alternator and idiot light

I have an idiot light wired between the key switch and the alternator but it stays on all the time, even though both sides are live. When I turn the ignition off,(injector pump is seperate) the light dims but stays on. How is that possible? The light is all plastic and not grounded.

Sounds like you have it wired way wrong. Plus if it is diesel the alternator can not back feed the ignition system

What is the correct way to wire it? Right now I have it wired like the diagram.
Diagram

Well if the light is staying on then you have it wired so that the ignition switch is not turning the excite off so you have the #1 wire hooked to the wrong place on your switch going by that diagram. I.E. switch off no way light can be on if wired as that diagram shows

Old is correct if the wiring is as shown, but it is not. the ignition wire to the coil goes to the same terminal on the switch. the alternator is bad. or the bulb is not the right wattage to stop the flow. The regulator in the alternator gives a ground to the voltage from the key when first turned on. (lighting the light and exciting the alternator.) when the tractor starts, the alternator puts out voltage, and that terminal is supplied from the regulator with charging voltage. (both sides of the bulb are now with 12v, and it does not light). The switch is shut off, the alternator trys to keep charging, but the ignition is shut off and no 12 volts to the coil. The 12v from the alternator is still in the light, but the only ground is through the coil. The light has not got enough current carrying capacity to run the coil, so the tractor stops. (unless it is a diesel then it will stay on until the tractor is stopped.) Gas start diesel run IH tractors do this. When the tractor stops, the light goes out because the alternator is not producing electricity any more. If it is diesel, the best method to make it work is to hook the #1 terminal to a two pole oil pressure switch, and from there to the battery Pos. this makes charge when running, and none when stopped. Jim

Does it charge? Have you checked it against a known good volt meter?

Rod

if that tractor has a pull kill switch throw that lite away you dont need it. Rely on the amp gauge to tell you its charging wite you exciter wire straight to the switch :switch on powers the alternator switch off no power to the alternator

Amp gauge isn't really an option as it's a 100 amp alt. It's wired with a volt meter but I have this light and would like to make it work.

It charges fine. Turn the key on and the volt meter goes to about 12.3V. About a second after starting, the gauge pops up to about 14.5. It holds at about 13.5 with all loads on. Also checked that with a multi meter.