Using a 1956 Wiring harness on 1955 for 12v conversion

Machinehand

Machinehand

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Oct 6, 2022
Thunderbird Year
1955
Hello Guys, I’m in the process of restoring a 1955. I have the car completely dismantled down to bare body and frame. The dash is out, dismantled and all wiring is removed. The dash harness is very brittle but intact. The front and rear harnesses are rough. I plan on replacing all three. I will also be converting to 12v.

My question to you guys that have done restorations and converted to 12v is:

Since I am replacing all three harnesses, and plan on going to 12v, would it make sense to just go with 1956 harnesses?

This car is very solid and will be a clean driver with an ECZ block, heads and intake from a 1957. I have no interest in entering it in any concourse events.

Thank you in advance
 

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I would go with the '56 harness. I'm sure you know all the items you have to change.
 
Something to consider is that the 6v. wiring is usually of a heavier gauge to lower potential resistance with less voltage available. It is usual to retain the 6v. wiring when converting to 12v. Things that need to be changed or have resistors installed inline to lower the voltage are the radio, heater motor, power windows, power seat, and starter. I know I am leaving something out. As headlights and bulbs can require change, The starter may survive if your car starts easily and it will spin the engine faster for startups. The same with the power windows, they go faster but can often survive. there is much about these conversions on various websites. The real problem areas are the continuous running radio and heater motor. Intermittent electrical items cam potentially survive but expensive to replace if fried. By the way while you have the dash out; paint the underside white. It will help you later when doing work under the dash is necessary.
 
Thank you Guys for responding.

Jack, you voiced my dilemma. I would rather have the heavier gauge wiring just for the added safety although I’ve never experienced any electrical issues in my other vehicles…that weren't man made! :mad:

I wonder how similar the harnesses are except for the wire gauge? I’m guessing almost identical.

My car has the power seat, radio, and heater…no power windows. The radio looks like new. I will probably install it and only plug in the light. I may remotely locate another radio, and use that one for tunes. I plan on installing a modern starter, so no worries there. I doubt I will use the fan but if I do, I could just keep it on low. I doubt I will be driving it in cold weather. Anyway, the heat from the exhaust will keep me warm. ;) As far as the seat goes, once I set it where I’m comfortable, I probably won’t move it. This is sad considering I just completely rebuilt and detailed the seat motors. Well, at least they will look good.

The rest of the conversion is technically simple including all of the re-bulbing.

Thanks for the tip on the under dash color. White will make visibility much better. Working on it out of the car is great, but installed…another story.
 
Machinehand said:
Thank you Guys for responding.

Jack, you voiced my dilemma. I would rather have the heavier gauge wiring just for the added safety although I’ve never experienced any electrical issues in my other vehicles…that weren't man made! :mad:

I wonder how similar the harnesses are except for the wire gauge? I’m guessing almost identical.

My car has the power seat, radio, and heater…no power windows. The radio looks like new. I will probably install it and only plug in the light. I may remotely locate another radio, and use that one for tunes. I plan on installing a modern starter, so no worries there. I doubt I will use the fan but if I do, I could just keep it on low. I doubt I will be driving it in cold weather. Anyway, the heat from the exhaust will keep me warm. ;) As far as the seat goes, once I set it where I’m comfortable, I probably won’t move it. This is sad considering I just completely rebuilt and detailed the seat motors. Well, at least they will look good.

The rest of the conversion is technically simple including all of the re-bulbing.

Thanks for the tip on the under dash color. White will make visibility much better. Working on it out of the car is great, but installed…another story.
Click to expand...
One more thing; If you are thinking of installing electric winds shield wipers the time to do that is when the dash is out. It is is much more difficult with the dash in place as I sadly know.
 
I would think the easiest way to go is the 56 harness. When I did the electrical systems over, I used the commercially available repro harnesses and have experience no problems with them. Done some 16 years ago.
 
If you are going to keep some items 6 volt, look into a voltage converter. I ran one years ago to keep a 6 volt dash in a Willys truck I converted to 12 volts.
 
74 Harley said:
If you are going to keep some items 6 volt, look into a voltage converter. I ran one years ago to keep a 6 volt dash in a Willys truck I converted to 12 volts.
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Good point. I believe 1956 changed to 12v gauges with the change to the 12v system. 1957 kept the 12v system but went back to 6v gauges with voltage regulators
 
s far as the gauges (fuel and temp) are concerned, you can use the the 55 gauges with a Instrument Voltage Regulator like what is used a 57 and up Fords and Thunderbirds. These are readily available..
 
paul5456 said:
s far as the gauges (fuel and temp) are concerned, you can use the the 55 gauges with a Instrument Voltage Regulator like what is used a 57 and up Fords and Thunderbirds. These are readily available..
Click to expand...
The fuel gauge and the temperature gauge receive power through the 5v. voltage regulator. this setup continued through the 60s
 
Machinehand said:
Hello Guys, I’m in the process of restoring a 1955. I have the car completely dismantled down to bare body and frame. The dash is out, dismantled and all wiring is removed. The dash harness is very brittle but intact. The front and rear harnesses are rough. I plan on replacing all three. I will also be converting to 12v.

