i have a nissan march 2003 (K12). when i turn on the headlights, it loose its engine power. alternator and battery are ok. anybody knows what would be the cause?

thank you in advance.

additional info (02/09/16): lets say i'm running at 100kph on the motorway and when i turn on the headlights, the speed is dropping and i can feel the engine is loosing power. even the accelerator pedal is already floored. this happens even i turn on the park light. if i only use the aircon during daytime, it is running smoothly. i also notice when headlight is on, the gear shifts fast compared if the lights off (headlight or park light).

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  • How do you know the battery and alternator are good? How did you test them? Commented Feb 7, 2016 at 13:35
  • What do you mean, "the engine loses power" does the engine stall?
    – Ben
    Commented Feb 7, 2016 at 15:15
  • 1
    Are you talking about loosing power when driving, or when the engine is idling?
    – HandyHowie
    Commented Feb 7, 2016 at 20:36

3 Answers 3


my first guess would be that there is a problem with your idle air control valve. when you turn on extra loads like headlights or the rear defrost it is the IAC which rev up the engine to compensate for the extra load.


2 things I would look for. Alternator as previously mentioned, and grounds.

Basic alternator test is to connect a multimeter to the battery while the engine is running. The voltage should be about 13.5-14.2v. Turn on the headlights to see if this value drops. If it falls out of range, your alternator is bad.

Second thing is bad grounds. When there isn't a good ground, electrical components will follow any route they can to find ground. Start with the battery, make sure the connections are tight and clean. Follow the ground (black) wire, it should go the body or engine. Make sure this is good and tight. Look around the engine wiring harness, there should be several places where it connects to the body. Make sure these are good. Lastly, check around the ECU and any other body control modules you may have. You may want to find a schematic for your car to help find the grounds. You will also be able to see the headlight circuit to see if it interferes with anything else (which it shouldn't).

Another thought. If you only turn on the tail lights (1 click), does it have the same effect? If you pull the headlight fuses and turn on the headlights, does it have the same effect? Wondering if there might be something weird going on in the headlight switch or someplace else in the circuit(tail lights, dash lights).


Here is something to try. Start the car, then disconnect one of the battery leads. Should stay running, if it does, turn on the lights. IF it dies, it is your alternator. Might have enough strength to keep it running with out too much of a load(without lights, a/c), but not enough with a higher load. That will also weaken the battery. The cheap way to load test your battery is to hook up a multimeter to it and start your car. IT shouldn't drop below 9.6vdc. Check your fluid levels in the battery(can see that it is not a maintenance free/AGM). IF it is low add only distilled water. Tap water will add minerals that will cause chemical changes inside the battery.

A friend of mine had a similar problem, til one day it barely ran at all and would not charge the battery.

  • 3
    It is not a good idea to run an engine without a battery. The output from the alternator is rectified AC, in other words, the current is turning on and off. The battery smooths this pulsing current, so without it, you could experience strange results, or possibly even damage.
    – HandyHowie
    Commented Feb 7, 2016 at 20:33
  • I do understand that. There are some systems that only take voltage from the battery for that very reason. It is a very low cost troubleshooting measure. Forgot to point out that the multimeter could be used when the car is on, lights off, lights on. The voltage should not be below 13vdc.
    – Tobin S
    Commented Feb 8, 2016 at 5:43
  • Also, check the various ground points. The one from the battery will be black, the one from the engine to body/frame ground will more than likely be a braided flat ribbon type cable, and sometimes one from the alternator to the body/frame. What to look for is if they are frayed, cracked, discolored(burned), and corroded. Aluminum looks white and powerdery, steel/iron is rust or reddish type, and brass or copper will be a greenish or white-ish powder. More than likely all three will be grounded to the same spot to save space and keep things in one place.
    – Tobin S
    Commented Feb 8, 2016 at 5:50
  • @HandyHowie isn't there any kind of filtering? There usually is a voltage regulator on the alternator, doesn't it smooth the current? Commented Feb 8, 2016 at 12:53
  • @IhavenoideawhatI'mdoing There is a regulator to control the voltage, but there would need to be some large capacitors to smooth the wave from the rectified AC. If you look inside an alternator, there are coils, brushes, diodes and a voltage regulator. The battery acts as the capacitor.
    – HandyHowie
    Commented Feb 8, 2016 at 13:29

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