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Generator to Alternator Conversion

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  • Generator to Alternator Conversion

    KENN called me with a problem. He purchased a new 130-amp Ford alternator and connected it according to correct Mustang schematics. It wouldn't work.

    More specifically, the GEN light shined brightly when he turned the key to 'on' and the engine started but the light stayed on and the alt would not put out.
    He took the alternator to his neighborhood auto parts store, they tested it and it came back, OK. Why does it work in the store but not in his Squarebird!

    So, he called me.
    I asked him about the GEN light wire (Yellow/Black) that formerly went to the 'ARM' terminal of the old voltage regulator.
    He said, 'Yeah, it's still on there with an extension wire that connects to the 'sense' wire of the new alternator.' So, he is using that 'ARM' screw clamp as a terminal.

    I said, "Take that skinny Yellow/Black wire off and let it dangle by itself. Now, turn the key switch on and look at the GEN light."
    KENN said, 'The light is off now.'

    That's right! You just removed the ground from that wire.
    When you disconnect the old generator, pull the harness completely out. The wire sizes are too small for an alternator so the harness is useless. KENN wants to use his old voltage regulator to house headlight relays and also to let it resemble a 'stock' look. That's ok because the original main power wire (Yellow) connects to the 'BATT' terminal and it powers the horn relay (and now the headlights through relay contacts).

    Back to the GEN light problem: I told him the Yellow/Black wire needs a simple butt splice and that wire continues until it is connected to the 'sense' wire of the alternator. Once he did that, his new alternator worked exactly as you would expect. Now, we're happy.

    The way our OEM (generator) system worked was, turn the key to 'on' and the GEN light shines. As soon as the engine starts, the GEN light goes off. If you throw the generator belt, the GEN light comes on. <--this has saved countless Ford owners from overheating their engine, especially 'Y-Block' owners that use one belt.

    Some owners called the GEN light a 'test light' and in a sense it is. If you turn the key on and the GEN light does NOT shine, there is a problem.

    The GEN lamp gets power from the battery through the key switch then it connects thru the generator's armature, and then to ground. When the gen is stopped or if it is rotating slowly the GEN light comes on. When the gen is rotating fast, it produces it's own 12 volts, THEN the lamp has 12 volts on BOTH sides and the light goes out. The GEN light compares battery voltage on one side and generator voltage on the other side. It is one of two dash lights that are not grounded. (The other is the OIL light.)

    New alternators respond nearly the same way; turn the key on and the GEN light shines. Start the engine and the light goes out. Throw an alternator belt and the light comes back on. We have come to expect this over many decades as 'normal' operation.

    What is different? Alternators are solid state devices. They need a 'sense wire' to know when the key switch is on. 12 volts from the GEN light is a perfect indicator that the key is on. This excites the internal voltage regulator. When the key is off, zero voltage is on the voltage regulator. That shuts down the alternator so it can't drain the battery.

    KENN tied his 'sense' GEN light to ground at the old voltage regulator. How? On the rear of the old regulators are two resistors that are connected to ground. When the 'sense' wire is grounded the alternator will not produce, just like having the key switch off. The GEN light shined brightly because one end of the lamp was grounded. As soon as he pulled the ground off that wire, 12 volts energized the new voltage regulator and the alternator worked just fine.

    If you want a better understanding of how the OEM charging system worked, follow the above information along with your electrical schematics.
    Charging systems are not magic but you need a bit of electrical and mechanical knowledge to 'tie them together' in your head. OEM generators and regulators are nothing more than windings of wire and relay contacts. They can be 'taught' to run current in either direction (positive or negative ground). Ingenious. Still, questions remain:
    Voltage Regulators disconnect the battery from the generator through contacts of the 'safety relay'.
    What energizes the voltage regulator's 'safety relay' coil?
    What does the voltage regulator control in the generator?
    Why 'polarize'?
    Why do cars have GEN lights but tractors do not? They use the same gen/voltage reg.

    I'm open to questions. - Dave
    My latest project:
    CLICK HERE to see my custom hydraulic roller 390 FE build.

    "We've got to pause and ask ourselves: How much clean air do we need?"
    --Lee Iacocca

    From: Royal Oak, Michigan

  • #2
    wow.... that's a very detailed bit of information, easy to understand and valuable info.... as always, thank you Dave.

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    • #3
      Awesome read. Thanks Dave.
      sigpic1999.jpg

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