Generator light

[70shortstep]

Almost finished with my truck😎. One issue I am having is the generator light stays on. With key on, not running, light is bright. At idle, light is dim. At highway speed light is very dim, and sometimes it goes off. I have an internal regulator alternator. Brown wire to terminal 1, terminal 2 jumpers to 12v at back of alternator. Back wire hooked to 12v terminal by battery.
13.6 v, engine off. 14.4 engine idle, 14.6+ revved up.
I know alternator is charging, not sure where else to look. Thanks for any help.

[VetteVet]

Originally Posted by 70shortstep

Almost finished with my truck��. One issue I am having is the generator light stays on. With key on, not running, light is bright. At idle, light is dim. At highway speed light is very dim, and sometimes it goes off. I have an internal regulator alternator. Brown wire to terminal 1, terminal 2 jumpers to 12v at back of alternator. Back wire hooked to 12v terminal by battery.
13.6 v, engine off. 14.4 engine idle, 14.6+ revved up.
I know alternator is charging, not sure where else to look. Thanks for any help.

This is just a theory based on how the light works, but the light is designed to illuminate when there is a disparity between the voltage output of the alternator and the state of charge of the battery. When the difference between the two is far enough apart, the light will come on, and it will vary in intensity, depending on how much of a difference it is.

Now for my theory, When you wire the red wire directly to the back of the alternator, you are getting the output of the alternator, and not accounting for the voltage drop on the circuit downstream from the alternator. This makes the regulator in the alternator demand more output from the alternator, and possibly accounts for the light staying on. It also might account for the way the light reacted to the varying RPMs of the engine.

I would suggest wiring the no. 2 terminal to the main junction of the wiring circuit, where the alternator wire, the battery charging wire, and the cab supply wire meet.

Your battery reading is about 1 volt high for a fully charged battery, but unless the alternator is overcharging it, it should not be a reason for the light.

Just for clarity, your alternator isn't one of those that can be used as a one-wire or three wire unit is it?

[RichardJ]

I see you have a 6 pg build thread. lots of very good work.
You build a test panel to fire the engine, but the only mention of the wiring after that was, "I did the wiring yesterday".

Is the wiring of your own creation, a original replacement with modifications of your own or a custom made for your own specific requirements?

>>13.6 v, engine off.<<
Can I assume 13.6 V immediately after turning the engine off, your voltmeter is good and the voltage settled down to 12.6 -12.5 V after an hour or so?

Many aftermarket harnesses are sold with the intention that you use a Voltmeter. No idiot light and no ammeter.
When they do that they do not install a resister in the Alternator Exciter circuit like GM did in the External Reg and the Internal Reg alternator wiring.

The first drawing is a simplified version of the original GM wiring for External Alt with resistor.

The second drawing has the Internal Reg Alt with a 10 Ohm, 10 Watt resistor wired directly across the idiot lamp. This also shows the wiring from Alt terminal #2 extended over to the junction block near the Battery as VetteVet suggests.

Don't forget, I'm taking a really wild guess here that you have a new harness that was built without the resistor. Otherwise, follow VetteVet's always good advice.

[RichardJ]

Just thought about something. Are those voltages you listed from the dash mounted Sunpro voltmeter you had of your test panel?

If so, use that gauge for driving reference only. Get a real Voltmeter Test instrument and read voltage directly on the battery. A $10 harbor freight would be more useful.

[70shortstep]

Thanks for the replies.
Vettevet, The voltage readings were immediately after turning engine off, so they are probably high. I will move the 2 terminal wire like you suggest. It has been a couple of years since I bought the alternator, I don't remember if it could be run as a 1 wire or not.

Richard, the wiring harness is original and was completely unhacked. The only alteration is removing the regulator wiring. I am running the original brown wire directly to 1 terminal. I am using a handheld voltmeter to take readings.

Where would I check the resistance wire? What colore is it and where exactly does it connect?

I am leaving town this morning and will be back tomorrow afternoon. I'll try the things mentioned when I get back. Thanks again!!!

[VetteVet]

If you look at the pictures Richard posted, you can see the resistance wire in the circuit between the Ignition switch and the alternator.

The charging light serves two purposes, it provides an indicator of whether the charging system is working or not, and it also provides the resistance to the alternator. This resistance is 10 ohms.

Gm realized that the bulb might burn out and open the resistance to the alternator, so they added a 24 gauge resistance wire as a backup to prevent the charging system from failing. The resistance wire is shown in parallel with the light in Richards picture as well. It is also measured as 10 ohms.

The charging light wire runs from the key switch ignition terminal to the dash cluster plug to the light, and the resistance wire runs from the ignition switch Accessory wire terminal, to the inside of the cab firewall block where it joins the wire from the light.

They continue on the engine side of the firewall block as a 16 gauge brown wire, which goes to the external voltage regulator as is also shown in Richard's first diagram. His second diagram shows the brown wire going directly to the alternator with the internal regulator. It replaces the external regulator and it's associated wiring. Basically this is all that's needed to convert alternators. The red wire in the plug with the brown wire, is the voltage sensing wire, which tells the alternator regulator what the circuit voltage is so it can adjust the alternator output. (simplified).

This was the basis of my theory to solve your problem because you've eliminated the sensing wire to the alternator. I'm curious to know if that's the case.

As you know, in order to measure the resistance on the light and the resistance wire, you would have to get up under the dash and connect the meter to each end of the wires in the circuit. This is fairly difficult when everything is in place.
There is an alternative way to do this which is much easier.
As we know resistance in a wire or a circuit causes a voltage drop, so all we have to do is measure the voltage drop on the brown wire, and we know that there is resistance in the circuit. Then you can assume that there is resistance in the circuit.

To do this just unplug the brown wire at the alternator and turn the key on. Measure the voltage on the brown wire and it should be the same as battery voltage.

Now plug it back in and measure the voltage with the key still on and it should show a drop, which tells you that there is resistance in the wire.Try connecting the meter leads to the plug and to ground. You may be able to get the voltage drop directly.

You may have to connect the meter leads to the ignition switch and to the alternator plug to get the reading. If so you could read the resistance directly but with the key off, and the meter on the ohms setting.

Here is a diagram of the resistance wire going to the key switch at the top of the diagram. The arrows point to the 12 gauge brown accessory wire and the resistance is just below it . They share the same terminal.

The charging light wire is shown in the middle of the diagram connected to the cluster plug on the right side, it's the second one down from the top.