7080 Alternator Light

when you turn on the SWITCH, it should put POWER to the light, thru the wire and into the alternator to ground... and the bulb turns ON..... When you start the engine and the alternator puts out 12- 14 volts, it pushes TOWARD the light and the light basically has 12 volts on each side and  no current flow so the light goes out...

steve(ill) wrote: when you turn on the SWITCH, it should put POWER to the light, thru the wire and into the alternator to ground... and the bulb turns ON..... When you start the engine and the alternator puts out 12- 14 volts, it pushes TOWARD the light and the light basically has 12 volts on each side and  no current flow so the light goes out... You need 12 v at the light with the key ON... and you need the power to flow thru the alternator to ground to get the light ON..... one of those two is missing.

Ok, with that operating principle in mind, I began to trace the wires again. There was power coming from the key switch, and when I unplugged the instrument panel harness and grounded the wire coming from the bulb, the light came on. Ok…the problem must be the alternator harness or the alternator ground. Took the plug from the alternator and grounded it. Spark and tripped circuit breaker. Hmm. Looked in the book…Alternator is supposed to have light green wire with black trace running to the generator light, and at the alternator terminal that wire connects with a brown wire, and the brown wire connects to a tan wire at the instrument panel harness. Tan wire runs to the key switch. So my problem with the light to start with was the brown wire was feeding power to the green wire and therefore the light, so it had power on both sides and no ground. Disconnected the brown wire and low and behold, the generator light now works as it should. Comes on with the key, goes out when the engine starts. But I can’t figure out what the brown wire is for (not the brown Tach wire, that’s different I know)

Edited by CrestonM - 28 Sep 2023 at 8:04am

Hurst wrote: I think you posted on the facebook page.  I am thinking you got an alternator with the "one wire" style voltage regulator.  If you unplug the 2 terminal plug from the side of the alternator and start the engine, does it produce ~14 volts?  If so, you have a self-exciting regulator in that alternator, and the charge light will not work with that style.   If this is your problem... Good news is they are super simple to change out and very inexpensive.  Look on ebay for a 10si regulator (just don't buy the "one wire" regulator), and use a finishing nail to hold the brushes back when you install the two halves together.  Takes all of 10-15 minutes once it's on the bench.  Pull the finishing nail (or piece of wire) out once the two halves are together (I've forgotten to do this and about pulled my hair out wondering why a fresh rebuilt alternator wouldn't work, then was like "doah, brushes are still pinned!"). Hurst

Must’ve been my uncle. He’s been helping me with this some but we were both stumped. Thanks for the tip, but it looks like it may have just been a power issue.

Hurst wrote: Sounds like the sensing and field wires at the alternator terminal were switched?  Kind of having a tough time following which wire went where.  If you just need to flip them, you can use a small screw driver or knife to release the spade from the connector and put them in a different position. Hurst

My apologies. I figured it might not be the best explanation. 4 alternator wires: brown (tachometer wire) orange (primary output wire). Those 2 are out of the question.
What I’m referencing is the plug with 2 terminals (#1&2 stamped on the alternator). One wire is light green, and is a power wire coming from the circuit breakers on the cab. The other terminal has a green/black wire and a brown wire together. Green/black goes to Alternator light, brown goes to key switch, so it’s also a power wire. I need to look at the book again but this brown wire doesn’t make sense to me yet as to why it’s there.

The number two terminal (stamped on the case) is the constant power. Number one is key switched power.

sounds like you got it figured out now.. I thought you were using ONE wire.. As long as you have TWO wires, its fine.. I have no idea why you have a third wire (brown) available.. It should look something like this below..  As mentioned, the #2 spade is always HOT, and normally comes from the  HOT wire on the key switch.. Some "aftrmarket" installs just jump to the BIG LUG for the power....... The #1 spade is excitation and can have a light bulb or a resistor in the line, and gets power ONLY when the switch is ON.

Edited by steve(ill) - 28 Sep 2023 at 8:32am

Thanks Steve, your diagram makes sense, and with the extra brown power wire removed, it works as it should. I will look at the book again and try to figure out why that brown wire is there.

YEA... i dont know that much about this specific tractor... Just a thought what the BROWN wire might be doing attached to the GREEN wire.