I believe my headlights have become dim and I am wondering how to check them. Either my eyes are going or they are. How can I know for sure which it is? I have a voltmeter and I’m not afraid to use it.

Skip Russo

I wondered the same thing a few years back. It turned out to be a little of both. Whenever an electrical component works questionably, the fault could be the electrical source, the circuit feeding the component or the component itself.

The electrical source is the charging system and battery. If the vehicle is starting and driving well, it’s unlikely that a problem lies here.

The wiring and switches controlling the low and high beam headlights can develop resistance over time. A corroded connector, oxidized switch contact or loose ground connection can reduce lamp performance by a noticeable amount.

Headlight bulbs typically work well or not at all, so they are an unlikely suspect. If polycarbonate headlight lenses appear cloudy or yellowed, polishing them with Brasso will help restore needed clarity.

To test the headlight circuit, try the following: Temporarily insert a small paper clip (one end unfolded) into each of the two or three terminals at the back of a headlight connector, making sure the clips do not touch each other.

Do not unplug the connector from the headlight. With the engine off and the low beam headlights turned on, measure the voltage between the positive and negative battery terminals.

Next, measure between the paper clips, trying differing terminals as necessary, until a similar voltage is seen.

If headlight voltage is within one volt of battery voltage, the circuit is acceptable, and it’s time to see your optometrist.

This test can be performed on each headlight, and again with the high beams selected.

If the difference between battery (supplied) voltage and headlamp (delivered) voltage exceeds one volt, further circuit testing is necessary to determine the cause.

Try this to narrow the search: With the headlights on, measure between the battery positive terminal and each paper clip until a small voltage (0.5 to 6.0 volts, not zero) is seen. This is the voltage drop of the positive side of the circuit, which includes the headlight switch, dimmer switch and a majority of the headlight wiring.

A reading considerably higher than 0.7 volts is unacceptable, and additional measurements across each switch and section of wiring will be needed to track down the source of resistance.

A reading of less than 0.7 volts means this half of the circuit is OK.

The ground side of the circuit is tested similarly – between the battery negative terminal and a paper clip showing a small but not-zero voltage. The maximum allowed measurement here is 0.3 volts. A greater reading is probably the result of a loose or corroded ground connection at the body.

For more information on automotive electrical system testing, please request an e-mail copy of my troubleshooting tips classroom handout.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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