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Old 06-25-2012, 01:47 PM   #1
mchammer187
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Default Will bad alternator diode cause parasitic drain and still charge battery?

I replaced my battery a couple weeks ago and have been having problems weak starting ever since. The battery was 7 years old so I figured it was toast when my car failed to start.

I know that my alternator is putting out 14.0 -> 14.2 voltages while my car is running because the onboard computer tells me that much. I've tested with lights radio ac blinkers on and it still maintains 14.x volts with the car running

I took the battery to auto zone (where I got the new battery) and they load tested itbattery and it tested fine (after it was charged). After they charged up the battery to full it did not start weak at all but after letting the car sit for a couple days the problem came back.

So I am thinking bad alternator diode or some other source of parasitic drain. I don't have tools but will get them if an alternator change is in order.
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Old 06-25-2012, 02:06 PM   #2
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It sounds like a bad alternator to me, it's possible that your old battery was fine but just not getting charged enough (although at 7 years old it's time to replace it anyway)

If the alternator was bad, then driving to autozone and then back could be enough to drain the battery - particularly since it sat for a few days after that.


For what it's worth, I have essentially zero faith in autozone/advance/etc battery and alternator testing stations. I have seen known bad batteries and alternators test good and vice versa. After numerous bad experiences (all at different stores and 'testing stations') I just don't rely on them at all anymore.

Just because your alternator is providing 14v to the OBD system doesn't mean it has enough amperage to charge the battery correctly. Based on what you've said, it sounds like either bad alternator or some parasitic drain, my money is on alternator.
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Old 06-25-2012, 02:17 PM   #3
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seems to be the case with my dakota..had the alt.tested and shure enough it has a bad diode. If I leave the batt connected the bad diode will flatten the battery.
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Old 06-25-2012, 06:33 PM   #4
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I had this very same issue with my Shelby about 15 years ago. The alternator charged fine and the car started fine as long as I didn't let it sit more than a day or so without running. I took the battery out to have it tested and noticed a rather large spark disconnecting it which means there is a load on the battery....on a 30 year old car that has not memory items or loads when the car is off. After some thinking I figured it had to be a diode in the alternator. Took it off, took it apart, and tested all the diodes only to find a bad one in the tri-diode pack. A few dollars later the problem was fixed.
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Old 06-25-2012, 08:04 PM   #5
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IF ... there is an alternator/generator/starter repair shop in your area ... you could remove the alternator and take it to them for testing and ... repair or exchange IF necessary.

IF ... the alternator is not defective ... then anything in the vehicle becomes suspect.

A clip-on DC ampmeter would be helpful to isolate the leaky path and component.

Last edited by 50 mpg by 2012; 06-25-2012 at 08:08 PM.
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Old 06-25-2012, 09:20 PM   #6
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Yes. a Shorted positive diode will drain the battery with the key off and out.
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