Edmunds Answers



  • zaken1 08/28/08 7:59 pm PST

    Sometimes there is more than one fuse box on a vehicle, or more than one fuse that protects a circuit. In some vehicles, there are fuse boxes both under the hood, and under the dash; in others, they are in compartments behind the upholstered side panels next to the driver and passenger side footrests. Are you sure you checked ALL the fuses that protect the lights? Also, I have seen fuses that look just fine, when inspected visually; but which have an open circuit when you try to run current through them. It is always worthwhile to try substituting a known good fuse.

    Another source of problems like this can be the connectors or the internal wiring in the fuse block. If there is an open wire or a bad connection inside the fuse block, power may not be getting to the circuits which control the lights. IT IS IMPORTANT TO BEAR IN MIND THAT YOU CANNOT RELY ON JUST READING A VOLTMETER, WHEN CHECKING OUT THIS KIND OF PROBLEM: Voltmeters typically draw so little current that they often will give misleading readings when checking a circuit that has developed excessive resistance. In order to get a reliable indication of the functioning of a circuit, it is necessary to connect the circuit to a load that draws a similar amount of current as the lights in the vehicle do.

    The best way to do that is to use a 12 volt trouble light, or a 12V pencil tester with a built in light bulb. Connect one wire from the light to a good ground (You can often get a good ground from a bolt which threads into the steering column, or into the framework which supports the dashboard). Remove the fuse which controls the light circuit. Turn on the lights or the ignition, so that the item you're testing would ordinarily be able to function. Touch the remaining wire from your test light in turn to each of the two contacts in the fuse box for the fuse you removed. If the wiring functions properly, the light should illuminate when the wire is touched to one of the fuse contacts, but will not light when it is touched to the other one.

    If the bulb does not light from either contact (and you're sure you're using a good ground), then there is an open circuit in the fuse box wiring, or in the connectors from the fuse box to the vehicle's wiring harness.

    One other issue which sometimes creates these kinds of problems is the battery ground circuit. On most vehicles, the battery ground cable goes from the battery's 'minus' terminal to a bolt on the engine. But since the engine is supported on rubber mounts, it often turns out to be electrically insulated from the vehicle body. Because of that, manufacturers place an additional ground wire between the engine and the vehicle body, or between the battery 'minus' terminal and the vehicle body. But it often happens that these additional ground wires get broken or are discarded when someone who does not understand their importance works on the vehicle. If that ground path is broken, it can create all sorts of strange electrical problems. So it is a good idea to make sure that there is a good ground path between the battery 'minus' terminal, the engine, and the vehicle body.

    I hope this information is helpful!


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