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John (or anyone else),

I am running a internal reg alt, and have my idiot light hooked up. i want to know if i swap the idiot bulb for a led and piezo buzzer, will that somehow screw with the charging? I also want to do the same thing with my oil and water idiot, but after i install electric guages. Is all of this possible?

Also, i wired in a Headlights-On-reminder alarm tonight that goes off with the lights on and door open. If anyone wants instructions on how to do the same, lemme know! :)

-Luke

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71 Malibu
19 yr old college student
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An answer on your idiot light question is no, you can not replace the lamp with an LED. You can replace it with a resistor but that doesn't help what you want to do. The idiot lamp serves another purpose. One side of it is tied to the IGN line off the ignition switch. The other side of the lamp is tied to a field output of the alternator. When you first turn on the key, the field gets voltage from the IGN line through the lamp filament. This voltage energizes the field so that the alternator will start producing an output. Basically a LED won't work.
There are a few ideas floating around out there to do something else besides an idiot light. I haven't given it much thought. Off the top of my head there are circuits out there that switch off items when voltage falls to a certain point. Some of these circuits are used in RVs to cut off power to things like refrigerators if the battery gets to low. May be able to see something like that on a RV site or maybe out of JC Whitney. Modify the circuit to switch at around 12.5 volts and activate your buzzer.
 

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Most cars have a resistor in parallel with the bulb. I know the GM shop manuals I have looked at usually show the resistor. It may not be an actual resisor but rather a piece of resistance wire.

With the connector out of the alternator just do a resistance measurement between the 2 wires going to the bulb. If it's a fairly low resistance (I'd expect maybe 50 ohms or less) then there is a resistor in parallel and you're all set.

The resistor is installed so the circuit will still work even if the bulb burns out.

Peter
 
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