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Terry, some people use what I refer to as an "idiot light" as opposed to an ammeter to tell when the alternator is working. When people convert to a GM 10 SI 3 wire alternator they need a voltage source to excite the lil No. 1 side terminal, and many simply add the idiot light in series with a wire from the ignition switch (or coil) to the alternators lil No. 1 side terminal. HOWEVER if the light they use is a lot less then 10 ohms (although it may still work excet for the run on problem) it doesnt provide the needed current limiting protection for the alternators internal diodes. THEREFORE I recommend a 10 ohm resistor (see below). A voltage source is required on the No. 1 terminal to excite the field for the alternator to kick in and start to charge, which is why one sees a voltage feed there regardless if direct off the coil or through an idiot light or a resistor and diode etc. Your same type of question came up earlier today on another board so I will copy n paste my same answer below. The diodes I use are 5 amp with a minimum of 50 PIV. Thats actually rated higher then required but it has bigger leads easier to wire in. Heres my other answer which applies to you: Jubal, In addition to Olds good advice, heres my take on your question: If it runs on, the very FIRST THING I RECOMMEND (if youre not using an idiot light in the circuit) is to install a safety 10 ohm resistor in the excitation circuit to the Alternators lil No. 1 side Excitation terminal. That serves as current limiting protection to protect the alternators internal diodes. In the event youre using an idiot light in that circuit, it can serve as the desired current limiting protection, but if its resistance just happens to be a lot less then 10 ohms, I would still add some resistance so theres a total of at least 10 ohms (idiot light plus resistance) in the excitation circuit. Nowwwww wwww its often the case once you add the desired 10 ohm resistor or an idiot light the run on stops. HOWEVER if she still runs on, then add a diode as Old recommended. Its acts as a one way current flow check valve so the alternator dont backfeed the ignition n she shuts off properly. IT MUST BE WIRED IN CORRECT POLARITY + Anode end to coil/switch, - Cathode end to Alternator or it wont work. The above is for the GM 10SI 3 wire Negative Ground alternators. Sure, Ive seen plenty with no currrent limiting protective resistance n they worked fine,,,,, ,,Ive seen some that run on while others dont,,,,, ,,,,but Ive seen some fried internal electronics on the Alternators which may well have beec caused by overcurrent or spikes or surges etc. Good Luck n God Bless John T Nordhoff in Indiana, retired electrical engineer
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