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  Mobile Mechanic Service


Jeff's Auto Repair of the Murrieta and Temecula area offers advise about common automotive concerns and answers to automotive faq's.

April 2010
Testing for a blown fuse using an incandescent test light.
The incandescent test light is the much less expensive type of test light that most people have, can afford, or justify spending the money on.  Price range is 5.00 - 25.00.  They have been around for 50 years or more.  It it the type that has the little light bulb inside and you can see the light bulb element.   The other type is an LED test light.  We'll talk about the benefits of using one of those in a later article.

So you have something electrical in your vehicle that does not work, or does not work properly, and you want to check your fuses first, before you check or move anything else.  Good.  This is a smart move.  I say this because I have been doing auto electric all of my life, and I myself, have been burned (not literally) by not checking fuses first.  I promise, every time, when you think you know what the problem is and you go to it, tearing things apart, forgetting to check fuses... in the end, it will be a fuse, and a lot of wasted time.

Ok.  Lift the hood and locate the battery.  Connect the alligator clip end of your test light to the negative side of the battery.  Then, prove that the test light is working by touching the probe end of the test light to the positive side of the battery.  If it lights, then proceed with testing.  If it does not light, then you need to test your test light.  Usually a bad connection inside the handle or at the connection between the wire and the alligator clip.  Anyway, assuming that the light lit up, turn the ignition on to the run position.  Do not start the engine.  Locate the first fuse panel, usually under the hood somewhere.  Visually inspect any fuses with clear windows, the ones that have no test points.  If they all look good, and the same, then start testing the fuses that have test points.  Test points are basically small portions of exposed metal at the top or ends of any fuse.  Touch the test light to one side of the first fuse, then to the other.  
    Now here is the meat and potatoes of this article.  It is real simple.  Both sides of any fuse should do the same thing.  If touching one side causes your test light to light up, then so should the other.  If one side does not light up your test light, then the other side should not light up your test light.  If both sides light up your test light then the fuse is almost definitely good.  I say almost definitely, because as with any rule, there are those once in a great while exceptions.  If neither side of the fuse cause your test light to light up, then it would be wise to simply remove the fuse and visually inspect it.  The smoking gun that your are looking for is a fuse that you probe one side and it lights your test light up and probe the other side and it does not light your test light up.  If you find this, you have found a blown fuse, and without pulling each individual fuse and inspecting it to find your problem.
If you found no blown fuse here, then go to the interior fuse panel, usually under or at the end of either side of the dash, and repeat the above procedure.  Don't forget, todays modern vehicles can have as many as 5 fuse panels, or as few as one Huge one.  Consult your owners manual for fuse panel, power distribution center, junction box location.  If you find a blown fuse, always replace it with the manufacturers specified value of fuse.  If your fuse repeatedly blows, then a short is present, and we'll cover what to do about that in a future article.
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