How to Tell If a Car Circuit Breaker Isn't Working

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Things You'll Need

  • 20-ohm, 20-watt wire-wound power resistor

  • 30A knife switch SPST (single-pole single-throw)

  • 6-by-6 inch board, ½-inch thick

  • Wood screws

  • Screwdrivers

  • 14 AWG stranded copper automotive wire

  • Diagonal pliers/wire cutters

  • Wire strippers

  • Needle nose pliers

  • 6-inch adjustable wrench

  • One set, 12-volt auto battery terminals

  • 3 alligator test clips

  • Soldering gun or heavy duty soldering iron

  • Soldering tip cleaning sponge

  • Rosin core solder

  • DMM (digital multimeter)


To test an automotive CB's ability to open a circuit under overload or short circuit conditions effectively, you need to be able to test it under load.

Image Credit: Jupiterimages/ Images

Automotive circuit breakers serve the same purpose as the circuit breakers in your home's service panel does; they protect the circuits against short circuits and electrical overloads. Some automotive circuit breakers (CBs), like the ones found in your home's CB panel, need to be reset manually after they trip out (or open) under a short circuit or overload condition. Others, like those found in many home appliances, reset themselves automatically after a specified length of time has elapsed. Automotive CBs, mostly of the automatic resetting kind, have replaced fuses in most automotive electrical circuits that are subject to occasional overload conditions.


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Assemble the Test Jig

Step 1

Clean the soldering gun tip by wiping the hot tip on the wet sponge. Tin the cleaned tip by applying a light coating of rosin core solder. A properly cleaned and tinned tip will appear bright silver in color.

Step 2

Mount the 12-volt, 30-ampere knife switch to the wood board. Cut two 48-inch and one 18-inch pieces of 14 AWG stranded copper wire and strip ¾ inch of insulation from both ends of each piece.

Step 3

Solder one end of the 18-inch wire and one end of the power resistor to one terminal of the knife switch. Solder an alligator test clip to the other end of this wire.


Step 4

Connect the other end of that 48-inch wire to the negative battery terminal. The negative terminal can be identified in two ways, the "-" (minus) sign on it and by the fact that the diameter of the hole is slightly smaller than that of the "+" (positive) terminal.

Step 5

Connect one end of the remaining 48-inch wire to the positive (+) battery terminal and the other end to the remaining alligator test clip.

Test a Circuit Breaker Under Load Conditions

Step 1

Open the knife switch and attach the battery terminals to a 12-volt, fully charged auto battery.


Step 2

Connect one alligator test clip to each of the blade terminals on the CB under test.

Step 3

Set your DMM to measure 12-volts DC and place the test probes across the power resistor. You should read the full battery voltage, if you read "0 volts," the CB is in the tripped position. If the CB is of the manually reset type, try resetting it. After resetting the breaker, you should read full battery voltage. If you do not, the CB is bad and should be replaced. Automatic resetting CBs may not always reset themselves after a severe overload and will need to be replaced.


Step 4

If the CB checked good, it is time to check to see if it will open properly under short-circuit conditions. Attach the leads from your DMM across the CB under test. It should read "0 volts," indicating that the CB is closed.

Unlike fuses, a CB does not always open a circuit, as it should. Under a severe short circuit condition, the surge of current may be strong enough to arc over inside the CB and fuse the contacts together and current will continue to flow in the circuit. Malfunctioning CBs can destroy electrical accessories, melt wires and cause electrical fires that destroy your vehicle.


Step 5

Close the shorting switch quickly while watching the reading on your DMM. If you do not see a reading of 12 volts as soon as the switch is closed, open the switch immediately to prevent damage to the test jig. You may observe a spark as the switch closes, bypassing the series power resistor and placing a direct short across the circuit. That is to be expected.

If the CB does not open and the DMM indicates this event by displaying the battery voltage, the CB is bad and needs to be replaced.



Jerry Walch

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.