I have noticed that at night with my car’s headlights switched on, the engine’s idling speed slows down, while the lights stay very dim and brighten considerably only when I rev the engine. Is there a problem with the engine or the electronics, or both?
The most likely cause for the symptoms described above is a low voltage supply. This will often cause the engine control unit to function a little abnormally, causing the headlights to be dim.
The first thing to check in rectifying the problem is the condition of the battery. If it is weak, replace the battery immediately. Sometimes, the problem may just be loose battery-terminal connections. Make sure these are clean and tight.
Another possible problem may be the earth cable, which can deteriorate over time and cause electrical issues. Inspect the cable and replace it if necessary.
Finally, check the condition of your car’s alternator, which should supply 14 volts to the battery when the engine is running at its normal idle speed.