel lights
- Terminal D1 = dome feed light
- Terminal D2 = dome light
Consult the service manual for terminal identification for the vehicle you are working on. If this is not listed in a separate chart, you should be able to identify the circuits from the wiring diagram.
- Connect the 12-volt test light across terminal B and ground. The test light should light. If not, there is an open in the circuit back to the battery.
Testing for battery voltage to the connector. Reprinted with permission of Ford Motor Company.
- Connect the test light across terminal A and ground. The test light should come on. If not, repair the circuit back to the fuse panel.
- Connect a jumper wire between terminals B and H. The headlights should come on. If the headlights fail to turn on, trace the H circuit to the headlights. Also check the ground circuit side from the headlights.
- Connect a jumper wire between terminals A and R. The rear lamps should illuminate. If not, trace the circuit to the rear lights. Also check the ground return path.
- Connect a jumper wire between terminals A and I. The instrument panel lights should come on. If not, trace the circuit to the panel lights.
The chart below indicates the test results that should be obtained when testing the headlight switch for continuity. use an ohmmeter or a self-powered test light to test the switch.
Steering Column-Mounted Switches
On some vehicles it is possible to test the steering column-mounted switch without removing it. The test is conducted at the connector at the base of the column. However, on some vehicles it is necessary to remove the column cover and/or steering wheel to gain access.