How to Use a Voltmeter to Test a Fuse Block

A digital multimeter (DMM) is one of the most useful test instruments in any automotive electrician's tool kit. Every DMM is slightly different but there are some general guidelines you can follow for its use. An old analog voltmeter will work too but be careful of polarity when using one of them or you can peg the needle and do permanent damage to the meter movement.

Hands using voltage meter
credit: Polka Dot Images/Polka Dot/Getty Images

Troubleshooting an Automotive Fuse Block

Step 1

Locate the fuse block on your vehicle with the help of a work light or flashlight The main fuse block is usually located underneath the dash on the drivers' side of a vehicle. There will also be a power fuse block located under the hood near the vehicle's firewall. The blocks contain two types of fuses, the older "glass cartridge style" fuses and the newer "blade style" fuses. The blocks may also contain re-settable circuit breakers.

Step 2

Clip the negative, black test lead to a good ground using the alligator clip attachment. Remember the car's metal frame is one side of its electrical circuit. If you have a meter that must be set manually, set the meter's "Function" switch to "DC Volts" and set the "Range" switch to a scale that will allow you to read 12-volts. On most manually ranging meters, that will be the 20-volt range.

Step 3

Locate the small holes on each end of the blade-type fuses; those holes give you access to the blades inside the fuse. Insert the pointed tip of the red test lead first in one hole and then in the other. If you read 12-volts to ground on one end but not the other, the fuse is blown and needs to be replaced. With glass fuses place the red test lead tip on the silver-colored end caps to check them. Double-check your ground connection if all the fuses read "0" volts to ground. If there is no power to one fuse but all the others are functional, the problem lies with the fuse block itself.