22-04-2020, 10:05 PM
Hi David H.
I'm assuming your car has the earlier Lucas SM5 switch panel which incorporates the ignition warning lamp. The main function of this lamp is to glow when the ignition is on but the engine isn't running, to save you leaving the ignition on inadvertently which would flatten the battery and/or overheat the ignition coil. It is effectively wired across the cutout contacts, so once they close there is no voltage difference and the light goes out. This is a standard widely used arrangement. There is no earth connection to the bulb.
The bulb centre "pip" is connected via a springy brass finger to the IGN terminal of the switchboard. The bulb threaded outer is connected to the +D terminal of the switchboard via an internal link (presumably brass strip). I have no information on the original bulb voltage or whether it has a series resistor like the later cars. If it does, a 30 ohm resistor is made using thin silk covered resistance wire wound in a layer or two round the outside of the bulb holder and a 2.5 Volt torch bulb is used. The resistance wire is interposed in series between the bulb outer and the +D terminal. It's quite possible that this wire may have gone open circuit, stopping the light from working. A test ohm meter would be able to show if this is the case but you might need to disconnect other wiring to get a sensible measurement.
Shorting out the resistance wire and using a 6 Volt bulb is often done, and appears to have no ill effects. In your case you would need to put an external link wire between the bulb holder outer and the +D connection. Only thin wire is needed; the max bulb current is a fraction of an amp.
If the Dynamo has healthy voltage output but the cutout contacts aren't closing, one thing worth checking is that the cutout has a good earth connection to the metal bulkhead. This may rely on fixing screws rather than an obvious wire. This earth is needed for the "voltage" coil which pulls the contacts closed when there is sufficient voltage from the dynamo. Once the contacts are closed and charging commences an additional "current" coil comes in to play to help the contacts to remain closed. In this way there is a difference between the closing and opening voltage, which prevents the cutout from chattering in and out.
I'm assuming your car has the earlier Lucas SM5 switch panel which incorporates the ignition warning lamp. The main function of this lamp is to glow when the ignition is on but the engine isn't running, to save you leaving the ignition on inadvertently which would flatten the battery and/or overheat the ignition coil. It is effectively wired across the cutout contacts, so once they close there is no voltage difference and the light goes out. This is a standard widely used arrangement. There is no earth connection to the bulb.
The bulb centre "pip" is connected via a springy brass finger to the IGN terminal of the switchboard. The bulb threaded outer is connected to the +D terminal of the switchboard via an internal link (presumably brass strip). I have no information on the original bulb voltage or whether it has a series resistor like the later cars. If it does, a 30 ohm resistor is made using thin silk covered resistance wire wound in a layer or two round the outside of the bulb holder and a 2.5 Volt torch bulb is used. The resistance wire is interposed in series between the bulb outer and the +D terminal. It's quite possible that this wire may have gone open circuit, stopping the light from working. A test ohm meter would be able to show if this is the case but you might need to disconnect other wiring to get a sensible measurement.
Shorting out the resistance wire and using a 6 Volt bulb is often done, and appears to have no ill effects. In your case you would need to put an external link wire between the bulb holder outer and the +D connection. Only thin wire is needed; the max bulb current is a fraction of an amp.
If the Dynamo has healthy voltage output but the cutout contacts aren't closing, one thing worth checking is that the cutout has a good earth connection to the metal bulkhead. This may rely on fixing screws rather than an obvious wire. This earth is needed for the "voltage" coil which pulls the contacts closed when there is sufficient voltage from the dynamo. Once the contacts are closed and charging commences an additional "current" coil comes in to play to help the contacts to remain closed. In this way there is a difference between the closing and opening voltage, which prevents the cutout from chattering in and out.