A 1971 electrical system iussue

Hi all,
The electrical system works very well but there is something I don't understand.
The ignition red warning light when I turn the ignition key will not turn on: is connected correctly, a white wire goes to the master switch and the white/brown to warning light assimilator.
The ignition red warning light continues to be (correctly) off even when the engine is running.
What is known is that the red bulb holder (plastic) fided  in the headlamp is not grounded but when I connect the bulb holder (the WN wire) to ground the red ignition warning light comes on regularly.
What is the problem?
Cio.
Piero

pierodn said:

What is known is that the red bulb holder (plastic) fided  in the headlamp is not grounded but when I connect the bulb holder (the WN wire) to ground the red ignition warning light comes on regularly.

What is the problem?

Click to expand...

The most likely cause is a faulty assimilator.

pierodn said:

I try to replace it with another one that I hope to work!.
Is there a way to check that the warning light assimilator works?

Click to expand...

The easiest way to check would be to replace it with another (known good) assimilator.
[Edit]This is assuming the assimilator wiring is all in order and that the three wires are connected to the correct assimilator terminals?

Wire -Terminal
WN - WL
GY - AL
R - E

If the Red warning light works when the assimilator "WN" and "R" wire terminals are bridged (Ign. ON) then the assimilator is almost certainly at fault.

A color diagram if that helps. Taken from an old Haynes.

Try disconnecting the white/brown lead from the WL terminal on the warning light assimilator. Then temporarily disconnect the white/brown wire to ground or a red wire. With the ignition switch 'on' the warning light should illuminate. If it doesn't then you need to trace the fault to something other than the warning light assimilator. To do a real quick check on the warning light assimilator using a ohm meter, you should read 0 ohms between the WL terminal and the E terminal. That tells you that the switch contact is closed inside. You could also check the resistance of the resistive wire inside the assimilator by hooking the ohm meter up to the AL terminal and the E terminal. If you read no continuity or infinite ohms, the assimilator is bad.