25-05-2020, 09:40 PM
Hi again David H.
The warning lamp brightness depends on the difference between battery volts and dynamo ouput volts. When things are working properly, it will have (6 - 0) = 6 V when the dynamo is stationary and will therefore be fairly bright. As the dynamo begins to turn slowly, it will have for example (6 - 3) = 3 V and will be dimmer. When the dynamo turns faster it will have for example (6 - 6) = 0 V and will go out. A bit faster still and it will have (6 - 7) = -1 V which may or may not be enough to see it start to glow again very dimly. Any faster and the cutout will close at just over 7 V, beyond this point the battery voltage will be the same as the dynamo output as they are shorted together by the cutout contacts. So no more glow.
BUT, if the cutout doesn't close at 7V, the dynamo output will rise quickly as its speed rises, as it has no load. So you could easily get (6 - 12) = -6V, then (6 - 14) = -8V etc etc. The bulb doesn't mind that the voltage is now the opposite way round to normal, it will still merrily glow and will get brighter and brighter as you rev up, especially on the winter setting.
So from the symptoms you describe, it would seem that the your cutout is faulty. You have already measured the shunt coil resistance which appears to be ok, so some other effect must be preventing it from closing properly even at voltages way over 7V. I am guessing that you don't have access to a variable voltage power supply to allow you to test it off the car (or at least disconnected) so you may be stuck with testing it by substitution. Any 6V Austin Seven cutout will do if you can borrow a known good one. The terminals may be in a different order but I am sure you could figure out which goes where.
Hope this makes sense. Cheers, John.
The warning lamp brightness depends on the difference between battery volts and dynamo ouput volts. When things are working properly, it will have (6 - 0) = 6 V when the dynamo is stationary and will therefore be fairly bright. As the dynamo begins to turn slowly, it will have for example (6 - 3) = 3 V and will be dimmer. When the dynamo turns faster it will have for example (6 - 6) = 0 V and will go out. A bit faster still and it will have (6 - 7) = -1 V which may or may not be enough to see it start to glow again very dimly. Any faster and the cutout will close at just over 7 V, beyond this point the battery voltage will be the same as the dynamo output as they are shorted together by the cutout contacts. So no more glow.
BUT, if the cutout doesn't close at 7V, the dynamo output will rise quickly as its speed rises, as it has no load. So you could easily get (6 - 12) = -6V, then (6 - 14) = -8V etc etc. The bulb doesn't mind that the voltage is now the opposite way round to normal, it will still merrily glow and will get brighter and brighter as you rev up, especially on the winter setting.
So from the symptoms you describe, it would seem that the your cutout is faulty. You have already measured the shunt coil resistance which appears to be ok, so some other effect must be preventing it from closing properly even at voltages way over 7V. I am guessing that you don't have access to a variable voltage power supply to allow you to test it off the car (or at least disconnected) so you may be stuck with testing it by substitution. Any 6V Austin Seven cutout will do if you can borrow a known good one. The terminals may be in a different order but I am sure you could figure out which goes where.
Hope this makes sense. Cheers, John.