Electrical Problems: Why Does My Headlight Dim As RPMs Drop?

MC Garage tips on diagnosing rpm-related modulation of the light system.

Q: I have a 1990 Kawasaki ZX-10 that I bought new and have put 80,000 miles on. The headlight is really dim but gets a bit brighter if I rev up the engine. I have a headlight modulator that clips onto the back of the bulb. With the modulator hooked up the bulb will modulate, though dimly, when it's not supposed to. More rpm makes the bulb brighter and stops the modulation. If the revs drop, the bulb gets dim again and the modulator starts up again. I had the modulator checked out by the manufacturer; it's okay. Even without the modulator, the headlight is dim at low revs. I've checked the idle speed, replaced the bulb, and checked all the wiring harnesses and the high-low beam switch. I even pulled the alternator and had it checked out. The battery is new too. What's causing the headlight issues?

Greg Thomas / Via email

A: Most faired sportbikes have a sub-harness for the fairing that connects to the headlights. Corrosion or dirt in the connector to the main harness might cause enough resistance to make the headlight glow dimly at low rpm. Check the resistance of the headlight bulb ground by measuring between the negative wire at the headlight plug and the chassis. (In the photo, the high-beam wire is #1, the low beam is #2, and the ground is #3.) You should see 150 ohms or less. Now check the voltage at the battery with the ignition on but the engine off. It should be 12 volts or so. Check voltage at the headlight plug with the ignition on. It should be nearly the same. If it's less by more than 1 volt, there's a wiring issue.

If the headlight circuit has a relay, check it and the connectors, and don’t forget to clean the bulb socket and remove any dirt or oil from the bulb itself with rubbing alcohol. It’s worth double-checking the voltage at the battery terminals when the engine is running just to be sure the charging system is operating properly. You should see 13.8 to 14.4 volts on the battery when the engine is running. If battery voltage is lower than that when the engine’s idling but rises to the correct value when you rev the engine, you most likely have a charging-system issue; check the regulator/rectifier first. Finally, remember the three most common causes of electrical problems are (1) grounds, (2) grounds, and (3) grounds.