How alternator light works, a more detailed description Options

[Tom]

After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.
This is how I think the alt light works:
When the key is on and engine not running, there is 12 volts + at the alt light power side coming from the fused side of fuse #9. The other side goes to a junction on the relay board with D+. With the key to off and a meter connected between D+ and ground at the relay board, the reading is 12 ohms. As soon as the key is turned to on, the reading jumps to 12.5 meg ohms and the light comes on. If the wire for D+ to the alt is removed, the reading stays the same and the light stays on. Removing the VR caused the reading to jump to infinity and the light goes out.
For the light to work, there has to be power to one side of the light and some resistance reading to ground for the other. Looking at the wiring diagram, one can follow the blue wire to the junction at the relay board at D+, then up thru the VR to a set of relay contacts, then down thru a ( resistor ?, not sure) and then down to the DF connection and on to the rotor where the current will produce a magnetic field. After the rotor, it goes to ground. When the alt spins enough RPM's, a voltage is produced and fed back to the VR, causing the relay to open and removes the ground path for the alt light.
I could be entirely wrong here, but this is what I see and my readings more or less confirm it. If you see an error in my thinking, please post and let me know.
Thanks,
Tom

[mikesmith]

- After posting, I took the racer out for a spin to check the drivetrain install (it's a caged racer but actually has plates and insurance). I was just cruising 5-10 minutes locally so didn't "excite" the alternator (not worried about battery level) but 5 minutes into the drive I glanced down and the dash was showing 13.5V.

How did this happen, and is it possible that this is a non-problem for a higher-revving race car?

Your description just begs more questions, sadly. Did the alternator self-excite? (residual magnetic field, current leakage into the field winding?). Is your battery just a monster (or are you running a 16V setup?), do you trust the dash's voltage reading?

- In the street car, I had a glowing GEN light that would be dim at idle and get brighter with RPM. I did some voltage checks and found that the B+ battery voltage was ~13.5V so it was charging fine, but the D+ voltage was full alternator output, from ~14.5V at idle to ~17+V at 3000 RPM. I drove it like that for a few months until I got motivated to replaced the alternator (the only thing the D+ was going to was the GEN light); a new voltage regulator did not resolve it, but a replacement alternator did.

Why is that? What failed in the alternator for that to happen?

If D+ is significantly higher than B+, then the high side of the main rectifier or something else between the stator winding and the battery (B+ terminal hardware, battery strap not tightly connected, etc.) is probably hosed.

Not sure about what you're doing with D+ only going to the GEN light... that's not how it's supposed to work. 8)

[GregAmy]

Your description just begs more questions, sadly. Did the alternator self-excite? (residual magnetic field, current leakage into the field winding?). Is your battery just a monster (or are you running a 16V setup?), do you trust the dash's voltage reading?

"Self-excite" is the only thing I can think of. But here's the rub: prior to my removing the replay plate and re-wiring everything, the VR was sitting on the relay plate, as normal. However, the stock dash was already removed and a Stack 8130 was stuffed in there, with no wires going to it for GEN/ALT light. I removed the Stack last year to install a Race Technologies DASH.

And yet...the alternator was charging. I *presumed* (but never verified) that the previous owner had stuff a resistor or bulb someplace in there, but was never curious enough to find it. However, today we know for a fact that since I removed the relay plate it's not connected.

And yet...?

And here's another rub. Some years ago I removed my VR from the replay plate on the street car; I described it in this thread here. At the time, I was not aware that the common knowledge was that the alternator would not "self-excite" without a GEN bulb..."and yet"...when I did that the voltmeter was showing charging. I only added that jumper wire in order to get a GEN light indication in case of failure.

So...I do wonder if it's actually needed.

Battery in the street car is a Miata AGM, and battery in the race car is a very small Deka ETX14 battery.

Not sure about what you're doing with D+ only going to the GEN light... that's not how it's supposed to work. 8)

Ok, you're correct. To clarify, the GEN light is the only D+ connection to the car. So I drove it like that, not worrying that it was going to fry any equipment in the car (the VR didn't seem to mind). I still have that alternator in a box in the garage, I'm likely to send it to an electrical repair shop to keep as a spare. But it wold be nice if it's a simple bench fix (while I'm handy with electtrical systems basics, I'm a mechanical guy, not an electrical guy).