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Changing Your Electrical System From Positive To Negative Ground...


Changing Your Electrical System From Positive Ground To Negative Ground Is NOT  Difficult

This is a quick lesson on changing the polarity of your 6-volt antique vehicle's electrical system from positive ground to negative ground. There is enough misinformation floating around on this subject to write a dozen books. My goal here is to teach you how to get it right the first time, and know why you did what you did.

First a little history lesson. Prior to 1955 when automotive electrical systems upgraded to 12-volts  and negative ground electrical systems became the recognized standard, some automotive manufactures like Ford build their vehicles using a positive ground electrical system, while others like General Motors (and the independents who bought their electrical components from General Motors) built their vehicles using a negative ground electrical system.

Technically, there is no advantage to either one. As an electrical engineer  you were taught electrical theory that said electrical current flows from the negative to the positive. A mechanical engineer learned just the opposite, that the current flows from the positive to the negative.

In the mid 1950's when automotive electrical systems were upgraded from 6-volts to 12 volts they also made negative ground electrical systems the recognized standard. This was done in part because of the modern solid state accessories being developed. Unlike mechanical driven  electrical accessories like heater blower motors that are NOT affected by polarity, solid state accessories like radios ARE affected and will be destroyed if they are connected to the wrong polarity.

Car radios began using solid state components to replace the old tubes in the late 1950's. Alternator charging systems (built using solid state components) were introduced in the early 1960's to replace the mechanical generators. Alternators provided increased electrical output and had less mechanical parts to wear out thus were more reliable and required less maintenance.

Most solid state devices are built using what are called diodes, which are simply one way electrical valves. They let current flow in only one direction. Without getting to technical you can imagine what happens when you try and send current backwards thru a diode in the wrong direction, things don't end up well and in some cases it causes the smoke to leak out and ruin your solid state accessory (like your cell phone).

Fast forward to today...and everything is built using solid state components. This includes your modern stereo radio, you ipod, your cell phone, and your GPS are are solid state devices built for negative ground.  I have power inverters available (see the parts section of the website) that will setup 6-volt electrical system current to 12-volts so you can power your modern 12-volt accessories with your 6-volt electrical system. These power inverters are designed to work with negative ground electrical systems, now you know why!

A Power Inverter Allows You To Power 12-volt Accessories From Your 6-volt System


When you reverse the polarity of an antique vehicle electrical system all that you are doing is changing the direction the current flows, but the current will still end up at the same place as it did that means your starter will not run backwards because the current from the battery still arrives at the same battery post on the starter that it did before. It is the same for your dash gauges and ignition coil. as long as you DO NOT change the wiring connections, all of those accessories will work as they always have.

So remember....DO NOT reverse to wires on the ignition coil ( an ignition coil is a step up transformer and takes battery voltage and steps it up to 35,000 volts where it is then sent to the spark plugs.) If you reverse the wires on the ignition coil you will reduce the output of the coil by 30 percent. That will appear as an engine miss at high speeds. If you are not aware of this common mistake you can replace all of the ignition tune-up parts only to still have the same engine miss at the higher rpms, that you had when you started. It should be the first thing you check.

Remember...if you do not change any wiring on the dash gauges they will work just as they always have.  Just like the ignition coil, just leave well enough alone!

Tech Tip - 
If you are converting your electrical system to 12-volts from 6-volts be sure and keep the original 6-volt senders in all of the gauges. Remember...your original 6-volt dash gauges are calibrated for 6-volts so they require a 6-volt sender. When you install a Runtz (see parts section of the website) the voltage will be reduced from 12-volts back down to 6-volts at the input of the dash gauge so you original 6-volt dash gauges and senders will work just like they did when your electrical system was still 6-volts.

Use A Runtz to make your 6-volt gauge and sender work with a 12-volt electrical system.

Using the gas gauge as an example if you install a 12-volt sender in the tank and connect it to your 6-volt dash gauge, your 6-volt dash gauge will read full all of the time. This is because the 6-volt sender works in a range of between 0 and 30 ohms while the 12-volt sender works between 30 and 90 ohms. So the top of the 6-volt scale is the bottom of the 12-volt scale.

