Converting your 6-volt Ford flathead car, truck, or tractor to 12 volts does not require replacing everything associated with the electrical and engine circuits. The original starter, switches, gauges and ignition points will handle the increased voltage. Part of the conversion includes adapting a larger 12-volt battery in the engine compartment and retrofitting a mount for an alternator. The least complicated aspect is changing the interior and exterior light bulbs. Roll out your box of standard gauge tools, along with a repair manual or wiring schematic of the electrical circuits in your Ford flathead vehicle.
Disconnect the battery cables from the 6-volt battery. Remove the battery retainers and lift the battery out of the engine compartment. Remove the old battery box from the engine compartment.
Mount the appropriate battery box for your new 12-volt battery. Modify the box as needed for a reliable installation. Install the 12-volt battery and secure it with the new retainers. Do not connect the cables at this point.
Loosen the wire connectors at the back of the 6-volt generator and detach the generator wires. Remove the belt adjustment bolt from the adjustment bracket. Remove the mounting bolt at the base of the generator and take the generator out.
Establish the best way to mount the 12-volt alternator. You may be able to use the existing belt adjustment bracket and modify the existing generator mount on the engine block. Make the necessary adaptations and mount the 12-volt alternator.
Reconnect the original generator wiring to the appropriate terminals on the alternator.
Install a new alternator v-belt on the fan and alternator pulleys. Adjust the belt tension to 1/4-inch deflection.
Locate and remove the 6-volt ceramic resistor at the firewall in the engine compartment. Disconnect the wires from the resistor. Install a new 12-volt enclosed resistor at the same location and reconnect the original wires to the new resistor.
Remove the 6-volt ignition coil from the coil bracket. Pull the coil wire from the cap of the coil. Remove the positive wire and ground wire from the coil. Reattach the positive and ground wires to the appropriate terminals as marked on a 12-volt coil. Install the coil in the original bracket and push the existing coil wire into the cap.
Disconnect the battery wire and ground wire at each side of the 6-volt solenoid. Pull the starter lead off the terminal on the solenoid and remove the unit from the side of the engine compartment. Install a 12-volt solenoid in the same location and reattach the battery wires and starter wire to the appropriate terminals.
Disconnect three wire leads at the old headlight relay in the engine compartment. Remove the relay and mount a 12-volt relay in the same location. Reconnect the original wires to the appropriate hot, ground and neutral connectors on the unit.
Remove the headlight trim rings. Pull the 6-volt headlight lamps out of the sockets and compare the plug-in tabs with the tabs on your new 12-volt headlight lamps. Most 6-volt lamps have two tabs, compared to three tabs on 12-volt lamps.
Loosen the screws at each headlight casing and pull the casings inside the engine compartment. Decide whether to install new casings or adapt 3-tab female connectors to the exiting casings and run an additional wire lead to the headlight relay. Complete the headlight conversion based on your decision. Install the new headlights and reattach the trim rings.
Replace the 6-volt tail light lamps with 12-volt lamps. Change out the interior overhead lamps and the instrument panel lamps with 12-volt lamps.
Refer to the repair manual or wiring schematic for voltage resistance values. Test the new circuits with a voltage meter before connecting the battery cables.
Remove the 6-volt components and steam clean the engine and engine compartment before making the conversion. Converting to 12 volts will not correct a bad starter, solenoid, ignition, or wiring. Evaluate or test these components and replace as necessary.
Do not rely on a voltage tester for resistance values. Use a voltage meter. Properly dispose of the old battery and ignition coil.