Generator light

[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.