Going Negative

Story by Paul Kile

In addition to their own particular brand of charm, many British classics also possess something else unique: a positive— ground electrical ‘system. While nearly all modern vehicles have electrical systems that ground the negative battery terminal to the frame, older British cars use a positive ground system, causing problems for owners trying to install modern electrical accessories such as audio systems, CB radios or cell phone chargers.

You could install a power inverter and electrically isolate the modern electronics from the car’s chassis, but this is complicated and risky. A better option is to convert your car to negative ground. Then you can easily install your new accessories. You’ll also be one step closer to swapping your generator to a modern alternator, providing the extra power you may need.

The process is relatively easy: Simply disconnect the battery terminals, switch any affected electrical components to negative ground, re-polarize the generator, and reconnect the battery terminals in the opposite direction.

Most electrical items on the car are unaffected by polarity changes. Lighting systems, diaphragm—style horns, turn signals, cigar lighters and starters will work just fine on either positive or negative ground. However, there are a few other things that need to be checked before making the switch.

Electric Fuel Pumps

If your car uses an SU electric fuel pump, it will be affected by the polarity change. Older SU pumps use an internal capacitor to limit arcing across the points, and these capacitors were set up for either positive or negative ground. These pumps will work for awhile with their polarity swapped, but the points will burn out after a few thousand miles of driving.

Later SU pumps feature a zener diode across the points, and while this setup does a much better job of protecting the points from arcing, the pump still will not tolerate reversed polarity. If you have one of these later pumps (identified by a cylindrical bulge on the plastic end cap of the pump), reversing the polarity will immediately fry the diode and cause the pump to stop working.

One remedy is to disassemble the pump and reverse the leads of the diode to prevent

this from happening. However, since fuel pumps seem to go bad more than any other component, this is a good excuse to buy and install a new pump that is designed for negative ground. Then, since a spare fuel pump is always useful, convert the old pump and put it into your spare parts bag.

Ignition Coils

Most ignition coils will work under reversed polarity, but the spark output will drop by 15 to 20 percent. This may not be a problem in temperate climates, but it could cause hard starting and misfiring in cold weather.

The easy solution is to reverse the wires to the primary connections on the top of the coil these are usually identified by “+” and “—” or “CB” and “SW.” This will only work with points—type ignition systems.

An older—type electronic ignition that was designed for positive ground (like a capacitive discharge module or Piranha or Lumenition optical trigger) will fry if switched to negative ground. You’ll need to replace your old ignition system with one designed for negative ground.


Most likely, you are switching the car to negative ground so you can install modern audio equipment. If you decide to keep the original radio, you must reverse the input to negative ground.

Many older radios that are compatible with positive ground have a slide switch or a reversible plug on the case that will switch the unit from positive to negative ground, without requiring any external wiring changes. Don’t forget to make this change before reversing the battery connections, or you will destroy the radio.

Dash Gauges and Tachometers

Some types of dash instruments, such as electric fuel and temperature gauges, will be affected by polarity changes. Others will not mind the swap, and determining whether or not the gauge will mind is relatively easy.

If the gauges are of the thermal element type—these move up slowly when the key is turned on—they should not be affected by reverse polarity. If the gauges respond quickly when the key is turned on, like the fuel gauge in an MGA, the connections on the back of the piece will need to be reversed. (If your gauges peg down toward the left when you turn the key to the on position, this is a sure sign that you forgot to reverse the wiring.)

Ammeters will still work under reverse polarity, but will show “discharge” instead of “charge.” You can reverse the wiring at the back of the ammeter also, but make sure you do this while the battery is disconnected.

If your car uses an electronic tachometer, this will need to be rewired for negative ground. There are several different types of Smiths tachs used on British cars, and people who are handy with a soldering iron should have no problem with the job. Your best bet is to search the Internet for “tachometer conversion,” as there are several very good step-by-step instructions available.

If you don’t want to risk ruining an expensive gauge, send it to a professional instrument shop, like one of those that advertise in this magazine. They can easily handle the conversion.

Small Motors

Most of the small motors found in the car will work fine with their polarity reversed.

Wiper motors usually work fine under reverse polarity, but don’t forget to remove the wiper arms from the spindles and test the system for proper parking position.

Heater motors, air horn compressors and electric windscreen washer pumps will usually run backward, so you need to reverse their power and ground wires before using them.

The Final Steps

When you’re ready to make the actual conversion, you will first need to re—polarize the generator. Generators retain a residual magnetic field in the stator coils. If this field is not reversed when the polarity is switched, the generator will put out a reverse charge to the battery.

To re-polarize, first disconnect the two wires from the terminals found at the back of the generator. (On most British cars, this will be a large brown wire and a smaller brown—yellow or brown—green wire.)

Before changing the battery connections, you may have to reverse the terminal connectors, since the positive and negative poles on the battery usually have different diameters. Check to make sure that the size of the terminal connector on the ground connection corresponds to the size of the negative terminal, and if not, change the positive and negative connectors.

Now, hook up the battery terminals in the new negative ground configuration, and re—polarize the generator. Connect a jumper wire to the positive (hot) terminal. Take the other end of the jumper wire and momentarily touch it to the small terminal at the back of the generator. You may see a small spark—this is okay. Now remove the jumper wire, reconnect the wires to the generator, and you are done.

Now you are free to add all sorts of modern audio equipment, cell phone chargers, satellite navigation and DVD players, but don’t go too far. After all, you want to retain that classic feel.

This story ran in an old issue of Classic Motorsports. Want to make sure you’re reading all the latest stories? Subscribe now.

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