System Voltage Low

Possible causes

• CommonFailing alternator or voltage regulator
• CommonSlipping, worn or broken drive belt
• CommonCorroded or loose battery terminals
• CommonCorroded earth strap between engine, body and battery
• CommonWeak or failing battery
• PossibleDamaged or corroded main charging cable
• PossibleParasitic drain flattening the battery when parked
• PossibleBlown fusible link in the charging circuit

What you might notice

• Battery warning light on
• Headlights dim and dimming further at idle
• Slow cranking, or failing to start after a short stop
• Engine cutting out
• Several unrelated warning lights
• Heater blower slowing when other loads switch on

What to check, in order

1. Check the drive belt first. A broken or slipping belt is quick to see, and on many engines it also drives the water pump - so a broken belt means overheating as well as no charging.
2. Measure charging voltage at the battery with the engine running. Little or no rise above the resting voltage means the charging system is not working.
3. Clean and tighten the battery terminals and earth straps, then measure again. Corrosion here is the cheapest cause and a very common one.
4. Perform a voltage drop test on the main charging cable and the earth path under load.
5. Have the battery load tested. A battery that cannot hold charge drags the whole system down even with a good alternator.
6. Do not simply fit a new battery and carry on. If the charging system is at fault the new battery will go flat too, and you will have paid for nothing.