The light is connected between the battery & the generator output. Before the engine starts, current is flowing from the battery (about 12.6 volts) through the light to the generator (zero volts). When the generator starts putting out, it's voltage rises to about 14 volts, the battery voltage also rises a bit, & with now nearly the same voltage on both sides of the light, it goes out.
In very rare cases, if a generator were putting out considerably more than 12.6 volts, There would be maybe 18 volts on one side of the light & 12.6 volts on the battery side & the light would again come on due to too high generator output.
First, check voltage at the large generator terminal to ground with a good meter & see if it rises to about 14 volts as the engine is speeded up. The voltage at the battery terminal should rise in a similar manner. If not, there are generator, regulator, wire connection problems, or a very loose belt.
There are several detailed references on the internet for checking the charging system. Here is one that may help. I'm not sure where it came from:
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How to test a Lucas dynamo
To test a dynamo, you need a multi-meter which can read 30 volts DC (Direct Current). Any cheap meter will do this job, as you will normally either get the reading you want, or no reading at all.
Before testing the dynamo, check the two cables connected to the rear of the dynamo. Gently pull on the spade connectors to ensure that they are firmly soldered or crimped to the cable. Any loose connection here will play havoc with the charging circuit.
Repeat the check at the other end of these cables which is at the voltage control box. This may be under the glove box on many small Fords, or mounted on the near-side inner wing, in the engine bay.
WARNING: The following procedure involves working in the engine bay with the engine running. Ensure there is no loose clothing and keep your hands and the connections to the test meter clear of the fan blades. Have an assistant available to switch the engine on and off.
Disconnect both cables from the connections at the back of the dynamo.
Set the meter to 30 volts DC.
Connect one lead from the meter to the chassis or body-work. Ensure a good electrical connection is made.
Connect the other lead of the test meter to BOTH terminals on the dynamo. You might need to twist a bit of wire around both terminals before connecting the lead from the test meter.
Start the engine and let it tick over. Connect the multi- meter's negative cable to the dynamo. Connect the meter's positive terminal to earth (somewhere on the car bodywork).
A reading of between 20 and 24 volts should be seen on the meter. This will vary depending on engine speed, but as a guide at about 750 rpm you should see about 15 volts, rising to over 24 volts at about 1,000 rpm. There is no need to rev the engine above 1,000 rpm when testing a dynamo. If the voltage reading is not present there is a fault.
* A reading of about 1 volt indicates a field winding fault.
Confirm a field winding fault in the dynamo as follows;
Set the multi-meter to Ohms and measure the resistance between the field winding terminal F and the dynamo body. Make sure you get a good connection between the multi-meter probe and the dynamo body. The resistance reading should be 6.2 ohms. If the reading is different from this suspect that the insulated coils are touching the dynamo body OR that the field coils are disconnected.
There is really nothing you can do about that except replace the dynamo.
* If the reading is 4 or 5 volts the armature windings may be faulty.
* If the reading is zero then either a dynamo brush is completely broken or there is some break in the testing circuit. Repeat the test.
If there is definitely no voltage reading then the dynamo needs new brushes or must be replaced.
If the reading is within the ranges indicated (15 - 30 volts) but you still suspect a problem in the charging circuit, test the voltage at the control box end of the F and D cables. (Remember to replace the F and D cables to the dynamo). If the voltage is other than 15 - 30 volts then there is a problem in the wiring loom.
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D
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
