If there is trouble without an obvious cause in any electrical component, test the
to find the cause.
A circuit tester is a useful and inexpensive tool for making electrical tests.
Checking a simple circuit is straightforward - the lighting circuits are among the simpler ones - but the electrical wiring in a car contains many interlinking and branching circuits, which bring complications.
All car wiring is colour-coded; unfortunately there are no national or international standards for colours. Colour codes for individual cars can be found in
, in the car handbook or in a service manual.
Study these diagrams so that you can find short cuts which save you having to check an entire circuit.
For example, if you know that the power for a suspect circuit comes from the
, and if other items fed from that
are working, there can be no fault between the
. So you can save time by starting at the ignition switch.
How to use a circuit tester
Connect the tester clip to the
and touch the probe to the
If the tester lamp does not light, the battery is dead (or the bulb in the tester has blown).
If it lights, try again with the clip earthed to the car body: if the lamp fails to light, the battery negative
is not earthed properly.
Earth the clip near the switch of the circuit being tested and touch the probe to the 'live' (battery) side of the switch. If the lamp does not light, the wiring between the battery and the switch is faulty, or a
If the lamp lights, turn the switch on and probe its other
side: if the lamp does not light, the switch is faulty.
If the switch works, leave it on, earth the clip near the component and probe the live side of the component. If the lamp does not light, the wiring from switch to component is faulty, or a fuse has blown.
If all of the checks so far are satisfactory, transfer the clip to the live side of the battery.
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If your car's instrument panel includes an
, it will tell you how well the charging system is working - the difference between the
going into the
and the power being used from it.
If a component that is fed with electricity through a
How car electrical systems work
to the feed terminal on the component, thereby bypassing the relay and supply wiring.