In Reply to: ammeter / light posted by Peter Nelson on October 23, 2018 at 08:11:18:
Like Jack stated, be sure to follow the correct wiring PICTOGRAM supplied by JMOR for your 8N. The May, 1949 MPC has the lighting diagram -see LINK below. Though you say it is 6V, is the circuit wired correctly throughout? The generator should be a 3-wire/3-Brush unit, with a Voltage Regulator, and ballast resistor in the circuit. Are you using original or repops of the original lights? The OEM wiring used 1 power wire to the lamps, and the ground was made to the metal body anywhere. A slight discharge on the AMMETER when the lights are ON is normal. The generator may or not be charging the battery. First is to verify ALL the wiring is correct. You can pull the battery, generator, and VR and take to your trusty local starter/alternator shop. He can bench test everything. If generator is in need of a rebuild, and he doesn't know these old Ford systems, I can do it -email is open. Sometimes with these early 8N generators they are missing the tiny belt tension arm, p/n 8N-10145, and thus create a non-charging issue because the fan belt isn't tight and the generator isn't charging properly. If you are missing this arm I have a bunch I made exactly like the original. If the battery is not sustaining a full charge under load, it won't have enough juice to crank over the engine and certainly nor run lights.
FORD '48-'50, PRIOR TO S/N 263843 GENERATOR & TENSION ARM: