Battery light is on alternator and battery tested good. What is causing this?

RACINGDAN

MEMBER
1999 MERCURY MYSTIQUE
112,000 MILES

I have a 99 Mercury Mystique that has a battery light on the dash. I have done the following so far. I have tested the battery and that is good. I have tested the alternator by taking it to a parts store. I am also getting 14.3 volts at the battery with everything that I can think of on in the car. I have checked the connections and clean them up as best I could with a wire brush on the battery terminals and all wiring connections. I have check the main 175 amp fuse with multimeter and that checks out ok. I have check the 15 amp fuse in the engine compartment and that is ok. I have check for shorts by having the negative battery terminal off the battery and using a test light one end hooked up to the negative post and the other to the terminal and the light didn't light. So that tells me that nothing is running off the battery and taking power. The only think I can think of doing to make that stupid light to go off would be to unplug the bulb for it. Any suggestions would be greatly appreciated.

4 Replies

FIXITMR

MECHANIC
9,990 POSTS

So when did this happen first? Any changes made by you just before?

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Monday, February 6th, 2012 AT 8:18 PM

RACINGDAN

MEMBER
2 POSTS

It been on for awhile now. I have had it in to a mechanic and a couple of other places and they say everything is ok. I also think there is nothing wrong with the alternator and I think it is something with the wiring going from the alternator to the instrument cluster. According to the wiring diagram in the manual I have there is a wire going from the alternator regulator (green and black) going to the instrument cluster the other one from that connector of the regulator is going to the battery. I was going to try and trace the green and black wire from the alternator to the instrument cluster and put a multimeter on it to see if its broken somewhere.

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Tuesday, February 7th, 2012 AT 4:31 AM

FIXITMR

MECHANIC
9,990 POSTS

Yes, that seems like a logical choice.

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Tuesday, February 7th, 2012 AT 7:41 AM

CARADIODOC

MECHANIC
33,864 POSTS

Hi guys. Here's a quick test. Unplug the voltage regulator connector, then look to see if the warning light is off. If it's not, that green wire is grounded and the generator shouldn't even be turning on as the current coming through the light is the turn-on signal. If the light does go out, suspect the regulator. Once the system is up and running, it puts 12 volts back onto that green wire to turn the light off. It knows to do that when half of system voltage appears on the white wire that's plugged into the side of the generator.

Most auto parts stores that test generators only look for output. Many do not test the warning light circuit.

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Tuesday, February 7th, 2012 AT 6:58 PM