How to carry on the task of continuity (test) if any of the electrical system of a car gets busted?

How to carry on the task of continuity (test) if any of the electrical system of a car gets busted ?

After, checking fuses & relays.

How to check a bad relay on a car ?


1 Answer

  • !
    Lv 7
    1 decade ago
    Favorite Answer

    You need a 12V test lamp - a small screwdriver with a short piece of cable ending with a croc clamp or a multimeter set for dc voltage, plus a basic understanding of electrical circuits. The first is cheaper and easier to use for simple go no-go testing.

    Locate the main components of the service you want to test: the service itself (lamp, fan, stereo, motor etc), relay(s) if any, controlling switch or sensor, the fuse, and expose them by removing cowls, panels etc. Locate the input (+) and output (- or ground) terminals, on newer cars usually enclosed in mutipolar connectors which make access difficult. A wiring diagram would help.

    Your goal is to pinpoint the input terminal with no voltage; one or more compnents upstream is/are the culprits and need repair or replacing.

    If the service works with ignition, switch both the service and ignition (just dash, not engine) on, otherwise service only. Start from the service tested (eg lamp) and work your way upstream (relay, switch, fuse).

    Connect the croc clamp (negative/black probe of the multimeter) to a bolt or similar metal part away from the tested component.

    Touch with the tip (or red probe of the multimeter) the (input) terminal you want to test. If you're testing a lamp which doesn't light up and there's voltage at its + connector, the lamp might need replacing or the ground connection is defective, so check the ground (-) terminal for continuity with the rest of the chassis. Lamps with twin-filament bulbs are a bit trickier to troubleshoot. If there is no voltage, the problem is upstream, so test the relay (if there is one) next.

    Most relays are of the three or four terminal variety, they can be either traditional or electronic ones. They use low-current circuits to control heavy loads. To do that there are two different circuits: the control one, connected to the hand-operated switch or sensor, and the operating one, conducting power directly from the battery to the service. Each circuit has one input and one output terminal and a ground terminal. There may be a common input terminal for both circuits, as in the three-terminal variety. You can tell the circuits by the wire gages - the thinner ones belong to the control circuit, the thicker to the power circuit. Test both thoroughly. Most traditional relays click while switched on/off, many electronic ones do not. If a relay whirs, vibrates or overheats, it needs replacing.

    Testing for continuity involves extra work - the component should be disconnected from the circuit.

Still have questions? Get your answers by asking now.