How to Identify Circuits Using an Automotive Test Light

If you buy a used vehicle that someone has made modifications on they may have cut or removed wires. It can be difficult figuring out where to reconnect them. In this article I will explain how to identify circuits with a test light. Using this method I was able to identify the wires for our rear brake lights and blinkers which had been removed.

Using an Automotive Test Light

An automotive test light is relatively simple and easy to use. There is a clip that connects to ground, which can be clipped about anywhere on the frame, providing there isn’t paint or something else acting as an insulator. You then touch the probe to each wire you are testing. With the circuit switched on the light inside of the test light will light up when you touch it to the correct wire. Now you have identified the wire!

1. Test Your Test Light

Especially if you have borrowed it from a friends old toolbox. Connect the clip to the negative battery terminal. Then touch the probe to the positive battery terminal. The light should light up, provided you don’t have a dead battery.

2. Turn on the Circuit

Turn on whatever circuit you are testing on to send current to it. If it is the left blinker, turn the left blinker on, if it is the headlight, turn the headlights on.

3. Connect the Ground

The test light can be used in either direction but the clip is meant to be clipped to the frame. The pointed tip of the probe can pierce through wire insulation, testing the wire without cutting or disconnecting it.

4. Test Wires

Probe any wires in the harness you suspect may be the circuit you are looking for. When the test light lights you have identified the wire.

Look for wires that travel towards the light etc you are working on.  Any section you can see with cut wires is always suspect. On our truck there was a bundle of wire going down the back of the cab that had been cut. Our truck used to be an RV, the previous owner removed the RV and cut the wires. Many of the wires were for electronics in the RV, but the rear brake lights, blinkers and license plate light wires were also in this bundle that had been cut.

4. Seek Help

As the old adage goes; “if you don’t know now, ask.” Google is my best resource when I come to a dead end. Our  truck chassis was built by Dodge, but then sold to a variety of different RV manufacturers, Winnebago etc. These manufacturers then made their own modifications to the chassis, adding or re-routing wire, etc. It took quite a bit of research but I was able to come across a wiring diagram that matched the colors that I was as seeing on the truck. I also purchased the Haynes repair manual from Napa for Dodge vans 1971 – 2003. That was also helpful.

Read my next post to see how I attached temporary taillights!

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