Headlights dim at idle
Headlights that brighten when you rev and dim when the engine drops to idle are showing you system voltage in real time: the alternator is not keeping up at low rpm. On a healthy car the voltage at the battery should sit at 13.8-14.5 V at idle with lights and blower on. Anything under 13.2 V points to the alternator, its belt, or a resistance in the main charging and earth leads; a battery on the way out makes it worse but is rarely the root cause.
Urgency
Visit a workshop soon
Possible causes, most likely first
- Weak alternator (worn brushes, diode or regulator)
Voltage at the battery below 13.2 V at idle with loads on, rising only above 1500 rpm. A failed diode shows as AC ripple above about 0.3 V on a multimeter set to AC across the battery; a worn regulator gives a reading that drifts with rpm. - Slipping or glazed drive belt
A shiny belt or a weak tensioner lets the alternator pulley slip under load at idle, often with a squeal when loads switch on. Check tension and look for glazing or cracks; on overrunning alternator pulleys check that the clutch locks in one direction. - Poor earth or corroded main cable
Resistance in the engine-to-body earth or the alternator B+ lead drops voltage before it reaches the lights. Do a voltage-drop test: more than 0.3 V between the alternator output and battery positive, or between engine block and battery negative, at idle with loads is too much. - Battery with a failing cell or heavy sulphation
A weak battery cannot buffer the dips and the lights flicker when the blower or wipers cycle. Load-test it or check that resting voltage after a drive is above 12.6 V. - Excessive electrical load for the alternator
Aftermarket amplifiers, light bars or heated seats that exceed the alternator output at idle. Switch off accessories one by one and watch the voltage recover. - Corroded headlight connectors or weak earth at the lamps
Only the lights dim while the dash stays bright. Measure the voltage at the bulb connector with lights on: more than 0.5 V below battery voltage means resistance in the lighting circuit or relay. - Low idle speed
If idle is below spec because of a dirty throttle or an idle control fault, the alternator simply turns too slowly. Check the idle speed against the specification first.
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What you can check yourself
- Measure battery voltage at idle with headlights and blower on: it should be 13.8-14.5 V
- Look at the drive belt for a glazed, shiny surface or cracks and press it to check tension
- Check battery terminals and the engine-to-body earth strap for corrosion or looseness
- Switch off all accessories and see whether the dimming stops