I have been troubleshooting a dead battery problem in a 1955 Thunderbird I just bought.  My 6 volt charger reads fully charged, so I noticed someone put a Duralast 56 DL battery in the car.  According to the Internet, a 56 DL is a 12 volt battery.  The car still has the positive ground.  Everything seems to be working including the radio, fuel gauge, clock, signal flasher, horns and lights.  Is it possible the former owner converted the car to 12 volts with a positive ground?

If you have a Voltmeter You can verify if it is in fact a 12 volt Battery. I guess it would work 12 Volt + Ground but the way to go is 12 Volt - Ground. There should be resistor somewhere to  cut Voltage to 6 for Gauges and Radio if it is 12 Volts. Are the Bulbs in Lights 12 Volt? Is the Coil a 6 Volt or 12 volt? You are right about the 56 DL being 12 Volts. I have never heard  of anyone converting to 12 Volts and leaving it + Ground. Interesting. On the Coil where the Small Wires attach there is a + on one Terminal and a - on the other. If its 12 Volt + Ground the terminal to the Distributor will be a +. If its - Ground and someone has the Battery hooked up backwards right now that Terminal to Distributor will be - on the Coil.

  

The coil is smaller in size than a regular sixties style yellow top coil.  It has two posts.  One post reads DIST and the other reads BAT.  The DIST side is connected with a wire to the side of the stock 1955 Load-A-Matic style distributor and the BAT side is connected to a ballast resistor.   The positive cable from the battery is connected to ground.  The negative side is connected to a 6 volt single small post solenoid.  Everything looks exactly as it should in a 6 volt car except the battery is a 12 volt battery.  I am currently charging the battery with a 12 volt charger.

That's an interesting One You have Phil. If its a 12 Volt System there should be a 12 Volt Style Solenoid with 2 small Terminals and a Resistor to drop the Voltage to Points. When You get it started after charging take a Voltmeter and connect to Battery. See what the charging Rate is from Generator. 12 Volt should be somewhere around 12.9-13.1. 6 Volt 6.9 approx. Interested to know what the story is with your Bird.

  

The battery is now charged to about 70%.  The car starts and runs fine.  A volt meter across the battery terminals now reads about 11 volts.  As a test, I started the car with the meter attached and the voltage increases slightly as I rev the motor.  This tells me the generator is working.  There are no markings on the generator or voltage regulator, so I can't tell if they are 6 or 12 volt units.  From what I have read, this is not suppose to work.  At a minimum I would have burnt out the radio, clock and gauges.   They all work perfectly.   I don't see any additional resistors, but I haven't looked under the dash yet.   I chould switch the polarity, replace the starter solenoid and coil, reroute the wire to the distributor to change the car over to 12 volts entirely. I'm worried that changing the polarity will hurt the radio and clock. but they are running on 12 volts now.  Is there any disadvantage to keeping the car 12 volt positive ground?

With the Car running it should show about 12.5- 14 across the Battery Terminals when You increase idle Speed. At Idle it would show about 12 as there is not much coming out of Generator until U raise Engine Speed. I would agree with You that its a 12 Volt System but it should not work without resistors for Clock  Gauges and Radio. The ignition should have a 2 Terminal Solenoid with a bypass Wire from the "I" terminal to the "Bat" Terminal on Coil to get a full 12 Volts to start Car. I would assume there is a full 12 Volts all the Time to Coil which will shorten Point Life because no Resistor to drop Voltage to less than 10 once the Car Starts. I am not sure what would need to be changed to make it  12 Volt Neg. Ground like almost every 12 Volt Vehicle. Maybe Someone else could answer that Question. 

  

The starter solenoid is definitely a six volt style with one post (no ignition post).  This car is running a 12 volt battery with all the six volt hardware and positive ground polarity and everything works.  I think someone swapped out the battery to a 12 volt unit some time in it's life with no ill effects,  The battery is not new.  I'm not sure why this works, but it exists.  I think I'll do a little more research before taking any action.  I have a feeling these old cars are a lot more flexible than we think.

I disconnected the clock and the old radio.  I'm going to change the whole car over to 12 volts and be done with it.  Thanks for the input.

Sounds like U already have 12 Volts. I would check the Gauges for a Voltage Drop Device. They will not work very long on 12 Volts so I suspect there is a Resistor somewhere. Maybe on the back of the Dash Panel. The only Year that Ford used 12 Volts to power the Gauges was 1956. After that there was a little Device on the back of the Instrument Panel to drop the Voltage to the Gauges.

  

I have determined this '55 T-Bird has been converted to 12 volts positive ground. I pulled out the radio and it is a 12 volt positive ground conversion. I disconnected the clock as I read reversing the polarity will harm it.   I suspect the generator, coil and voltage regulator are also 12 volts.  The car runs great and everything works fine.  The starter solenoid is the 6 volt style with only one small post. I disconnected the battery cable while the car is running and the engine stays running, so I assume the generator is working.   It looks like all I need to do is to switch the polarity from positive ground to negative ground and I should be good to go to the next step.  I know I need to re-polarize the generator when I do this.  I have never done this before, but it doesn't look hard.  Any pointers?

After that's done, I am going to replace the 55 distributor with a rebuilt 57 distributor with points and change the intake to a 57 with a Holley carb. I will also replace the starter solenoid and reroute the "I" wire to the ballast resistor.  I have a set of '57 "G" heads to go on the car later.  My goal is to end up with a 12 volt negative ground car with a 1957 Ford ignition and top end.  Does all this sound like I'm on the right track?