[OG68]

I did a little research this morning and I went to my incomplete restoration with pre-conceived ideas of how this system works. But as I found out and with further reading the Diagnosis manual more in depth, I then drew out the circuit on paper and the A-HA moment popped into my head.

This is what I now understand; See drawing
At the Ignition switch Accessory position, Battery power is applied to the ACC terminal. One wire (brown) feeds the wiper, radio, heater fuse circuits, the other (Brown/White, +- 22 ohm) provides a reduced voltage path to illuminate the GEN light.
Since the circuit that normally supplies 12 volts to the indicator lights is de-energized in the ACC position, the Oil Pressure circuit provides the ground path. So current through the indicator light runs opposite of normal.

When in the Ignition Switch is in the run or IGN 1 position, (engine not running) battery power is supplied through the IGN 1 terminal through the GEN light and through the voltage regulator and Alternator field windings to ground. Bypassing the 22 ohm Brown/White resistance wire, the GEN light shines brighter. The Oil Pressure light will also light but not the Water Temp.

With the ignition switch is the RUN or IGN 1 position and the engine is running, voltage is generated and fed through the VR to terminal H of the firewall connector to the GEN light. Since this voltage is equal to the supplied voltage on the opposite side of the GEN light, GEN is dark.

If there is a failure of the charging system, battery voltage again is greater than supplied voltage from the VR. The GEN light again has a ground path and lights brightly.

For the bulb test circuit;
In the start position, only the Temp and Oil Press lights come on. To do this the ignition switch provides a ground for the water temperature light. Oil pressure will be zero at initial cranking and provides its own ground until oil pressure comes up. The oil light will eventually go out if cranked long enough
The GEN light is lit when in the 'Run' position while transitioning to the start position but not during starting.

And since current flows both ways in the GEN light circuit a polarity sensitive LED bulb would not work properly.

I wish my car was running to verify this, but its how I read the schematics, diagnosis manual, and service manual and verified with an ohm-meter.

I welcome critiques and please point out any errors that you see. Maybe someone can verify this with their own 68 Tempest/LeMans/GTO

[tom s]

I use a VR similar to this on my 63 Lemans.Tom

[dataway]

OG, certainly sounds right to me. I'm very close to functioning exactly as you describe ...

I AM getting a very dim GEN light in ACC, also getting dim (but a tad brighter) in the IGN run position, and I'm getting the full bright light in the transition to engaging the starter.

If I had to imagine a problem ... it would be a bad connection/compromised 22 ohm wire ... as my "dim" is pretty freakin dim.

Your explanation of lack of LED function makes perfect sense, and exactly how mine behaved with an LED bulb.