What is it for and is it necessary????
A generator cutout provides a path for current flow from the generator to the battery and permits charging the battery when the engine is running. When the engine is stopped the cutout opens and prevents the battery from discharging through the generator. A voltage regulator or a diode can be used as a substitute and are available through the supply houses. If the cutout were to fail open the generator would suffer serious damage and require expensive repair. If the cutout were to fail closed the battery would discharge through the generator and thus lose its charge. The available voltage regulator (VR) replacement for the cutout provides better control of the battery state of charge and provides a fail safe current path for the generator in case the VR fails. At least that is my understanding of the situation.
It's necessary. If you don't have one, your battery turns your generator into a toaster when the engine is off.
The original cut out is an on-off switch that is not a voltage regulator. There are several modern options to replace the original cutout.
Not only will it turn your generator to a toaster when not running, but it could also melt the Ammeter or other wiring. Could even cause a fire.
It's a good idea to periodically watch your Ammeter to see that it is charging when you are driving, and not discharging when you turn off the engine. A large like 20 amp fuse in the circuit will keep the battery from melting the generator while parked, but if it goes open while driving, you could still melt the generator because it has to charge the battery, and without the battery, it will send a high voltage into itself causing it to melt down. The battery actually becomes a voltage regulator with the 3 brush generator.
A diode can also go open or short causing the same problem. I am told that the voltage regulator sold by the vendors has a circuit that will ground the generator if the voltage regulator fails but I have no experience with one.
For the short daytime trips I usually make in my T it doesn't matter if the generator doesn't work. It runs on magneto and I rarely use the ligthts. I get used generators from swap meets and can usually get one to work by swapping parts until I have a spare that works, so I don't worry about an "expensive rebuild". Even though it's possible to have a melt down, and it does happen sometimes, I haven't had one in years of driving Model T's and Model A's. I did have a 35 V8 which used to blow the light bulbs at night, because the battery cable was no good but it didn't cause the generator to burn up. I replaced the battery cable and the light bulbs and solved the problem.
Excellent snyposis of the overall situation.
Ron the Coilman.
Thanks guys....sounds like a voltage regulator is really the best solution though.
Since my company makes the Voltage Regulator I sure don't want to talk you out of getting one ha ha but I do want to chime in and set the record straight. Diodes NEVER fail OPEN. They always fail shorted. They can be exploded to an open circuit by massive over current but assuming something near "normal" usage - they fail shorted. I totally agree that one large fuse is a very good idea but mainly to protect against wiring shorts from frayed wires and errant tools while tinkering. Since the T system is essentially a 20 amp max system, the normal fusing guidelines call for fusing a circuit at 1.25 times its "carry" current hence a 25 amp fuse would be my recommendation. Since fuses CAN cause trouble because they are yet another connection point, I really don't recommend more than one main fuse since the system is so simple. I don't think it is very likely at all that a stuck mechanical cutout or shorted diode type cutout can cause your T to catch fire and I say that because the T generator winding represents about 1 ohm of source resistance so when the cutout sticks - the battery being 6.3V and the generator being 1 ohm - the discharge current will be 6.3 Amps and the power it dissipates in the form of heat will be a shade over 36 Watts which isn't very much and unlikely to generate enough heat to do much to even harm the generator since the same parts that are getting the 6.3 Amps are the ones designed to put out over 15 Amps. Astuck cutout or shorted diode cutout certainly WILL run your battery totally dead in several hours assuming it was fully charged and one good total discharge will certainly do in your battery or remove serious life out of it. Finally I want to say that it is totally true that the Fun Projects Voltage Regulator is designed to FAIL SAFE by shorting the generator to ground and opening up the battery side. It will give up its life to save the generator in the event that something is wrong with the electrical system and the generator is not responding properly either due to itself being haywire or the car being screwy. There is also a special circuit inside each VR that will automatically override the normal regulator circuit whenever the VR senses that the battery has become disconnected. The VR will completely shut down the generator and then periodically check to see if things are normal. If they are it will reset itself and all is well. If the battery is still not connected - it will go back into shut down mode. This circuit is designed to save your generator if at all possible. Since the battery is disconnected when this happens - there is little it can do to save your battery. It is NOT recommended that you ignore this condition and let the VR operate this way since eventually the VR will overheat and FAIL SAFE if you ignore it.