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Old 06-29-2017, 05:23 PM   #1
Maclab58
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Default 12 volt negative ground conversion

What is required to convert my cabriolet to 12v negative ground?
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Old 06-29-2017, 05:44 PM   #2
Synchro909
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Default Re: 12 volt negative ground conversion

Hope I don't forget something but here goes.
You'll need a battery, 12v alternator or generator, ignition coil (or ballast resistor), a few light globes and I suggest a resistor each for the horn and windscreen wiper if yours is electric.
When I converted, I changed the field windings in the starter motor around so they are all in series rather than a combination. A kit is available for this but I do my own using the original field coils. This results in 4 times the resistance in the fields, thus reducing the current getting to the armature and slowing it down. Many say that the starter will be fine as is and I have seen many that have - for a while. I took the safe route. Another way of slowing the starter is to use small gauge battery cables so you lose a few volts in them.
It's not a big job and you can get replacements anywhere when needed.
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Old 06-29-2017, 07:07 PM   #3
Charlie Stephens
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Default Re: 12 volt negative ground conversion

One thing that usually gets left out until the end is the ammeter. Needless to say you will need to reverse the wires on the ammeter to get it to read properly. There is another thing you need to do with the ammeter. There is a wire that runs to the ammeter. It is probably 10 gauge. Run a second wire in parallel to this wire and connect it to the other terminal on the ammeter. Now half of the current will go through the ammeter and half will go around it. Your ammeter will now only read half of the current that is going to the battery. This is something you will need to remember when reading your ammeter. Since your new system is probably going to put out twice the current that the ammeter was designed for this is what you will want. There are also reproduction ammeters designed to handle 30 amps but I have been told that all of the reproduction ammeters leave a lot to be desired.

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Old 06-29-2017, 07:36 PM   #4
J Franklin
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Default Re: 12 volt negative ground conversion

Seems like a lot of work for doing the same function.
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Old 06-29-2017, 07:55 PM   #5
BillLee/Chandler, TX
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Default Re: 12 volt negative ground conversion

This might help: http://cedarcreekas.org/TechTopics/1...rn_Signals.pdf
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Old 06-29-2017, 08:32 PM   #6
Charlie Stephens
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Default Re: 12 volt negative ground conversion

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Be sure to check out the "brite" bulbs from Ron Francis Wire Works, https://www.ronfrancis.com, (800) 292-1940 once you get to 12 volts.

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Old 06-29-2017, 11:13 PM   #7
Mike V. Florida
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Default Re: 12 volt negative ground conversion

this is from Kens garage;

Battery

Any good 12 Volt battery will do as long as it will fit your battery box and hold down. I selected a 72 month, 675 cranking amp, universal battery.
Alternator

I installed a GM 63 Amp Alternator with an internal self exciting voltage regulator. The hook up is one wire, the same as the existing generator. You will need to make two small brackets to mount the alternator or you can use two shackle bars and drill out for the proper size bolts. An alternator pulley can be purchased from your friendly Model A parts supplier.
Starter

No modifications are necessary to the starter. Just touch the pedal and enjoy really fast starts.
Ammeter

I changed the standard 20 amp ammeter to a 30 amp ammeter as an alternator will peg the standard meter. You must reverse the leads on the ammeter due to the change in polarity of the battery. Your neighborhood Model A parts supplier carries the 30 amp ammeter.
Headlights

Since I installed an alternator I opted to go all the way for headlights and convert to Halogen bulbs. The conversion is relatively easy. You can purchase a kit to modify your existing reflectors or you can purchase new reflectors with the Halogen sockets installed. You must install an alternator to successfully operate Halogen bulbs.
6 Volt kits are available. What a pleasure to be able to really see while driving at night.
Ignition

You could install a 12 volt coil and a resistor to reduce the voltage to the points.
I elected to install a PerTronix IGNITOR electronic ignition system. This is an extremely easy system to install as it fits entirely inside the distributor; no outside boxes are required. Kits are available for the standard Model A or B, early V8, and Mallory distributors. Both 6 and 12 kits are available.
Kits are furnished with a new coil as the correct coil resistance is very important for proper operation. Kits are distributed by: Remund Ignitions, Inc.
P.O. Box 857
Lemon Grove, CA 91946-0857
(619)460-3620
Horn

Getting the horn to work properly was the most perplexing part of the conversion. I tried several "voltage reducers" from the local parts stores but none could handle the current demands of the Model A horn. I finally purchased a 0 to 2 Ohm variable power resistor with a 100 watt rating from an electronics supply house. I selected a variable resistor in order to optimize the voltage to the horn.
The resistor I purchased was am Ohmite D100K2RO ($15.87) with two Type 12 brackets ($0.55 ea.) (Newark stock nos. 13F671 & (2) 13F099) from:
Newark Electronics
1-800-463-9275
Branch offices of Newark are located throughout the United States. Minimum order is $25.00. An alternate approach would be to puchase a 12 volt horn.
Light Bulbs

The following bulbs can be used with a 12 volt system: Cowl, Tail, and Dash Lights - #89, 6 cp
Dome Light - #67/97, 4 cp
Stop Lights - #1156/10, 32 cp
Turn Signals

If you have a Signal-Stat turn signal it will be necessary to change the flasher to a Signal-Stat #180 and the bulb to a #1445.
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Old 06-30-2017, 01:35 AM   #8
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Default Re: 12 volt negative ground conversion

Some people seem to prefer to use a ballast resistor on the coil, to keep some of the heat away from vital coil components.
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Old 06-30-2017, 11:55 AM   #9
Growley bear
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Default Re: 12 volt negative ground conversion

To what end is a 12V Negative ground system conversion?
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Old 06-30-2017, 07:12 PM   #10
joel rapose
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Default Re: 12 volt negative ground conversion

Remember....the polarity connection from the points to the coil must match the battery ground polarity.
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Old 06-30-2017, 07:37 PM   #11
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Default Re: 12 volt negative ground conversion

LED tail lights, so they see when I'm stopping and sealed beam headlights so I can see when I'm going
Problem solved
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Old 06-30-2017, 07:57 PM   #12
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Default Re: 12 volt negative ground conversion

I purchased a kit which came with alternator, coil, ballast resistors etc., in fact everything you need except light bulbs.
At the time, a few years ago it wasn't expensive, and being a speedster, I had bought replica headlights which came with 12 volt halogen bulbs which give good light. ('28-'29 style in stainless steel which has the softer look of nickel plating)
I converted the taillights to LEDs from an on-line supplier, and these are almost too bright if that's possible?
From memory, only the dashboard light proved a little challenging in finding a 12 volt replacement.
I had the starter motor checked over by an auto electrician and new brushes fitted and he said it would be fine with 12 volts so long as it wasn't used for extended periods of continuous winding.
The motor has a 7:1 head and the starter copes effortlessly.
So far 6000 miles later, no problems and plenty of charge for night time driving.
Keith
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