17-04-2018, 08:04 PM
(This post was last modified: 17-04-2018, 08:06 PM by John Cornforth.)
Hi Alan
The ignition warning light has one side connected to the +6V supply to the ignition coil (downstream of the ignition key switch) and the other side connected to the dynamo output. When the engine is not turning (or only at cranking speed) the voltage from the dynamo is near zero, so the light has 6 volts across it and it lights up.
Once the engine starts and speeds up the dynamo voltage rises, the voltage across the light therefore diminishes and it wil go dimmer. At sufficiently high engine speed (typically 1000 RPM) the cutout contacts close, the voltage across the light is then zero and it will go out completely.
The purpose of the light is to remind you not to leave the ignition switched on for too long without the engine running, as this can (eventually) overheat the coil and flatten the battery.
If the light isn't glowing with the ignition key switch on and the engine stopped or only at cranking speed, then you have a problem and the car will not start, so you need to fix this aspect before tackling anything else.
I suggest getting hold of a test meter (they are remarkably cheap these days) and checking the voltage between the SW (switch) side of the ignition coil and ground, you should see 6 volts with the ignition on. Most likely cause of your trouble is wear in the ignition switch internals, so try jiggling the key or turning it off and on again. As a work around it's possible to wire an external ignition switch in parallel with the original, leaving the rotary section of the switch to deal with only the lighting and charging tasks.
Cheers
John
P.S. Shameless plug time - see my Ruby circuit diagram on the Dorset Austin Seven club website technical section, it's not that different to the 1932 era cars.
The ignition warning light has one side connected to the +6V supply to the ignition coil (downstream of the ignition key switch) and the other side connected to the dynamo output. When the engine is not turning (or only at cranking speed) the voltage from the dynamo is near zero, so the light has 6 volts across it and it lights up.
Once the engine starts and speeds up the dynamo voltage rises, the voltage across the light therefore diminishes and it wil go dimmer. At sufficiently high engine speed (typically 1000 RPM) the cutout contacts close, the voltage across the light is then zero and it will go out completely.
The purpose of the light is to remind you not to leave the ignition switched on for too long without the engine running, as this can (eventually) overheat the coil and flatten the battery.
If the light isn't glowing with the ignition key switch on and the engine stopped or only at cranking speed, then you have a problem and the car will not start, so you need to fix this aspect before tackling anything else.
I suggest getting hold of a test meter (they are remarkably cheap these days) and checking the voltage between the SW (switch) side of the ignition coil and ground, you should see 6 volts with the ignition on. Most likely cause of your trouble is wear in the ignition switch internals, so try jiggling the key or turning it off and on again. As a work around it's possible to wire an external ignition switch in parallel with the original, leaving the rotary section of the switch to deal with only the lighting and charging tasks.
Cheers
John
P.S. Shameless plug time - see my Ruby circuit diagram on the Dorset Austin Seven club website technical section, it's not that different to the 1932 era cars.