Light bulbs are not polarity sensitive and will work on either positive ground or negative ground. You do not have to specify the polarity of your electrical system when you buy headlight bulbs or taillight bulbs for instance. Once you understand what you are doing and why... it becomes much easier to sort out the fact from the fiction.

Now you also understand why I build my 6-volt alternators as negative ground alternators. Because all modern accessories are negative ground I try to save you the social embarrassment of changing your apple iphone into a road apple because your electrical system was the wrong polarity. It can be a pretty expensive lesson.

If you stick with a generator charging system you will need to get a negative ground voltage regulator and polarize it to the generator to make your generator charging system work properly. Most everyone upgrades to an alternator with the internal regulator to gain the increased output at idle and low rpms which gives you the bright headlights and the fully charged battery for easier starting. Because everything is a bolt on it makes perfect sense.


Keep in mind that electronic ignitions ARE solid state... and ARE polarity sensitive so if you reverse the polarity you will also need to replace the ignition module inside of the distributor to match the polarity of your electrical system. Original ignition points and condenser are fine as long as you do not change the polarity on the ignition coil. When you upgrade to a 12-volt electrical system you only need to change the ignition coil to a 12-volt coil. The points and condenser will be the same for both 6-volt and 12-volt.


Reversing the polarity of your positive ground electrical system is simple! All that you need to do is reverse the battery cables (negative cable from the battery is now ground... the positive cable from the battery goes to the starter) then reverse the wires on the amp gauge or in the case of Fords...straighten the loop of wire out going thru the back of the amp gauge and you are done!

Fords used an inductive style amp meter which means that as the current passes thru the wire, the magnetism created is what moves the needle in the gauge. So you want the amp wire to come in from the opposite direction to make the needle more in the correct direction. If you fail change the amp gauge wires around...nothing bad will happen, the dash gauge will just read backwards. That's all there is too it!!

Ignition Coil Wiring

If your original ignition coil is positive ground the positive terminal of the coil should be connected to the distributor. The negative terminal of the coil should be connected to the ignition switch.

When you upgrade to 12-volts and replace your original 6-volt positive ground ignition coil with a modern 12-volt ignition coil... the wiring connections should be as follows...the positive terminal on the ignition coil should be connected the the ignition switch, and the negative terminal on the coil should be connected to the distributor.

Remember...if you reverse the polarity of your 6-volt electrical system, and keep your original 6-volt positive ground coil, you DO NOT reverse the wires on the original positive ground ignition coil. The negative terminal of the original positive ground ignition coil is still the battery terminal (that connects to the ignition switch) and the coil itself does not care which direction the current traveled from the battery to the negative terminal of the coil. The coil will still work the same as it always has.

If you are installing a Fifth Avenue 6-volt alternator and you are reversing the polarity of your electrical system to negative ground, the yellow exciter should connect to the negative terminal on the coil, the same terminal that goes to the ignition switch.

Buyer Beware...
When I first learned how all of this worked back in my younger days... there was a local shop who advertised that they would do a changeover of your electrical system from positive ground to negative ground for $150.00. They would say "have your antique car at our shop no later than 8:15 am in the morning and we will try and have your car done by 4:30 in the afternoon. With that time frame, one would assume that it was a big complicated job best left to professionals.

Now you know it took them less than 15 minutes to do the job. I have written hundreds of tech articles over the years and every time I explain how to reverse polarity, I get hate mail from a few shops who say ..."Hey...whats the big idea away a trade secret like that...who do you think you are...?"

Who am I...? I am the guy who explains to to do it yourself and how to get it right the first time and not pay some shop $150.00 for 15 minutes work. In case you are slow in math that is $600 an hour.  OUCH! We all deserve better!

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Since 1987, Fifth Avenue owner, Randy Rundle, has been making antique, classic and special interest vehicles more reliable and fun to drive.