Auto Repair Help
Carl’s Corner – Auto Repair Help

By Carl O’Reilly

The “Battery” or “Charge” light remains on after the vehicle is started.

Although this light is relative to the battery, it is usually an indication of a charging system failure. The light should come on when the key is turned to the on position as a bulb check. Once the engine is started, the alternator begins to charge the battery and turns the light off. When this light comes on, the usual cause is a defective alternator or voltage regulator.

Prior to an extensive diagnosis you should check all the vehicles fuses to ensure that none are blown. Be aware that some vehicles employ two fuse boxes. One is located in the passenger compartment and the other is located under the hood. Consult your owners manual for fuse ratings and identifications. When replacing blown fuses always use a replacement fuse of the same amperage as the one being replaced. Ensure the battery terminals are clean and tight prior to diagnosis. Diagnosis and verification of this symptom requires the use of a voltmeter. This can be obtained at any auto parts store. When the key in the off position the battery voltage should be around 12.6 Volts. Once the engine is started the battery voltage should increase to 13.0 – 14.5 Volts as a result of the alternator charging the battery. If it does not, the alternator and/ or the voltage regulator are most likely the cause of the malfunction. Most newer vehicles utilize an alternator that incorporates and internal voltage regulator. Earlier vehicles use a seperate voltage regulator and alternator. Consult a vehicle specific service manual or parts professional for positive identification.

On vehicles that use an integrated alternator, a voltage regulator replacement of the alternator is necessary. Replacing the internal regulator on the alternator is possible, but usually requires special tools and a working knowledge of internal alternator operation. It is usually more cost effective to replace the alternator with a remanufactured unit. On vehicles that use a seperate alternator and voltage regulator, vehicle specific testing procedures will be necessary to isolate whether the alternator or voltage regulator is defective. Auto part stores can often test an alternator to determine whether it is functional once it has been removed from the vehicle.

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4 Comments on "Diagnose The Battery Or Charge Light Came On"

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3 years 1 month ago

I have a 2008 Mustang V-6 and the battery light comes on and goes off. I have had two auto parts stores check my battery and alt. and say eveything is good. My readings according to them are: Volts 12.76 U, Meaured 622 CCA, Rated 500 CCA, Temp 107. I have also cleaned the ground connection, but not the pos. on the battery/cable. There is no corrosion on the post at all. I have not check the fuses yet, but no indication that any have blown.

Any suggestion on what I need to check next?

Thank you for your help.


1 year 5 months ago

How do you prevent fuses to be blown?

8 months 19 days ago

The air bag light of my Mitsubishi car is not going of, auto repairs said I should ignore, any help.