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I have a 2002 mitsubishi magna, battery light came on intermittently one day and a few months later the car died mid drive, battery completely dead. i got a jump start which lasted about a minute until the car died again. purchased new battery which got me home. battery showing 12.43v which drops to 11.98v when idling. getting 11.95v from alternator (testing from alternator B post and housing, also from battery neg to alternator B post). seems to be no AC leakage. fuses all good. wiring seems all good. voltage to alternator from battery is correct (connector tested). is the alternator just producing low voltage because the alternator is faulty? i should mention it is absolutely covered in oil from an oil leak, wiring is dirty but working, could oil inside the alternator be the cause?

  • Is that alternator reading with or without the battery connected? And is it at idle or at say 2000 revs? – Mike Brockington Dec 31 '19 at 15:18
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Your alternator should be producing 14 volts, 12 volts is way below what it should be producing. It's likely the alternator or voltage regulator (usually part of the alternator itself), especially if it did get a load of oil in it, but you should check the belt is in good condition and isn't loose before you replace anything.

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If the alternator belt has lots of oil on it, replace it - it may be slipping. Clean the area fully, and try again. If the alternator isn't producing more than about 12.5 volts (13.5 is normal - but it will depend on the battery load), you'll have to replace the alternator.

The oil could have caused the problem, but on a 2002 car, it could also just be old age that killed it, with a bad battery pushing it over the edge. I don't think disassembling the alternator to clean out oil would be worth the effort - just pull it and replace.

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    Bad batteries do an excellent job of killing otherwise functional, but old, alternators! – Brian Knoblauch Dec 30 '19 at 17:02
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Your alternator is dead and does not produce electricity anymore. You are running your electric system on battery power - that's why you got home with a new battery (they sell them charged).

Oil is a good reason for an alternator to die (brushes spark and wear out quickly when oiled).

99.9% sucessful strategy to fix the situation:

  1. Charge both batteries. The old one is probably just depleted and otherwise ok. They will both die if left depleted for an extended period (say, a week)
  2. Fix the oil leak.
  3. Have the alternator (or the whole engine compartment) steam-washed and replace the brushes (they usually go with the voltage regulator).
  4. Replace the belts (all of them). They also fail after being exposed to oil.

In the meantime, you can run your car for small trips (1-2h) if you charge the battery beforehand.

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