Bela, as I understand it, the regulation is wholly dependent on the battery.
There is nothing else in the system, anyway. The cut out is just that - it cuts the dynamo out of the system if it isn't making 6 to charge the battery, and cuts in when the voltage hits 6 as required. Or 12, if the system has been modified.
The dynamo may not be regulated, but it is limited - it should only give about 8 to 10 amps at 6 volts, this is decided by how much you take off and feed back into the field windings.
So the power in the system (volts times amps) is under control. And there must be an upper limit to what the dynamo gives in volts. I doubt it would run a 48 volt system.
To repeat: there is the dynamo, the battery, and the cut out. Nothing else in the generating system.
Sorry folks, I must be particularly dim....
The warning bulb holder has a central terminal on a sprung brass link from the IGN terminal on the back of the switchboard. This is live when the ignition is on. It doesn't have any insulation that I can see, but simply passes through a small hole in the switch panel... the bulb then completes the circuit to (earth?) by the threaded holder (via the solder blob on the outside of the bulb under the glass)?
Unless I make a physical earth from the threaded holder - Or the blob - (& thus to the bulb), the bulb will not light when the ignition is turned on.
Is the threaded brass holder supposed to be the earth return? The wiring diagram for the 1932 RN front tank saloon -which is as near as I can find for an AG tourer (which few authorities (like Rinsey Mills!) acknowledge exists! -
simply shows the brass link to the bulb base, other diagrams (Dorset Cut-Out Explained) show the feed coming from IGN , round the bulb and on to a terminal "D" on the back of the switch panel. However I think this is for a later version....
The engine starts & runs fine, but the warning light - if I earth the threaded holder - stays on. I imagine because I have completed the circuit artificially.
What turns off the warning light? does the cut out shut off the brass strip feed somehow?
I will try & take some pictures to add to the fun.
I think I will change my tag to:-
Perplexed of Stratford!
Sorry...no! But...
That is the separate lamp fitted in the later 30's and up to the 60s....my TR had one & I had a brand new one until I sold it on ebay because I would never need one again.....I don't know if the holder on my AG has such a wire resistance wire. It does have a cotton cover so maybe it is the same system.I still can't see how it returns to the D+ terminal though! More head scratching.
Thanks for the idea .
D
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.
Here's a couple of photos, which may help. In the front view you can see a modern resistor I have fitted to replace the defective original wound round the bulb holder.
23-04-2020, 11:18 AM (This post was last modified: 23-04-2020, 11:41 AM by David.H.)
Thanks Dave....well,sort of! That looks as if I will have to take all the wires off and dismantle (again) my rather fragile panel. I will initially try & put a connection to D+ as suggested by John C. This may not be as easy as I think with my limited soldering skills! I do at least have a good solid earth wire at the cut out from the E terminal!
Well I connected a wire as suggested & the light lit up.....but no sign of it going out as the revs go up. Now it did work before I took out all the hideous wiring! I will try cleaning the cut-out contacts with some contact cleaner...They are nice & springy & look ok...... Will it help to now what voltage should I expect at D+ on the switch board and D+ on the cut out, with engine running? And indeed at SW on the switchboard which is from the F1 dynamo ? How can so few leads be so difficult...or perhaps there is something serious amiss?
David , way back in the 1920s/1930s Lucas used a 2.5volt 0.2amp MES bulb . This was a torch bulb and easy to obtain.The current ie 0.2 amp is the important part not the voltage of the bulb,as it is in series with the dynamo windings.I have replaced the resistance wire with a 27ohm .6 watt resistor
Are you seeing a charge on the ampmeter,?if so then it is only the wiring to the ing lamp that needs connecting.
Once some one puts extra wring in it can cause problems as you have found out.
Best of luck you will get there in the end.
