Alternator idiot light

[H20rider]

I want to add a warning light for low/no battery charging. I have a stock 3 wire alternator that is externally regulated. Where is the best place to tap in to accomplish this? If off the voltage regulator, which wire (color)?
Thanks in advance for any help.

 

[Broncobowsher]

Look as stock wiring for an idiot light car. '78 Granada or something like that. Copy it.
Pretty sure it just uses the 4th wire on the regulator.

 

[H20rider]

Thanks….good idea? Just don’t know which end of the row is 4th.

 

[H20rider]

Yes…green and red stripe wire is the one. As far as adding a parallel resistor, I see that10-15 Ohms…but there is no mention of rated wattage. Anyone know?

 

[okie4570]

1965 F100 had a both a Alt and Oil dummy lights. Might look at that wiring as well. It was identical to the bronco gauge cluster other than the dummy lights, just swap them out? lol

 

[Broncobowsher]

1965 F100 had a both a Alt and Oil dummy lights. Might look at that wiring as well. It was identical to the bronco gauge cluster other than the dummy lights, just swap them out? lol

Double check you are dealing with an alternator and not a generator. I had a '62 but it was a generator. Some of the 70's trucks also came with warning lights. Gauges were an option.

 

[Viperwolf1]

Yes…green and red stripe wire is the one. As far as adding a parallel resistor, I see that10-15 Ohms…but there is no mention of rated wattage. Anyone know?

Not much. As I recall the resistor is only there to excite the regulator if the bulb burns out. How many watts for the bulb?

 

[Broncobowsher]

The indicator bulb is the same as the turn signal or high beam indicators

 

[H20rider]

Actually, to clarify…when I look for 15 ohm resistors, I fine one’s that are rated at 15ohms but at different wattage:

15 ohm Resistor 1/2w (0.5Watt)​

25W 15 Ohm​

Etc.

I’m just using an small led and I already have an actual gauge in place.
I suspect that the resistor iis meant to be slightly less that the resistance through the bulb. In that case it buffers the light and avoids momentary light flashes during short drops in charging that occurs at low idle, etc. if that is the case, with an led that is very low resistance, the resistor is superfluous….and, if any thing a resistor may need to be in series with the led to minimize random flashes.

 

[Broncobowsher]

I think the 15 Ohm rating is incorrect. Placing that across a light bulb (or LED) is pretty much a shunt.
Went looking at a bunch of searches and something in the 560 Ohm rating shows up all over the place. That is small enough that a light bulb in parallel could actually light up.

V=IR is the starting formula. V is Volts, I is Amperage, R is resistance. A 15 Ohm resistor on a 15V circuit is 1 amp. Watts = Volts x amps, so 15W. That is massive. A 560 Ohm resistor will only 0.4 watts. A very managable heat load to handle. Here are a couple of references to the 560 Ohm resistor.
https://www.ford-trucks.com/forums/1110224-4th-wire-on-a-1g-alternator.html
https://www.garysgaragemahal.com/#nabble-td64593
This one lists 510 Ohm http://1969stang.com/forum/index.php?/topic/59902-1g-to-3g-alternator-conversion/
For this basic of a circuit the exact value doesn't really matter. Just anything in that range.

I did see the 15 Ohm reference on a Mustang site. No doubt you found it too. But general electronics engineering kicks in here and tells me the 500-560 is probably the right number for a safety shunt around the light bulb.

 

[Rustytruck]

I thought our brake warning light doubled as an alternator light its on when you turn the key on and goes off when you start the truck.

 

[Broncobowsher]

I thought our brake warning light doubled as an alternator light its on when you turn the key on and goes off when you start the truck.

I've never had a working brake warning light on a Bronco. But the common thing in most old cars/trucks was to have every light come on as you start as a bulb check. Either as function (charge, oil pressure) or triggered during crank.

 

[okie4570]

Double check you are dealing with an alternator and not a generator. I had a '62 but it was a generator. Some of the 70's trucks also came with warning lights. Gauges were an option.

That very well could be I'll have to look again. Looks like 64' was the last year fomoco used generators, 65' being the first year for alternators. The 65' f100 gauges still says "GEN" on it though

 

[Viperwolf1]

I thought our brake warning light doubled as an alternator light its on when you turn the key on and goes off when you start the truck.

It's just a brake imbalance light. The Ignition switch provides a ground for it in the crank position only as a bulb check.

 

[H20rider]

I think the 15 Ohm rating is incorrect. Placing that across a light bulb (or LED) is pretty much a shunt.
Went looking at a bunch of searches and something in the 560 Ohm rating shows up all over the place. That is small enough that a light bulb in parallel could actually light up.

V=IR is the starting formula. V is Volts, I is Amperage, R is resistance. A 15 Ohm resistor on a 15V circuit is 1 amp. Watts = Volts x amps, so 15W. That is massive. A 560 Ohm resistor will only 0.4 watts. A very managable heat load to handle. Here are a couple of references to the 560 Ohm resistor.
https://www.ford-trucks.com/forums/1110224-4th-wire-on-a-1g-alternator.html
https://www.garysgaragemahal.com/#nabble-td64593
This one lists 510 Ohm http://1969stang.com/forum/index.php?/topic/59902-1g-to-3g-alternator-conversion/
For this basic of a circuit the exact value doesn't really matter. Just anything in that range.

I did see the 15 Ohm reference on a Mustang site. No doubt you found it too. But general electronics engineering kicks in here and tells me the 500-560 is probably the right number for a safety shunt around the light bulb.

That is very helpful. Thank you.