Intermittent Battery Light. Voltage is fine. - Ford Mustang Forum
 
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post #1 of 5 (permalink) Old 03-03-2013 Thread Starter
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Question Intermittent Battery Light. Voltage is fine.

Ok so I have been wearing my eyeballs out for 2 weeks reading up on this one. I've read Cliff's troubleshooting flow chart and wiring diagrams and ran the checks. ALL is fine. No corroded connections, fuses and links are good. Grounds clean etc. So here is my story, which is different than other examples I have found on the net.

Symptom: Battery light comes on and goes off randomly. Might stay on for 5 minutes, might stay on for 2 hours. NEVER does it from a cold start or during short trips. ONLY happens on long trips.

Car has never stopped running or had problems starting.

I have my GPS hooked up with the Gauges display and the voltage is a solid 14.2 volts or so. NEVER lower than 13.8V and up as high as 14.5 volts. Same voltages when light is on or off. It reads this off the OBDII port via Bluetooth.

Starting conditions:
Battery 1 year old measures 12.5V static and 14.3 volts with car running.
All connections super clean and tight.
I've followed the flow charts and everything seems good.

What I HAVE NOT done is load tested anything.
I don't trust these big box stores to not short something out and make things worse.

NOW here is the ONE clue that might be the key to the puzzle. When the light does come on, it SEEMS to me that I am accelerating strongly, like getting on the highway AND only with a hot car.

Bumps do not turn the light on BTW.
The gauge nor the voltage read out on the GPS which comes from the OBDII port does not fluctuate when the light comes on.

Like I said, this is a weird one.
So, before I waste $200 on an alternator, anyone have any ideas?

Like I said, strong acceleration seems to aggravate it, but not all the time so I may be providing false information here.

Any ideas?

TIA
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post #2 of 5 (permalink) Old 03-03-2013
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Bad diodes maybe?

Take it off the car and run it to Autozone or one of those other places you don't trust (lol I don't blame you... the sh!t I hear coming from those guys mouths when they're trying to upsell stuff haha) and have them bench test it.


2001 GT Convertible

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post #3 of 5 (permalink) Old 03-04-2013 Thread Starter
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Quote:
Originally Posted by SoCal2V View Post
Bad diodes maybe?

Take it off the car and run it to Autozone or one of those other places you don't trust (lol I don't blame you... the sh!t I hear coming from those guys mouths when they're trying to upsell stuff haha) and have them bench test it.
I think that's what I am going to do. I did read some posts about the diodes going and the alternator "rippling" which I assume means it's putting out "dirty DC" which will register fine as far as voltage is concerned but not good for the electrical system.

Maybe I'll hang my Oscilloscope on it and take a look. The problem is it never is in the failing mode when I'm in the driveway

I'm going to take it off this week and get it bench tested. I suspect you are correct and it's weak.
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post #4 of 5 (permalink) Old 03-09-2013
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many regulator problems in those alternators. usaully battery light comes on at first at high rpm. changed alot of alternators for intermittent battery lights.

412 rwhp 417 tq, 1999 gt , stock 4.6 sohc, 2.1 kennel bell, upr rear control arms, strange 10 front and rear, borla s type, built diff, 3.73, stage 3 snow performance kit.
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post #5 of 5 (permalink) Old 03-10-2013 Thread Starter
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Replaced the alternator with a rebuilt from Advanced Auto because they had a great sale going on :kooky:

So far all is well.
I took it on a trip today and tried to aggravate it by accelerating a lot and never got a battery light so it looks like it's fixed. I see tons of messages on the net about people with this or similar problems. Battery light intermittent, may come on for an hour maybe not for 10 seconds. Voltage is always 14.0 or higher and car runs and starts fine.

They all seem to end the same way though, with the car dead on the side of the road.


Seems it may run for months like this or days so I didn't want to take a chance.

Bottom line, if all looks well, connections, battery, grounds, belt and so forth an intermittent battery light with good voltage might just be the alternator.

Thanks to everyone for the help!

P.S. The car seems to have more get up and go than it did with the original alternator. Probably the placebo effect but I'm happy!
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