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Q: battery& alternator on 1994 Mercury Grand Marquis

Had problems with dash lites tail lights , changed light switch everything ok with lights but now the battery lite on dash stays on, battery checks ok. There was a burnt wire on light switch could that be the problem ? Had battery checked they said ok. alternator checked ok
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Really need to inspect the wiring diagram for the dash. Does you battery stay charged? If so, then it is most likely an Instrument Cluster problem, something may be using the battery light circuit to create a false ground because its own ground is weak. It is called 'back feeding' or an 'unreported ground'.
I forgot to say the battery does not stay charged I used the car today after I had charged the battery. Idrove about 5 mi stopped at light the battery was dead the car would not start.. Auto Zone said battery was ok but alternator was not working properly. Installed new alternator, but have the same problem. Battery does not stay charged .Had problem with tail lights , dash lights parking lights ,had burnt wire in light switch ,installed new switch but did not change wiring module .Could that be the problem? Is there a fusible link between alternator and battery? The battery lite stays on in the instrument cluster. Sure could use some help. Thanks a lot
With respect to dandd - just happened to see this response and wanted to reply from my perspective: Your replacement alternator could even be bad. Sometimes this happens - out of the box, onto the car, defective. Not uncommon, unfortunately. Also, did you swap the voltage regulator or did the new one come with it? The regulator could be bad, if you just swapped it. OR, as described in my initial response, the rectifier connector on the alternator that actually sends the signal to the instrument cluster and is integral to verifying charging could have a melted or damaged terminal. Definitely inspect it for damage and check the wire connector plugging into it. I feel like this is the right track. Good luck.
When you replace your headlight switch on this car, you almost always need to replace the wiring connector, as it is likely melted from overheating. This may or may not be related to your battery light...but still should be inspected and repaired as necessary. Check the terminal and connector at the alternator for corrosion and overheating as well. Good luck.
Today checked problem again .Same story checked all wiring cannot find problem.
Changed alternator but same problem. Went back to Auto Zone had battery and alternator checked the battery was low,but ok alternater was not putting out Still drains battery. When you start car there is a intermitten buzzing about 5 different times then quits.Its a blue box behind glove box. Battery lite still on in dash. HELP HELP THANKS
OK. Since your original concern was a headlight switch issue, and your headlight switch usually overheats, and wiring is usually affected, I would start back testing the wiring from the headlight switch into the loom through to the instrument cluster area to see if there is a short to ground.

First, go to the engine compartment and find the fuse box under the hood. Check the starter solenoid/alternator fuse - 30 amp. With the four square relays top to bottom on the right side of the box, the fuse you are looking for is the fourth from the top of the left row. After you verify this fuse is good, pull the 3 wire plug from the voltage regulator (on the alternator). Then turn the key on. Your battery light should be OFF. If it is ON, you have a short to ground somewhere on the light green and red wire. If it is OFF as it should be, you probably still have a problem with your regulator/alternator and/or connector under the hood. I have a hunch the battery light will remain on when you pull the harness plug...indicating the short. See how that goes...

Your blue box buzzing behind the glove box...could be your air bag diagnostic monitor. Not entirely sure -- but the location is consistent. The buzzing could also be due to a wire short in the same harness affecting the battery light...if in fact it is a short after all. Does your Air Bag light flash or stay solid? Does it prove out?
Checked 30 amp fuse OK. Pulled 3 wire plug at alternator, started car, the battery lite is off. Blue box keeps buzzing 5 different times before it shuts off . Air bag lite is off .
Plugged 3 wire plug back in ,started car , battery lite comes back on. Keeps buzzing at blue box. Very frustrating. Didn't want to take to garage like to fix problems on my own. Thanks for your help
Just examining the wiring diagrams briefly, it appears the only box present behind the glove box would be that air bag diagnostic monitor. To confirm, see if there are two plugs that plug into it - top and bottom. I'm nearly certain this is the box you are seeing. I'm guessing also that your Air Bag light does not light up. This is because when the monitor detects a problem with the air bag light, it will beep/buzz 5 you indicated it does.

This air bag diagnostic monitor buzzing and likely inoperative air bag light may or may not be a clue into what is causing your charging system issue.

The next step is to check the White with Black stripe wire coming out of that 3 wire connector and plugging back into the alternator. This can cause the light too. But overall, I'm hearing from you that your charging system does not work...which at the end of the day, if you've checked the connectors, the wiring at least as far as you can see, and so're back to a alternator/voltage regulator charging issue. I am still wondering if you have replaced your voltage regulator, or have swapped it from alternator to alternator, as the regulator can definitely be the cause.
Thanks for the info. They tell me at the parts store that the voltage regulator is built into the alternator. Hopeing that you are right and that might be the problem. THe air bag lite used to come on intermittently. But when took off I dash panels to replace the light switch I replaced all dash lite bulbs thats when air bag lite on dash stopped And the beeping started. This all sounds screwy to me Thanks for your help.
I feel your pain. I recently changed out the wire harness and switch for my 92 Grand Marquis and now the dash lights and ashtray lights stay on lol.
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