Glow plug circuit.

[IppoJ]

Can someone explain how the "cold start warning light" circuit works?
I know there is a resistor in parallel to the indicator light but when I check voltage to ground the outside of the light holder is ~6vdc and the center conductor to ground is 12vdc.

Is there something that 'indicates' the engine is cold or does the light just come on when the glow plugs have voltage?

[roverp480]

Can someone explain how the "cold start warning light" circuit works?
I know there is a resistor in parallel to the indicator light but when I check voltage to ground the outside of the light holder is ~6vdc and the center conductor to ground is 12vdc.

Is there something that 'indicates' the engine is cold or does the light just come on when the glow plugs have voltage?

The light comes on when the glow plugs are operated, but the intensity alters as the plugs heat up , see extract from Owners Manual ( Part No. 4482 Publication TP/317/H. Oct. 1965) . The light delay is because the resister coil changes resistance as the current passes. If later 12V heater plugs are fitted they are wired in parallel and the light is again on when the the plugs are switched on, but the light intensity is constant

The 6V on one terminal sounds about right as you are tapping into the wire between the battery and the frame halfway along a line of resisters . There is the dropper resister ( as you mention) one side and the heater plugs the other , so you are seeing 6V between the two.

[IppoJ]

Perfect, thank you.
And, I can still buy the owner's manual!

I sincerely appreciate the help.