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How to test an electric switch

We demonstrate how to use a multimeter to determine if a switch is functioning properly

When an electrical problem pops up in a vehicle or machine, you need to know if the switch that activates the problem circuit is working properly. A multimeter will help you figure that out.

Set the multimeter’s dial to the (Ohm) position. The meter’s internal battery will then send a tiny current out through one of the leads. Attaching the meter leads to each end of an electrical circuit or individual component will allow you to measure any resistance to current flow. Things like rusty parts or partially broken wires will increase resistance. (Be sure the circuit or component you’re testing is disconnected from any other power source.)

The setting also helps you establish circuit continuity, which means it will clearly indicate if there is a complete break in the wiring. And it can help you trace wires.

To demonstrate that, we’ll use those features to test an ignition switch. In this example we need to not only figure out if the switch works properly, but also which posts on the rear are live with each key position. The switch we’re using belongs to our ongoing Jeep restoration series, Project CJ3A.

The switch only has three key positions: accessory, off and run. It is meant to be used along with a separate starter button, so it has no “start” position.

When the meter leads are connected to the posts of this ignition switch with the key turned on, current is able to flow back through the other lead and the readout changes to indicate how much resistance there is. The 000 readout indicates a complete (closed) circuit with no resistance.

When the meter leads are connected to the posts of this ignition switch with the key turned on, current is able to flow back through the other lead and the readout changes to indicate how much resistance there is. The 000 readout indicates a complete (closed) circuit with no resistance.
photo: Scott Garvey

By placing the multimeter leads on the unmarked posts at the back of the ignition switch, we can establish which posts are activated in each key position. The meter allowed us to determine that the centre of the three posts is where power from the battery needs to connect. With one meter lead on the “power in” post, it’s just a matter of touching the other lead to each of the remaining two posts with the key in different positions and checking the readings.

If the post is live and current is flowing, the meter gives a digital readout of the amount of resistance. The closer to 0, the better the circuit. If there is no current flowing (an open circuit) our meter’s readout indicates that by displaying a “1.” That might be a little different on other meters, which may display “OL” instead.

On this switch when the key is in the “accessory” position, battery power is routed only to the left post. When the key is in the “run” position, current flows to both the right and left posts. The meter readings also let us know the switch is in excellent condition with virtually no internal resistance.

Now we now know how to route wiring to and from the switch. From the right post (the “run” key position), we’ll connect leads to the coil in the primary engine ignition system and the alternator. From the left post (the “accessory” key position), we’ll connect any vehicle accessory systems. That will allow them to work with and without the engine running.

About the author

Machinery Editor

Scott Garvey is the machinery editor for Grainews.

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