How to Find the Cold Side of Brake Switch Wiring

by Curtis Von Fange
rosa oldtimer heck image by Matthias Nordmeyer from

When the brake pedal on an automobile is depressed, a brake switch closes and lets voltage flow to the brake lights. This switch has a "hot" side (the terminal which has battery voltage always present), and a "cold" side (which delivers voltage on demand to the brake lights). When replacing a brake switch, or when checking the related circuit, it is important that you know the hot and cold sides. Being aware of which is which will help you avoid blowing fuses from or melting wires when working on the brake lighting system.

Step 1

Put on your safety glasses and gloves.

Step 2

Locate the brake switch on the vehicle. It is usually underneath the dash just above the brake pedal. Look for a small switch attached to the brake linkage that has two wires connected to it.

Connect the pigtail of the test light to a good ground. If using a voltmeter, connect the negative lead to a good ground. With the brake switch disengaged, probe one of the two wires at the connector with the positive lead of your tool of choice. The hot wire will either light the test lamp or cause the voltmeter to read the battery voltage of the vehicle. The cold lead will have no response on the tools.


  • Working underneath a dash can be tight. Using a mirror and troublelight will help you locate tucked-away wires and switches.
  • Sometimes it can be difficult to backprobe a switch. Using a small paperclip bent at a 90 degree angle can help get an easier hookup for the voltmeter or test lamp.


  • Be careful when backprobing connectors that are hot. You can blow a fuse if you accidentally ground the test lamp probe.

Items you will need

  • Safety glasses
  • Light weight gloves
  • Test light or voltmeter

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