Alternator Problems...

Last week my dad and I tested the alternator charging, because the AMP light wasn't coming on, replaced the bulb, still didn't come on, and the hand-held volt meter read 11.98ish, I took the ALT off, my dad took it to get tested, tested good. Our next step was to check the voltage regulator, I was putting the ALT wires back on exactly how I found them and I went to go start it, ran good (still charging issue, but ran like its supposed to), I went to go turn it off and it wouldn't shut off, key was turned all the way off, then I looked at the dash and the lights were on even though the key was off, so I double checked my wires 3 times and they were correct. Does anyone know what I did wrong?

 

I believe the alternator or voltage regulator will have a diode that prevents it back-feeding battery voltage to the ignition when the engine is off. Maybe the charging test didn't see that or if it was defective.

 

Should be a resistor instead of a diode? Reliable silicon P-N junction diodes were not affordable until maybe the '70s, and a resistor (or incandescent bulb) works fine for points ignition. Certainly 60s consumer gear got germanium diodes that are too bulky and unreliable for automotive service.

Understand how this works - the ALT light is connected from the ignition switch to the regulator. When the engine is not running, the regulator connection is a ground (sink of current). Turn the key on and the ignition switch is a source of current, which goes through the ALT bulb to the sink at the regulator. The bulb lights. The engine starts running. The regulator now becomes a source of current supplied by the alternator. Connect two sources through a bulb and no current flows. The bulb goes out.

When you shut off the ignition, the alternator continues to hold the regulator terminal high, as a source. The alternator can continue to power the ignition via that connection, unless there is something in the current path to block that back-feeding into the ignition switch. With points ignition, having the bulb in there is enough resistance to starve the ignition and prevent back-feeding. The current through the bulb is enough to light the bulb, and to start the alternator charging; not enough to power the ignition. A diode works too, but with points ignition, the ALT bulb should be enough.

If you have the ALT bulb in there, and it works like you expect,and the engine does not shut off, you've made some additional connection between the ignition switch and the regulator. Suggest you trace out the circuit with your multimeter, a big sheet of paper and some colored pencils, and see what you have.

 

After much checking and double checking by Ayden We found a short in the ignition switch. Charges like it should. Now to chase the short. I will have to say the Ayden is really getting good at this stuff. Proud Dad!