Hi Wortluck,
Here's a rundown that may help...
The dynamo makes a VERY small amount of electricity when spinning if the "F" wire is not connected. When the "F" wire is connected, on a normal regulator setup, the "F" wire gets power from (via the regulator) the "D" wire.
The connectors at the dynamo end are not great, and rely upon the wiring being passed through the small ferrules, and bent back along the full length of the ferrule, which is tapered. When the ferrule is pushed into the hole in the dynamo, it locks. I had problems here, as well as a failed regulator.
So if the "F" wire is loose anywhere, (the green wire), the dynamo gets no field current, and no output results. The current flowing through this wire is not a lot, about 2 amps max on a 6V system. The original Lucas regulator switches this on and off RAPIDLY, and modern electronic regulators do the same. The "mark/space" ratio of this signal (how long it is on, compared to how long it is off) controls the dynamo output. Long "ons" and short "offs" result in the dynamo giving a high output, and vice versa.
The first thing to do is check the output of the dynamo, using a headlight bulb. A 12V bulb on a 6V system, and a 24V bulb on a 12 volt system. You join the "F" and "D" wires together, having removed them from the regulator. Individually insulate the REGULATOR "F" and "D" connections to prevent accidental connections while testing. Start the bike and run at a fast tickover, THEN connect the bulb between the joined "F" and "D" wires, and earth. The bulb should light, and quite brightly. Do not do this for long, stop the engine, because you are now giving the field coil of the dynamo more than it is used to, or wants... If this test fails, repeat at the dynamo itself. If OK, the wiring is faulty. If no good, then I am afraid the dynamo is dead
The reason for not connecting the bulb before starting the engine is that it is a non-linear resistance. When off, it is very low, and "steals" the meagre amount of electricity that the dynamo is making, meaning that the field coil does not get energised. If you do connect the bulb before starting, you will find that it does not glow, and as you increase the revs, it will suddenly come on very bright, and could even burn out. (As the bulb warms up, its resistance increases, the demands it places on the dynamo reduce and this allows the field to get current, so the dynamo starts to produce a high output).
If the bulb reliably glows when connected, (and it will probably glow dim, even at tickover), the next test is done with another wire which is joined to the already joined "F" and "D" wires, and, with the engine already running at a fast tickover, put the other end of this wire onto the "A" terminal of the regulator. (Which must still have it's "normal" wire connected, and you now have "F", "D", and "A" all joined). You should then see a charge on the ammeter. Don't do this for long, and disconnect the wire to the "A" terminal BEFORE stopping the engine.
Start the engine and leave it running at TICKOVER with a bulb connected as above (and glowing dimly), and start pulling the wiring around. At this point, you may wish to have the seat off, and the headlight out... Again, don't do this for too long, although at tickover, the dynamo is getting almost its full rated field current. So it is getting "kicked" not killed...! If you don't find any loose connections, stop the engine and reconnect everything normally. If it does not now charge, it looks like the regulator.
Now... Interpreting what the ammeter is telling you. Electricity flows through it in both directions, but not at once! If electricity is flowing from the battery, towards the bike's electrical system, (lights) the ammeter will show a discharge. If the electricity flows from the system (dynamo) towards the battery, the ammeter shows a charge. And if everything is working normally, the dynamo makes just the right amount to run the lights, plus some for the battery, a small amount if a charge is needed, and "next door to nothing" if the battery is fully charged.
The dynamo regulator responds very quickly to load changes. It works on voltage, and wants to see a certain voltage on the system. If that voltage is low (battery being drained by the lights) the regulator instructs the dynamo to generate more electricity. It does this by supplying more power to the dynamo "F" connection. So it won't wait for the lights to produce a discharge for a while, and then decide to charge the battery, it will immediately sense that the lights are taking power (and dropping the system voltage), and "up" it's output.
If you are seeing a discharge with the lights on, that stays that way for a while at a fast engine speed, and then suddenly see a small charge, the charging system has an intermittent fault. What has happened here is that the charging system has failed, the battery has started to run down, and suddenly the charging system has sprung back into life, detected a low battery voltage, and pumped in some charge. The system is sensitive, and will react to less than 1/10 of a volt.
If (with the engine above tickover) you see a discharge with the lights on, and don't see a charge when they are off, the charging system has failed!
If all is working well, run at a "50mph in top" engine speed with the lights off. There may be a small charge. If you "whack" on the headlight, you will see a momentary discharge, and the needle will rapidly climb back up to the middle, if not go slightly into the charge side. This can be seen because when the bulb is off, it is cold, and has a very low resistance. At the instance of "switch on", it totally overloads the electrical system, (for a few thousands of a second, that bulb is possibly demanding 150 watts!!!) and although the regulator and dynamo have responded, they can't keep up. As the bulb warms up, the overload disappears, and all is well.
Dynamos.... Clean commutators, decent brushes (check that the copper "pigtails" are not loose in the carbon brush) and check all connections.
Regulators. Original Lucas, clean and adjust as per service instructions/Haynes manuals. Electronic regulators. Black art. If dead, replace!
Well, that was a "war and peace" of the charging system, hope it helps, I am gonna shut up now!
Regards,
Stan.
