What to Do For a Clicking Noise in a Fuse Box

by Robert Moore

Clicking noise in your car’s fuse box is caused by a relay that is turning on and off rapidly. This can be caused by a computer failure, resistance in the ground wire for the control side of the relay or high resistance in the power supply to the control side of the relay. To find out the source of the problem, you’ll need to run some quick tests using a multimeter.

Tips

If you don't have a multimeter, you can purchase or rent a low-end model from your local auto parts store.
If you don't know how to use a multimeter, St. Lawrence University has published a good tutorial.
Dodge trucks built between 1992 and 1995 have a common problem with the PCM. The main computer board cracks, and it causes the ASD and fuel pump to click on and off constantly.

Locate the problem relay.

Open the lid on your fuse box and take a look at the relays -- the small black cubes scattered around the fuse box. Hold the handle of a screwdriver to your ear and touch the top of each relay with the tip of the screwdriver. When you hear a distinct, repetitive ticking through the screwdriver, you’ve found your troublesome relay.

Tips

Relays control a high-current circuit using low current. It makes it possible to control things like the headlights and the fuel pump without running high-amperage current into the passenger compartment.

Try swapping the relay.

Remove the relay from the fuse box -- It pulls straight out. Swap it with another identical relay with the same number of terminals from the fuse box see if that relay also clicks. If that relay also clicks constantly, you need to run more tests. If that relay works fine, then the original relay that was in that spot is faulty. Simply replace that relay with a new one.

Identify the terminals on the relay.

Look at the terminal labels on the relay. They are identified as 30, 85, 86, 87 and 87a, if it is a five-terminal relay. Terminal 30 receives constant battery voltage. Terminal 85 is the relay's connection to ground, 87 and 87a are the output terminals for whatever the relay controls and 86 is the power to the control side of the relay.

Check for constant ground to the relay.

Identify the slot in the fuse box that terminal 85 plugs into. Set your multimeter to read voltage on the 20-volt scale. Plug the negative lead of your multimeter into the slot for terminal No. 85. Touch the positive lead to the positive battery terminal. You should see constant battery voltage. If the voltage flashes, repair the ground and ground wire that connects to terminal No. 85. If you get a constant voltage, continue to the next step.

Warnings

When touching the test leads to the battery terminals, hold the leads by their plastic handles to prevent accidental electrocution.

Check the quality of power to the switched side of the relay.

Plug the positive lead of your multimeter into the slot for terminal No. 86. Turn on the ignition key, then touch the positive lead of the multimeter to the negative battery terminal. If the voltage varies, the problem lies in whatever is supposed to activate the relay -- the computer or a switch, for example. Inspect, the wiring for terminal No. 86 and make any repairs as needed. If you do not discover any damage to the wiring for terminal No. 86, replace the component that controls the relay.

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