Before 1968, most British sports cars adopted a positive ground electrical system, despite the fact that automotive manufacturers everywhere else in the world had switched over to negative ground many years before. Other than for cars displayed at the highest levels or where originality is a primary consideration, switching from positive to negative earth simplifies the addition of aftermarket electrical components and can accommodate alternator for more efficient charging.
Surprisingly, very few of the electrical components fitted in British sports cars are sensitive to polarity. Motors will sometimes run in reverse (not the starter, but sometimes the heater motor or wiper motor), while the generator and coil require minor attention to work properly with negative earth.
There are apocryphal stories that positive earth vehicles corrode more quickly than their negative earth counterparts. Although this could be true, it is more likely that the propensity for some cars to suffer from electrolytic corrosion gave rise to this belief.
Procedure
If the car is fitted with a positive earth alternator, you must first locate a negative ground unit before proceeding further. Otherwise, follow the steps below:
1. Reverse the batteries.
It is usually best to remove the battery from its holder, turn it 180° and reconnect it. Problems will be encountered with the original Lucas ‘helmet style’ clamps, as the battery posts are sized (the larger one is the positive post). It is wise to replace the battery terminals on an annual basis anyway. The helmet style clamps can be melted off the power cables to preserve the length of the cable.
2. Polarize the generator.
Remove the lighter gauge wire (usually brown with green or yellow with green on early models) from the ‘F’ (field) terminal on the back of the generator. Now, with a length of wire, flash between the ‘F’ terminal and a hot lead. The hot lead can be the positive post of the battery, the hot lead on a fusebox, or an ‘A’ or ‘B’ terminal at the regulator. Flash the wire only long enough to see a spark. DO NOT LEAVE THIS CONNECTED, even for several seconds. Just flash it a couple of times. Reconnect the proper wire to the back of the generator.
Now start the car and ensure that the ignition light is working correctly. It must act just as before, with the light remaining illuminated until approximately 1000 rpm, after which it should dim completely. Flickering in the light is an indication that the voltage regulator is faulty. If the ignition light should remain on when the car is turned off, IMMEDIATELY remove a battery clamp or the larger wire from the back of the generator (‘D’ – dynamo). If the condition is ignored both the wiring harness and generator will burn up.
The voltage regulator has a certain longevity, and sometimes, if the unit is about ready to fail, the switch in polarity will push it over the edge.
3. Reverse the coil connections.
Reverse the coil connections: if the coil is original, then it will be marked CB (contact breakers) and SW (switch). The SW terminal should now be connected to the distributor. In the case of the modern coil, the negative terminal should be connected to the distributor. This allows the spark to jump from the central electrode to the ‘L’ on the spark plug.
4. Test the heater motor.
Find the leads to the heater motor. Turn the heater motor on and judge the amount of air coming from the heater vents. Then, reverse the heater motor connections and again judge the airflow. Leave the wiring in whichever configuration moves the greatest amount of air.
5. Test the wiper motor.
Ensure that the wiper motor is working properly. Since most wiper systems are gear driven from the motor, it makes little difference which way the motor runs – although the thrust of the armature should be away from the commutator end.
6. Ammeter/Voltmeter connections.
If the car is fitted with an ammeter or voltmeter, the connections must be reversed.
7. Fuel Pump.
Most of the original fuel pumps (AC, Harting and SU) fitted as original equipment are insensitive to the polarity, but recent SU pumps, including the solid state versions that are now available, as well as some aftermarket units, are voltage sensitive. If you encounter one of these pumps or a transistorized unit, it’s probably best not to change the polarity of the car until another pump is found that can operate with negative earth.
8. Clock.
Some clocks are sensitive to polarity, while some are not. Until you attempt the conversion, you probably won't know, so keep that in mind.
9. If the car is fitted with a mechanical tachometer this section is inapplicable. An RVI tachometer is labeled on its face whether it is positive or negative earth and is driven by a pulse from the hot side of the coil. Modern tachometers are driven by the pulse from the distributor side of the coil, and are much improved over the older RVI units. Two changes are necessary to make the tach work: (1) the wires must be reversed at the ‘white wire loop’ at the back of the unit; and (2) the power and earth connections must be reversed on the inside of the unit.
a. The wire in the ‘white wire loop’ comes from the key switch and travels to the hot side of the coil. Select one of the wires and tag it with two pieces of tape for identification. Then cut the wire between the pieces of tape, and cut the other wire to the same length. Reverse the connections (now there is one piece of tape on each wire), and solder them (remember that this is the power lead for the coil and is unfused). Tape up the connections carefully. When later replacing the plastic block on the back of the tachometer, ensure that the metal band around the block is carefully positioned. This is a necessary half of the electromagnetic pickup
b. To reverse the power wire and earth wire on the inside of the unit, it is necessary to remove the chrome ring, glass face, and the glare shroud. The chrome ring is usually removed with great difficulty by prying at it with a small screwdriver at the tabs. Then, remove the two screws in the back of the unit that hold the internals to the case, and allow those internals to drop carefully into your hand. Don't bend the needle! The spade terminal that carried the green or white wire is the power lead. Just next to this is the earth connection. One of the two leads is a resistor. Unsolder the wires and resistor from their positions and reverse them, resoldering the connections. Reassemble the unit after cleaning the glass.