I research this when I first purchased my 55 TBird about 6 months ago. As it turned out my car had already been converted to 12 volts which I didn't realize. My car had a 12 volt battery, but the ground was positive. I reversed the cables, repolarized my voltage regulator and it has worked ever since. I believe my car still has the 6 volt starter and I haven't had a problem so far. All my gauges work. My radio had been converted to a positive ground and I'm afraid to hook it up. I also have the clock disconnected for the same reason. There are dozens on articles on the Internet about these conversions. Here is the text from the one that I found the most helpful. It came from the
H.A.M.B. website. I hope I am not breaking any rules by posting it here.
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Some of you are making way too big a deal of the 6 volt to 12 volt conversion on the 55 Thunderbird. For instance, you don't, or should not need a wiring diagram. You don't need to rewire the car. All the wire for the 6 volt system is twice as large as need be for 12 volt service. The only thing you need to change under the dash is the dash light bulbs and maybe the cigar lighter if you still smoke. The polarity for the clock must be changed from positive to negative ground. You may need to remove the clock to do this. There is a little tab that is designed to be positioned for positive or negative ground. Position it for negative ground. Major damage will occur if it is powered up in reverse polarity. You must install a 125 ohm 10 watt resistor in series with the 6 volt clock to drop the voltage to 6 or 7 volts. Of course the radio will need to be replaced, converted to 12 volt, or powered through a voltage dropping resistor. It is not polarity sensitive. You do not need to change any switches or gauges on the dash. The fuel and temp gauges will work on 6 or 12 volt and are not polarity sensitive. If you miss some of the dash lamps you can change them after they burn out, make sure you put in 12 volt lamps. The direction lamp flasher must also be changed for best performance but you can change it later. The fuses may need to be changed to values smaller to the ones rated for the 56 or 57 Thunderbirds. The tail lamps, the back-up lamps and license plate lamp will need to be replaced with 12 volt lamps. Now for the firewall forward: You do not need to change the battery cables, the horns, the horn relay or the starter. If converting to a generator install a 12 volt one and a 12 volt, 30 amp voltage regulator and wire it up exactly as the 6 volt ones. Or do the alternator conversion. This is done same as a 12 volt generator to alternator conversion. You will need to install 12V head lamps and park and direction lamps. No changes in lamp wiring or the light switch is necessary, leave it alone or repair it as necessary. Of course you will need a 12 volt battery; there are 12 volt batteries that will fit the 55 battery carrier with little or no modifications. The Motorcraft BTX-56 fits nicely and has 550 cold cranking amps. Just make sure to connect the battery to negative ground. The 6 volt coil can be used but the polarity must be changed, just switch the battery and distributor wires or install a 12 volt coil same as used on 56 through 73 Fords or get good used ones from salvage yards. The ones used on the 60 through 64 Falcons are good to use, they have yellow tops. Now for that very minor wiring change that has everyone confused. Install a 12 volt starter solenoid such as the ones used on the 56 through 73 Ford products. They are still cheap at AutoZone. It will have the extra I terminal, I as in ignition. Run a wire from the I terminal of the 12 volt starter solenoid to the battery terminal of the coil, remove the ignition wire that is presently attached to the coil and add a new wire about a foot long to the battery terminal before securing the nut. Now connect both the new wire attached to the battery terminal of the coil and the original ignition wire from the ignition switch to a new ignition resistor, Mororcraft #DY-35. When the job is done, before you start the engine, remove the field wire from the voltage regulator and strike it (draw an arc) to the battery terminal of the voltage regulator in order to properly polarize the generator then reconnect the field wire to its proper position on the voltage regulator. The heater blower motor, the seat motors and window motors will work on 12 volts but they will run very fast. The seat motor running too fast should be no problem. The heater motor running on low speed will be acceptable. Do not hook up the high speed wire so you do not inadvertently run the motor on high as it will burn out the motor. The window motors will require some expertise in use in order to prevent mechanical damage. If you want, you can install 12 volt window motors, but it is not required and they are expensive. You might try using a dropping resistor in the motor ground wire circuit to drop the voltage but I have not determined what the correct value would have to be. It will have to be a very high power resistor, maybe 1000 watts and it will give off some heat. I would recommend just using the window motors as they are and release the window switch before the window gets to its stops. This may take some practice but it is easy enough and like I said before, it will take some practice and finesse. When making a 6 V to 12 V conversions on a 1955 Thunderbird it is difficult to find a battery that fits the 1955 tray and hold down without modifications I have found that a Group 56 12V battery fits the tray and allows the hold down to be used without modification. The length of the battery is slightly shorter but covers well without showing. In fact it looks like it was made for the tray and hold down. Reprinted with permission from Gil Baumgartner
Footnote: If the 6 volt gauges do not work when the 12 volt conversion is made install a 10 ohm 10 watt wire wound resistor between the gauges and the power supply to gauges. If the gauges still do not work it may be necessary to replace the gauges and the sending units with 1956 12 volt units. Wire wound resistors can be purchased at Radio Shack.
