|The MGA With An Attitude
CHANGING FROM POSITIVE TO NEGATIVE EARTH - ET-202
First things first. Changing your MG electrical system polarity is easy and cheap, so don't even think about a polarity converter for your new radio. The Positive Earth Society is for concours buffs. Anyone else should feel free to change.
Disconnect all battery cables from the batteries. Lift batteries, rotate 180 degrees, drop them back into the carriers, and clamp them back down. About then you should notice that the connector posts are different diameter, the positive post being larger, so the cable clamps don't fit the same. For two 6 volt batteries, the interconnecting cable can just switch ends and be re-installed. For the long hot cable running from the right side battery to the starter switch up front, you will have to remove the original negative post clamp (cut cable and re-strip if necessary) and install a bolt-on positive post clamp (about $4). For the chassis ground cable, just pick up a new cable of appropriate length at your local cheap parts store or discount department store (maybe $6). For a single 12 volt battery, position the grounded post at the front near the frame, just in case you should happen to short the post or connector to the chassis with a tool you won't weld or burn the parts (or yourself).
Positive post bolt-on connector, left. -- Negative post cast connector, right.
Install the hot terminal first, while there is no ground connection on the battery, so you can't accidentally short the wrench to ground. Install the grounded cable clamp last, where it doesn't matter if you touch the wrench to ground. If the connectors in the pictures appear dirty and corroded, they are a little, but looks can be deceiving. While the bolts are a bit corroded, the bolt-on clamp has a solid bite on the copper cable. The rest is mostly accumulation of a little dirt as the posts and connectors were thoroughly cleaned and coated with silicone grease at assembly. All is well here with very tight electrical connections.
Next you need to re-polarize the generator. For this you can use a jumper wire to pickup power from a hot terminal on the starter switch or fuse block (brown wire), and make a momentary connection to the field terminal on the back of the generator (the smaller terminal). Or even easier, just momentarily short together the A and F terminals on the control box. See charging circuit. Just a quick spark is enough. Connection of a half second or less will do.
To be nice to the spark plugs, you should switch the two small wires on the ignition coil to make the spark jump the right direction across the plug gap. The engine will run as well either way, but the plugs should have a longer life with correct polarity. See ignition circuit. Original positive earth coils may be marked "SW" and "CB" or "IGN" and "DIST". These are the terminals that will be switched. The white power wire will end up on the CB or DIST terminal, while the wire running to the distributor will end up on the SW or IGN terminal. Newer coils may be marked "+" and "-", which makes it easy. Just connect the hot white wire to "+" and the other wire to "-".
The windscreen wiper motor may run backward, but this would only make a minor difference in the parking location of the wiper blades. NO problem there. If necessary just R&R the wiper arms on the spindles to make them park where you like.
Original MGA heater motors are not polarity sensitive and will continue to run in the right direction. These motors have two black wires. If your heater motor has one black and one colored wire, then it's a replacement motor, probably from an MGB. That one will probably run backward when the polarity is changed. The rotation could be in either direction depending on the intended original application of the heater motor. You can try the wire connections either way to see which way blows more air. Or you can pull the 4 inch air hose off the front of the heater box to see the fan inside. The fan should turn clockwise (looking through the connector pipe from the front) for all MGA except the Twin Cam. The MGA Twin Cam has a mirror image heater box, and there the fan should rotate anti-clockwise.
Original MGA fuel pumps are not polarity sensitive and will continue to work properly with no wiring change. However, nearly any aftermarket electronic fuel pump would be polarity sensitive, and you would need to switch the wires. Additionally, many MGA have had a new fuel pump installed at some prior time, and the replacement unit may be polarity sensitive. The original fuel pump had bare contact points. Later replacement pumps had a capacitor across the contact points for arc suppression. This is a small metal can with a single wire on each end, similar to the "condenser" capacitor in the distributor. Such capacitors may have been installed other places in the car for radio noise supression. These are also not polarity sensitive, and these fuel pumps do not need any wiring change.
The picture here shows a capacitor type fuel pump on the left
and a diode type fuel pump on the right. Click picture for larger image. More recent replacement type fuel pumps may have a diode across the points for arc suppression, and these are polarity sensitive and do require an internal change. Powering these up with reverse polarity can blow the diode, either open circuit or shorting to ground. The diode is usually a tubular lump of plastic with two wires coming from the end(s). These pumps commonly have a bump on the end cover to allow space for the large diameter diode, but some may just have a taller cover and still be flat on the end.
Positive earth fuel pumps originally have a blue tape around the body or sealing the joint between the cover and the body of the pump, and the red lead of the diode connects to the points contact blade (as shown in the picture). Negative earth units originally have a red tape, and the black lead of the diode connects to the points contact blade. Any of these pumps could be missing the tape. If you're not sure what's inside your fuel pump, the only way to find out is to remove the end cover for a look. If your fuel pump has a diode, you will need to reverse the diode wire connections (clip and solder if necessary). You may also buy a new diode with the proper terminals to allow installation without cutting the wires. See additional notes on fuel pump polarity. Do NOT switch the external wires at the fuel pump. The original black chassis ground wire is electrically connected to the body of the pump. If you apply power to this wire you create a shorting hazard between the pump body and any nearby metal chassis part.
If you happen to have an original (optional) AM radio in the car, that would be polarity sensitive, but many of those have a polarity plug on the back to reverse the input polarity capability. Otherwise most people are switching to negative earth specifically to install a modern electronic radio or some other modern electronic device, or an alternator, all of which will be negative ground. The likelihood of anyone accidentally having a very early issue positive earth alternator installed on an MGA is pretty much negligible.
That's about it. Nothing else (original) in the MGA is polarity sensitive. If everything started as original you simply repolarize the generator and switch the primary wires on the ignition coil. All lights and controls, flasher and fuel gauge will function as well with reverse polarity.
One thing to watch out for would be if soneone had previously installed a positive ground electronic ignition module. This part would have to be replaced when changing the electrical system to negative earth.
Some people find the need to install an electric tachometer. Starting with the 1965 model year the MGB had the five main bearing engine which does not have the mechanical tach drive. The 1965-1967 MGB electric tach has a face appearance similar to the MGA tach, and may be used in the MGA when installing a five main bearing engine. This tach is originally positive earth, and must be converted to negative earth operation if you change your electrical system to negative earth. See next page.