Testing for a blown fuse used to be a simple case of getting your mug close enough to the fuse box to see which ceramic fuse had a separated fuse wire or which glass tube was black and burnt. With the advent of blade fuses (and mini blade fuses) you could no longer do this. Sure you can pull blade fuses out and visually check them but it is much easier and more accurate to use a testlight or multimeter.
Old Ceramic/Plastic Fuse and Glass Fuse and the newer Blade Fuse and Mini Blade Fuse.
If you don’t have access to a test light or multimeter you don’t have to spend much to get one that will do everything the home mechanic needs. Below is a selection of what we used in a workshop environment.
The Fluke meter is a great meter that has been used for wiring up efi computers and the like but it’s around $100 worth. The one is the middle is literally the cheapest one I could find at Supercheap at the time (about $20). I wouldn’t rely on it or accurate voltage readings but for testing for power or continuity it works fine. On the right we have a test light made from an indicator bulb and holder, a couple of lengths of wire, some connectors and an alligator clip. It works great and costs nine tenths of bugger all to make (less than $10). At the bottom we have another Supercheap product, about $15 for this test light. Any of these will do for checking fuses and help you find minor electrical faults and well worth having in your toolbox.
So armed with your test light or multimeter you first need to find a good earth point, the negative terminal of the battery or a metallic part of the engine is best if you are working in the engine bay or a paint-free bolt or metal part of the dash if you are working inside. Set your meter to the appropriate range (usually 20V DC) and test your meter or light by probing a couple of fuses. If there’s no sign of life there’s a good chance your earth point is no good so have a look around for another one. Keep in mind also that the component in question will have to be switched on to check for a blown fuse, for example if it’s a head light or park light problem you’re chasing then typically they will need to be turned on before there is voltage at the fuse, the same with something that is on the accessory or ignition circuit, there won’t be power at the fuse with the ignition turned off.
Probe points on a blade and mini blade fuse. The fuse amperage rating can also be seen here.
You may be able to isolate the fuse in question by looking at the fuse block diagram but I think it’s a good idea to run your meter or light over all of the fuses just in case there’s others that are blown that you weren’t aware of, it only takes a few seconds to do. Once you’ve found the culprit (the blown fuse will show voltage on one side and not the other) pull it out and replace it with the SAME AMP RATING FUSE. This is important because the components and wiring of a circuit can only handle a certain load and if you go upping the amperage of the fuse and there is an abnormal load on the circuit you run the risk of component damage and if the load on the wiring exceeds the wire’s rating there is a good chance of the insulation melting and an electrical fire starting.
The same can be said for a dead-short or short-to-earth. If you replace a fuse and it blows again as soon as the circuit is activated DON’T be tempted to start stuffing higher amp rating fuses in there or bits of wire (I’ve seen a cut down nail used before!), there is obviously a problem that needs to be rectified. We’ll take a look at how to diagnose a short to earth in the near future.