My question to you guys that have done restorations and converted to 12v is:

Since I am replacing all three harnesses, and plan on going to 12v, would it make sense to just go with 1956 harnesses?

This car is very solid and will be a clean driver with an ECZ block, heads and intake from a 1957. I have no interest in entering it in any concourse events.

Thank you in advance
Click to expand...
In a word, NO. Yes, that will work but think of this. Wires carry current or amperage. If you have lower voltage, (6V vs 12V) it takes twice the current flow or amperage to do the work in a 6V car than a 12V car. Therefore, if you purchase the correct 1955 factory harness segments, you will actually have heavier wire than you would with a 1956 harness. If you do not care about originality, you may want to look at an after market harness. There are some advantages to these. They will have a fuse panel and a more modern system. This is good unless someone wants to restore the car to it's original state, then all the modifications will have to be removed.
 
Converting to 12 volts as stated in the first post will not require heavier wires
the 56 harness is a good idea imo
 
CSPIDY said:
Converting to 12 volts as stated in the first post will not require heavier wires
the 56 harness is a good idea imo
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You are absolutely correct. My point was that the factory 1955 and the factory 1956 harnesses are similar in price. If you are using an original harness, then you are better off with the 55 harness because it will be overly adequate for the 12 volt system and, should the owner ever decided to take back to original or he sells it to someone with that intent, there is no need to rewire the car again If you don't care and want a 12 volt system, then an after market Painless or EZ Wire kit will do the job better and give you the ease of a more modern system with fuse box.
 
If you have the specific electrical manual and Not just the mechanical shop manual, Everything is done in sections to be able to get more detailed. You may have to trace the harnesses by hand, eye and camera.
Who know what happened before you.
 
Machinehand said:
Good point. Before I started this project I assembled a complete set of 1955 shop manuals as well restoration manuals. When you completely dismantle a car, reference is everything. My memory Is pretty good, but not that good!
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Take pictures at every step and label, label, label. It's fun taking it apart but putting it back together is a 'whole different breed of cat.'
 
Machinehand said:
Good point. Before I started this project I assembled a complete set of 1955 shop manuals as well restoration manuals. When you completely dismantle a car, reference is everything. My memory Is pretty good, but not that good!
Click to expand...
Now with cell phone cameras and other digital camera options it is so easy and so helpful to take pictures of everything possible in the disassembly process.
 
Machinehand said:
Hello Guys, I’m in the process of restoring a 1955. I have the car completely dismantled down to bare body and frame. The dash is out, dismantled and all wiring is removed. The dash harness is very brittle but intact. The front and rear harnesses are rough. I plan on replacing all three. I will also be converting to 12v.

My question to you guys that have done restorations and converted to 12v is:

Since I am replacing all three harnesses, and plan on going to 12v, would it make sense to just go with 1956 harnesses?

This car is very solid and will be a clean driver with an ECZ block, heads and intake from a 1957. I have no interest in entering it in any concourse events.

Thank you in advance
Click to expand...
Machinehand,
You didn't mention which ignition system you're planning on using, points or an electronic conversion? The main difference being the Ballast resistor is not needed for electronic conversions.

Either the '55 or '56 wiring harness will be fine. As previously mentioned the '55 will have heavier gauge wires than necessary. And the '56 harness is more applicable to 12v. It will have the built-in Brown wire connection that connects the "I" terminal of the starter solenoid to the "+/ Ign" terminal of the ignition coil... for bypassing the ballast resistor while starting.

'57 heads... their oem Temp sensor is smaller and not compatible with an oem '55 dash gauge. Assuming it was working before and you still have it, save the '55 Temp sensor and re-thread the bushing in the head to accept it. 3/8 NPT
If you don't have the original Temp sensor and want the dash gauge to function, you may need to get a reproduction Gauge and Sensor pair that is made for 12v conversions. Sold by places like Casco, Hills, Prestige, etc.

If you can keep the original dash gauges and their sensors, they should work fine on 12v, they are not voltage and polarity sensitive. You can use the '57+ style dash gauge regulator if needed.
Both the clock and ignition coil are polarity sensitive and need their wiring reversed.
55-56 & 57+ style temp sensors.jpg
.
 
Last edited by a moderator:
dmsfrr said:
And the '56 harness is more applicable to 12v. It will have the built-in Brown wire connection that connects the "I" terminal of the starter solenoid to the "+/ Ign" terminal of the ignition coil... for bypassing the ballast resistor while starting.

'57 heads... their oem Temp sensor is smaller and not compatible with an oem '55 dash gauge. Assuming it was working before and you still have it, save the '55 Temp sensor and re-thread the bushing in the head to accept it. 3/8 NPT
Click to expand...

Great feedback. This is why I am leaning towards a 56 harness...the Brown wire for the solenoid and the simplicity. I like the idea of having the heavier gauge on the 55 but it is not necessary. I have updated ignition systems in the past to electronic so this will simplify things. This is my plan for the Tbird as well. I hate splicing wires if I don't have to.

The heads and intake I have are 57's so this would apply. I know the gauge works because I restored and bench tested it. I have not tested the sensor but it's still in the head so it must be a 57. I will probably end up getting a repro gauge and sensor.

Thank you for the great insight
 
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