ANSWERS: 1
  • The headlight relay is a very important component in any vehicle's electrical system. It connects the headlights with the electrical power source that allows the lights to operate. The headlight relay is relatively easy to test, provided that you know what to look for and take the proper safety precautions.

    Background

    The headlight relay is located in the relay/fuse box, usually located near the car battery. It is usually the largest relay, and is located in the top left corner of the relay/fuse box. There are two possible ways a relay can fail. First, there can be some sort of internal problem with the relay. Second, there could be something wrong with the various types of circuits that provide it with power. By testing the relay itself, you can eliminate the first possibility and focus on the second. Before you test the relay, test the fuses using a test light. This will ensure that the relay is receiving power in the first place.

    Testing the Relay

    Open the car hood and open the relay box. You will need a helper to sit in the driver's seat and turn the ignition key. Put a finger on the headlight relay. Either when the ignition key is turned to "On" or when engine begins to start, you should feel a "click." If you do not feel a click, this is a good sign that something is wrong. Check the relay for corroded connections. Try the test with a new relay. If it continues to be a problem, then further testing is required. Use a test light or a multi meter to test the relay outlet. Turn the ignition on, then remove the relay and test all the sockets. There should be four of them, and two of them should have power. If this is not the case, check the fuses again. If the fuses check out, consult a car wiring diagram, available in most car repair manuals, to find out where the relay receives its power from. If there is not problem, then a further test is needed. Next you will test the relay's trigger circuit. Connect the test light lead to the positive battery terminal. Turn the ignition on then connect the probe to one of the relay terminals. Flip the headlights on. The test light should illuminate. If it does not, the relay trigger circuit is the problem.

    Bypassing the Relay

    If all of these tests check out, turn the ignition off. Every relay has essentially the same terminals. These terminals are demarcated by numbers. You want to locate Terminals 87 and 30. Making sure the ignition is off, run a small piece of 14- to 20-gauge wire between the two terminals. This bypasses the relay and allows you to test the surrounding wires and components for shorts. Use the test light to do so.

    Source:

    Saab9000.com: How to repair the headlight beam control relay

    2carpros.com: How Do I Check a Car Relay?

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