Alternator warning light wiring

I have rewired my gauge cluster, described in another post. However, my new alternator warning light is staying on all the time. I know that I have good charge coming from the alternator, evidenced by both my digital multi-meter, and the actual voltmeter in the car. In the other post, I said how I need to figure out the alternator warning light wiring, as the light was not hooked up yet. So I consult the 1981 EVTM and find this:



So instead of the wire going from a 12v source, through the light and to the alternator like I'm used to, I find basically the same set-up, except it goes from a 12v source, through the light, and to the voltage regulator. The incoming R/LG wire is hot in key-on, goes to the light, then the other side of the light has a LG/R wire going to the voltage regulator. So I cut the LG/R wire close to the voltage regulator, and connected my wire to it. My wire comes from dashboard power, goes through the light, and terminates at the LG/R wire entering the voltage regulator. Should work, right? But no, denied. My light stays on after engine start, even when the dash gauge is showing 14+ volts. So the alternator is clearly charging at that point.

One possible problem is that the alternator warning light is an LED, not an actual light bulb. But I'm pretty sure it is "resistored up" to look like a light bulb as far as load. I'm kind of at a loss to explain why the light is staying on. Anybody have any ideas?

Thanks!

Your LED doesn't properly work in that application.

Remember, that a diode will only pass current one direction.

With a filament in this circuit, the bulb goes out because potential on each side is the same.

Another issue with an LED is that it doesn't take much voltage difference to illuminate them.  But it takes a lot more voltage differential on an incandescent bulb to get it hot enough to have a visible glow.  So put an incandescent in and see what happens.

If it's on all the time, Gary, he is tapped to the excite wire and the other side is grounded.

Hmmm I was afraid of that. It's hard to find dash indicator lights that are not LED anymore. I'll have to do some searching. Thanks guys!

Jim - Going back I see Pete said "My wire comes from dashboard power, goes through the light, and terminates at the LG/R wire entering the voltage regulator."  So his "light" shouldn't be truly grounded.  But perhaps the "dashboard power" isn't on at this point and we are trying to power things in the dash?

Pete - What is your "dashboard power"?

Gary Lewis wrote

Pete - What is your "dashboard power"?

It's a key-on 12v power source that supplies power to the gauges and the OP warning light. All new wiring, not factory stuff. The actual path of this circuit is:

Battery + to fuse to relay (controlled by key-on source from factory wiring harness, I think R/LB if I recall correctly) to Alt Warn light (and other gauges/lights, all in parallel) and then back to the voltage regulator LG/R wire.

I think that approach will work with an incandescent light.  But maybe not with an LED.  Hopefully you can find a bulb with which to test.

I be the electron and follow that.

Remember in the stock wiring scheme we run a 512 Ohm resistor in parallel to the bulb so the truck will continue to charge if the bulb burns out.
I would think that's a minimum amount of forward power that an LED would have to pass.

Not sure why or how your led is getting the alternator to charge, Pete, but I'm not too familiar with what the fender regulator is doing V the integral regulator of the 2&3G alternators.

ArdWrknTrk wrote

Remember in the stock wiring scheme we run a 512 Ohm resistor in parallel to the bulb so the truck will continue to charge if the bulb burns out.

Ah, good catch, I did not even notice that resistor until you pointed it out.

I found some actual incandescent warning lights on ebay, I'll see if they act different when they get here.

Well, I got the incandescent warning lights in the mail. But I'm stumped by the results of my testing with it.

I have a quick-disconnect connector set on the LG/R wire that goes into the VR. If I unplug it, I can hear the load on the alternator ease up, and the voltage drops from 14 to 12. A voltmeter on both ends of that wire reveal that, with the truck running, the VR end has no voltage (well, almost none, the meter reads like .01). The truck end of the wire has 12v. So supplying that 12v to the VR is instrumental in getting the alternator to charge.

This seems completely backwards to me. I thought when the alternator was spinning, both ends of this wire would read voltage, thus making the "no potential" situation and thus no alternator warning light. But that's not what the meter reads.

Then to really confuse myself, with the engine running I hooked one end of the incandescent light to battery +, and the other end to the LG/R wire going into the voltage regulator. The alternator started charging again, indicating that current was being drawn through the light bulb. BUT THE LIGHT BULB DID NOT LIGHT!!! What the heck! Well, I guess it should not be lit if the alternator is charging, so maybe that's normal. Let's turn the engine off and it should start to glow, right? Nope. Nothing.

I give up. Guess I'll be ok with a good voltmeter in the dash, which I have.

Electricals are not my favorite. As someone once asked, what do you need a ground/return side for? I mean, you send current down a wire, and the energy is used to light a light bulb or whatever. So what the hell comes back on the return side? Good question...

Thoughts welcome.

How many watts/what resistance is the bulb you have?

ArdWrknTrk wrote

How many watts/what resistance is the bulb you have?

That I do not know. It's a tiny little bulb. Here is what I used.

https://www.ebay.com/itm/Pair-Amber-Pilot-Dash-Indicator-Warning-Lights-12V-Vintage-Classic-Hot-Rod/122306778605?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908103841%26meid%3Db180d132574a415eb339c4661c7ca9e0%26pid%3D100227%26rk%3D3%26rkt%3D10%26mehot%3Dpp%26sd%3D123560127545%26itm%3D122306778605%26pmt%3D1%26noa%3D1%26pg%3D2053904%26brand%3DUnited+Pacific&_trksid=p2053904.c100227.m3827

It says each light has about 27 ohms of resistance, but nothing about wattage.

Let's turn the engine off and it should start to glow, right? Nope. Nothing.

If the motor was off AND the key then you are right.
What if the motor was off and the key on did the light light?
Remember as a test to see if the light is good or not it should light motor off / key on.
Dave ----

Ohms law works in all ways.  😉

I usually assume 14V running.

ArdWrknTrk wrote

Ohms law works in all ways.  😉

I usually assume 14V running.

So that would be just under 2 watts I guess? What does that tell us?

That it's using .14A and at 98 ohms resistance is well below the 512 ohm resistor that will excite the alternator.

So I don't know why it's not showing when you first turn the key to 'run'

ArdWrknTrk wrote

That it's using .14A and at 98 ohms resistance is well below the 512 ohm resistor that will excite the alternator.

So I don't know why it's not showing when you first turn the key to 'run'

Ah, now I see where you were going with that, but there was no resistor in this scenario, the light bulb was not "the path of least resistance", it was the only path.

I'm stumped too. Oh well... I'll watch the voltmeter.

Thanks all, for your input.

I had no idea how much current the amber bulb filament was passing, but I did have a good idea of the threshold current the alternator needs.