Question about an alternator "idiot light"

[J Persons]

Originally Posted by Haz331

Is it necessary to wire a resistor in series with the switched voltage source? If I do need one, what size resistor should I use? Thanks

Actually it's not in series, but wired parallel. And yes, you do need one. Around 500 ohms. What it does is provide a backup load to the alternator exciter, in case the idot light burns out. The alternator won't "exccite" without a load on it. The way the idot light works is: When the key is turned to on, battery voltage goes to ground through the alternator, and lighting the idot light. When the engine starts and the alt starts producing, it sends current back up the wire to the idot light, and cancels the batt signal. I use an idot light as well as a volt gauge on my car. It's useful in that it will show an instant failure, such as a thrown or broken belt, that you may not notice with a gauge.

Here is a diagram of a 1G Ford alternator and wiring, showing the charge indicator lamp and resistor. Most alternators operate the same, just some of the wiring is different, depending on internal or external regulated, etc. Lots of good electrical stuff at www.fordfuelinjection.com
John