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How do I test a relay.?

My headlights don't come on all the time. I have to play with the switch every morning. I checked the switcha and it is good. The fuses are good and it is both headlights so I bellieve that MAYBE it could be the relay. I have went through the wiring with a test light and cannot find where the problem is.

Is there a way that I can check the relay before buying another one?

Update:

Sorry I left this out:

The car is a 1994 Honda Accord. I have a 1999 Ford Mustang also. I will check to see if they share any of the same relays.

I just don't want to buy a part that I don't need, its a waste of money.

8 Answers

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  • 1 decade ago
    Favorite Answer

    Remove the relay from the car, and test the contacts it with a digital multimeter. Use the ohms scale. Good contacts should read 0 ohms when closed, and infinity when open. Check the coil; it should measure near 0 if good, infinity if "open." Next, check to see if the relay actuates by applying 12 vdc to the coil. If it passes all these tests, the relay is good.

  • vrrJT3
    Lv 6
    1 decade ago

    Many relays have a built in diode in parallel with the coil. This diode absorbs the voltage spike that would otherwise occur if the diode was not there.

    So, if you apply voltage to the coil, be sure to observe the correct polarity. Otherwise, you can forward bias the diode and burn it up. Of course, this all depends on the type of the diode, and the kind of relay.

    I've had several problems with relays working intermittently. It's like a switch with corroded contacts. Sometimes you flip the switch and it goes on, sometimes not.

  • 1 decade ago

    You don't say what kind of vehicle you have, but many have the same relay for other electrical devices. If you have some that are alike, try them in the headlight relay socket and put the headlight relay in the other. If you then have trouble with the other device functioning, the relay is bad.

  • ?
    Lv 4
    5 years ago

    Well, if everything is working, then the relays are OK. If, for example, the high beams aren't working, I'd change the high-beam relay first. (assuming the bulbs are OK) Unless you have a good knowledge of relays and how they work, leave that to a technician.

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  • 1 decade ago

    i mechanic for a living,,and not really,,there's not a good test you can do,,because half the time when you test a relay it will show good,,as long as its not under a load,,id just replace it,,because there not that expensive,,they can be tested,,but by the time you do this ,you could have a good one out on it,,and be done with it,,good luck ,hope this help,s.

    Source(s): been a certified mechanic for 36 yrs.
  • (A)
    Lv 7
    1 decade ago

    Nelson could be correct.Another possibility might be burned out light elements.Switch is more than likely the problem.

    Source(s): AutoElec.Tech.
  • 1 decade ago

    You have bad contacts in your headlight switch

  • Anonymous
    1 decade ago

    Got a multimeter?

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