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Discussion Starter #1
I'm contemplating replacing the incandesant bulbs in the dash cluster (illumination and idiot) with LEDs.

Did a search, but no results.

I thought I read somewhere the charging circuit/diode trio needs the resistance of the "Gen" bulb to function correctly. Is this true? If so I'll have to devise a way to fool it to use the LED for an indicator.
 

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True but if you check the schematic you will see a parallel wire running from the ignition switch. It's purpose is to provide the excitation voltage in case the light goes out.
Make sense? Also they stopped using the extra lead later in the 70s but I don't know the reason.
 

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Discussion Starter #3
Thanks John, after your reply I dug out my schematics. I see the secondary wire, and "resistor" in parallel to the bulb.

Just in general Ohm's law, if the bulb and "resistor" are in parallel (two components), the total impedance is 1/2 of the given components.

Would I be correct in assuming that the regulator has some "give" in the amount of resistance needed to be seen, due to this? Or should I measure the impedance of the average bulb and factor this into my LED replacement to keep everything happy?
 

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Way too much thought into it...Doesn't the filament change resistance when it's cold verus hot?
(Works just fine without the light bulb.)
 

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I'm wondering, will an LED work in the GEN light circuit. The original light works because the bulb has a lower resistance than the "backup" resistor wire. Not sure what the LED will do with a parallel circuit. Sure would like to know if the LED works in this circuit. So if and when you make the change, feedback will be appreciated.

Your question "the charging circuit/diode trio needs the resistance of the "Gen" bulb to function correctly"

If you have the original external regulator 1D alternator, it does need voltage at the #4 terminal of the regulator for operation. It's not resistance that is needed it just needs voltage at regulator terminal #4 when the ignition switch is on. The factory installed the resistor wire as a backup, as stated. Guess the engineers didn't want anyone stranded with a non working charging system because a $.29 light bulb burned out. The backup wire has a much higher resistance than the light, allowing the light to function as a warning light but still have a backup. This alternator doesn't have a diode trio.

If you have the SI alternator, 1st generation internal regulated alternator, it doesn't need the lamp/resistor circuit or voltage on the #1 terminal for operation. Without the lamp/res circuit the SI alternator will not start generating until the engine is revved above idle. Your typical "one wire alternator" There are two advantages to using the lamp/res circuit. 1- is having a warning light to indicate a faulty alternator and 2- with voltage present on terminal #1 of the alternator it will start generating at idle. The SI alternator has the diode trio

The CS alternators need the light/res circit.
 

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Discussion Starter #6
Spent some "tinkering" time today in the basement shop. Built 5 white, 3 red, and 2 green LED bulb replacements. Take a look...(that's one of my junker bezels and a hacked harness for the tests)


Still using a bulb in the "GEN" socket, I'll have to try an LED in the car and see if it charges/indicates.

I'm really happy with how it turned out. My camera doesn't do justice to the actual thing. Check this, total amperage draw with everything on (no GEN bulb) = 1.58A (!) Just the dash lights wide open is .57A . I used the bases of a bunch of dead 1895 bulbs, a few resistors, and some Hi-Intensity LEDs I had left from my taillight project. I figure that each "bulb" costs about $1.30 (no labor), but they'll NEVER need replacement!
 
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