How can I make my Generator light function on an alternator? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-30-2008, 11:44 PM
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How can I make my Generator light function on an alternator?

I am rewiring my 1954 Ford Customline. It has a Generator light on the instrument panel, but my system runs off of an alternator. How can I use this original light and make it work with the alternator? What do I run a wire from to make it read for the alternator? Also, does anyone know how to tell if an Alternator is 80 Amps or more because the wiring harness that I am using requires that you install a bypass wire from the alternator to the starter solenoid if my alternator is 80 Amps or more.

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Old 12-01-2008, 10:16 AM
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You need to run a wire from the alternator output terminal, the big one one the back, to a relay. The relay should be energized by the alternator. Then if you want the dash light to stay lit when the alternator is charging, then use the Normally Closed (NC) terminal to connect the light. If you want the dash light to stay off while the alternator is charging, then use the Normally open (NO) terminal. This set-up will have the dash light come on when the key switch is turned to the on position, then have the light go out once the engine is running and the alternator is charging. Good Luck.
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Old 12-01-2008, 11:22 AM
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What type of alternator are you using? A one wire alternator will require using a relay like what Yknot described above and a diode in the relay control wire to keep the battery from letting the relay stay closed.

If you are using an early ford or GM alternator with or with out an external voltage regulator you can run the exciter wire through your generator lamp ..once the alternator is charging the current will flow back to the light, in effect it will no longer be a ground, and the light will go out. If you have problems with the car running on after the key is shut off, add a diode in the exciter wire so that current can only flow towards the alternator. This will keep the alt from backfeeding the ignition.

You should wire a resistor in parallel with the light, so if the bulb burns out you will still have current flowing and the alt will charge the battery.

You should run the alternator "BAT" terminal to the battery lug on the starter solenoid regardless of what the max output amperage is..use an 8 gauge wire .Then wire the fusebox feed from that starter solenoid lug..It's just safer that way.

Hope this helps,
Mikey
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Old 12-01-2008, 05:55 PM
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It would make it a lot easier if you told us more about about the type and year engine you have in your Ford. Most alternators have similar wiring, but there are a few differences in the details.

If you are using an engine built before the early 70's, the stock alternator is probably not more than 80 amps. However, you may need to find a parts number to know for sure.

Powerrodsmike's suggestion to use an 8 gauge charge wire is a very safe alternative. You should also use a 12 gauge fusible link at the point where it connects to battery power (usually at the starter or the positive battery terminal).

Bruce
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