How to Repair Auto Headlights That Dim When Idle

by Robert Russell

If you notice that your headlights dim or fluctuate in brightness when the engine is idling, then the primary culprit is your vehicle's charging system, particularly the alternator. The good news is that your headlights are probably OK. The bad news is that your alternator is not functioning properly. The problem may be as simple as a loose or slipping alternator belt. The worst scenario may require replacing the alternator.

Step 1

Start the engine and turn on the headlights. Allow the engine to idle, checking the headlights for fluctuations or change in brightness. The headlights should not be greatly affected by revving the engine.

Step 2

Turn off the engine and open the hood. Locate the alternator, which is adjacent to the engine.

Step 3

Inspect the alternator wires. Loose or corroded wires will prevent the alternator from functioning properly.

Step 4

Press down on the alternator belt with your finger. If it gives more than 1/2 an inch it needs to be tightened. If the belt is dry or cracked, it needs to be replaced.

Step 5

Connect a voltmeter to the battery. Attach the positive, red lead of the voltmeter to the positive terminal on the battery. Attach the negative, black lead to the negative terminal on the battery.

Step 6

Restart the engine and check the voltage. The voltmeter should read between 13.5 and 14.5 volts. If the voltmeter reads13 or lower, there is a problem with its ability to hold a charge, thereby dimming the headlights.

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