Where's The Fuse?

One of the most common questions asked in online auto repair forums is "Where's the fuse that goes to...[whatever]?" The reason this is so common is because in today's automobiles, 80% of all the problems experienced by the car owner are electrical in nature......(some electrical device is not working). It may be a power window, a power door lock, a radio, a tail light or a navigation system, just to name a few.

Here's the problem with that question:

Contrary to popular belief, fuses are NEVER the CAUSE of an electrical circuit problem. Blown fuses are ALWAYS the RESULT of an electrical circuit failure. I am continuously hearing the phrase "It's just a blown fuse" or "I think it's just a fuse issue". My response to that is WHAT??? There is no such thing as "just" a blown fuse! Fuses don't just go around blowing themselves out in an effort to make your day miserable. Fuses are electrical SAFETY VALVES. Any time there is a blown fuse, this means that there was an electrical overload that was threatening to burn your car to the ground!

In today's cars and trucks, there are literally miles of wire connecting all of the electrical components. Some are high amperage circuits that power things like motors and some are very low power circuits running on less than two volts for things like oxygen sensor circuits. All of these circuits, however, are connected to the vehicle battery in one way or another. All of these wires are bundled together neatly in what is called a wiring harness.

The fuse has the job of allowing enough power to run the circuit, while being weak enough to be the first part that will burn in the event of a short circuit or overload. This is so the wires themselves do not burn. When the fuse blows, it turns the electrical power off to the offending circuit.

A short circuit or electrical overload causes the wires to overheat. When the wires heat up, the plastic insulation melts off of them. The wire can then burn through the insulation in the wire next to it and cause it to overload. The next thing you know, you have a hundred different wires burning in the same vehicle because of one little short circuit.

The fuse that blew just finished preventing this from happening. It gave its life to stop your vehicle from lighting up like a bad dream from "The Abominable Torch Visits The Fireworks Factory". To replace this fuse and not find out and correct the reason WHY it blew in the first place, will only cause another innocent little fuse to have to give up its life to keep your vehicle from toasting. In this case, the first fuse will have died in vain!

If you are going to try to fix an electrical problem in your vehicle there are a few things that you must have to get the job done.

1. You need a fairly good understanding of how an automotive electrical system works. If you don't have this understanding you are headed for certain disaster and disappointment. Many community colleges as well as several online educational resources offer training in this area. I highly recommend getting at least some basic training. Even if you don't use this training to fix your own car, it will help you to understand what is going on at the shop when you take your car in for repair.

2. At the least you will need basic electrical system testing equipment. A digital volt/ohm meter and a 12 volt test light with a long cable will do in most cases. A set of long test leads, alligator clips, a few jumper leads, and some back-probing pins are also recommended. You want the long leads because it is always best to connect directly to the battery ground when testing power circuits and directly to battery positive when testing ground circuits. You can waste a lot of time and effort otherwise. Also be sure to get QUALITY equipment. I have seen a lot of misdiagnosis as the result of using a cheap test light that only works when it wants to.

3. You will need the wiring diagrams for the vehicle you are working on. This falls under item #1 above...you also need to know HOW to USE a wiring diagram. In the old days, automotive electrical systems were pretty simple and you could poke around with a test light until you found what you were looking for. In today's computerized cars and trucks, you have to know EXACTLY what you are poking on. Probing a wire with the wrong piece of equipment or with the wrong set-up can cost you a computer module in the blink of an eye.

Simply replacing a blown fuse will not fix your car. If you are not willing to go through all this and you are experiencing an electrical problem in your vehicle, I highly recommend that you don't waste your time and effort looking for the fuse. Probably 95% of all automotive electrical failures do not include a blown fuse anyway. The phrase "Check the fuse" is highly overrated.


-DTTECH
ASE Certified Master Automobile Technician

Other articles by dttech:
My Car Won't Turn Over

What Else Could Be Wrong?