How to Install a 3-Prong Rocker Switch in a Car

By Jan Benschop

Rocker switches are used almost universally in cars
i Jupiterimages/ Images

How you wire a 3-prong rocker switch depends on whether it is illuminated with an LED or neon bulb, or whether it is an SPST (single-pole, single-throw) or an SPDT (single-pole, double-throw) switch. If you don't know what kind of switch it is, a meter that tests resistance can tell you. It can also help you determine which prong goes to the switch bulb ground and which prong switches the device you are turning on or off. On all rockers, the center prong attaches to the power source.

Examine all surfaces of the switch, especially the sides and the metal terminals. Wiring schematics are frequently etched or molded to the plastic body. If there is no schematic on the switch, look for stamped numbers or letters (1, 2 ,3 or A, B, C) on or near the terminals themselves. If a printed sheet came with the switch, use the terminal stamps with any possible printed schematics.

Use an Ohm meter or multimeter to identify switch poles if no marking or documentation exists. Set the meter to resistance and turn it on. Touch the black and red test leads together and use the thumb wheel to zero the meter. If it is digital, it will autoset.

Hold one meter lead to the center pole on the switch. Hold the other to an end pole -- it doesn't matter which one. Note the reading on the meter. Flick the switch and note what happens. If there is no change in the meter reading, the end pole you are testing is bulb ground. If the meter swings to zero, that pole will connect to the device you are controlling with the switch and you can wire the switch only one way. If the meter swings to zero on either end pole, you have an SPDT switch, which changes power over from one device to another.

Drill a hole to mount the switch, or use a plastic-cutting saw bit to cut a rectangular switch hole. Run all switch wiring through the hole.

Strip 1/2-inch insulation off the ends of all wires you are connecting to the switch. Insert the end of the wire going to the power feed through the hole in the center pole. Insert the wire connecting to the switched device through the switched terminal. Insert the ground wire through the ground terminal. The other end of the ground lead is wired to common vehicle ground. If the switch is an SPDT with three poles, there is no indicator lamp inside the switch. On 3-pole SPST rockers, the ground lead grounds the indicator lamp only.

Snap the switch body into the hole, indicator lamp pointing up.