More or less everything electrical in a car works using a circuit. The source of energy is the ‘+’ terminal on the battery which reaches the device – any device – through a fuse.
Fuses are safety devices which are designed to overheat and melt when their rated current is exceeded. They also weaken with age and can, literally fall apart, or burn out at a lower current. A broken fuse is obvious and is probably the first thing to check as it requires very little taking things apart.
You’ve noticed the item not working and the next thing to do is find which fuse it runs off – your workshop manual or owner’s manual will tell you. If The fuse runs several items and they are not working either you’ve probably found your culprit.
Let’s say the fuse is broken. So what’s caused it? You have no idea. The only thing you can do is take a fuse of the same rating and put it in place of the original. Some fuses are connected directly to the battery and others receive power via the ignition switch. Certainly switch off all devices that receive power through the fuse and also, if it applies keep the ignition off while doing the replacing.
- If the new fuse blows while replacing it or when the ignition is switched on and you know all the devices are off then there is a short somewhere after the fuse. All you can do is trace it manually. Look for likely points it could naturally come apart – such as with a loom connector – on the wiring diagram or, if the fault has appeared soon after another event, could a recent welding repair or accident damage have broken the insulation and made it touch the car’s body. Any ‘live’ wire, carrying current to a device, that touches the car’s body will cause a short
that will blow the fuse instantly.
- If the new fuse remains intact switch on devices connected to it one at a time, switching each off again before trying the next one. Most fuses only run one device so there won’t be much of this. If the fuse blows at one device this device is the culprit. Check it out.
- If the fuse doesn’t blow the old fuse had probably weakened with age and there is no real fault. If the fuse blows again later there is an intermittent fault and good luck to you! Try and remember what was on and what was off when the fuses blow with intermittent faults. One device on all the time suggests it’s that, random ones means a wiring fault not related to a device.
If the fuse isn’t broken try twisting it in its holder and switching things on. If that doesn’t work replace the fuse. If that doesn’t work it’s got to be another part of the circuit. If the fuse feeds several devices and none of them work the fault is in the wiring that is common to all of them – usually only on the battery side of the fuse. Make sure the reason they don’t work isn’t because you haven’t switched the ignition on. It happens. Often you’ll find a terminal has come off on the power rail as mentioned above.
Fuses can sometimes be repaired with tin foil wrapped around them. It may get you out of trouble but may cause a whole lot more – even to things catching fire and that is never a good idea in a fast moving enclosed space with many gallons of volatile fluid a few inches from your chest. Tin foil won’t blow like the fuse did so make sure you know exactly what caused the fault before doing this.