The MGA With An Attitude
FUEL PUMP Polarity -- FP-101

At 11:31 AM 7/18/02 -0700, Max Heim wrote:
>on 7/18/02 10:57 AM, Kelvin Dodd at wrote:

>> Swapping the leads around creates the potential for a dangerous short circuit of the unfused power feed which would severely damage the wiring harness.
>> The original feed lines to the pump on early cars have a braided steel sheath, which are electrically in contact with the pump and are in close proximity to the body. In your experience you have not had a problem, but this suggestion could cause a catastrophic situation in other vehicles.
>Huh... then how come the description of negative ground conversion in the Moss catalog doesn't mention this?

I think I can clarify that (some).

The original fuel pump on my MGA was plumbed in with all rigid steel lines. The fuel tank is mounted in metal bands with rubber packing, and the filler neck to the tank is connected with a rubber hose. The fuel level sender unit in the fuel tank finds a reliable ground connection through the steel fuel line to the body of the fuel pump, and the fuel pump has a ground wire to the chassis of the car. When installing an aftermarket fuel pump, making the line connections to the fuel pump with rubber hoses will often break the ground connection for the fuel tank, and the fuel gauge may not work right (and you may also end up with a static spark hazard).

Original flex fuel lines at the carburetor on the MGA had steel braid covering and threaded brass end fittings and actually made a ground connection between the steel fuel line and the carburetors. Original flex lines on the MGB also had steel braid coverings. I know some of the flex lines on the MGB have push connectors with hose clamps and may not be electrically connected where they attach to the steel lines on the body (or the carbs), but I believe they are all electrically connected with banjo fittings at the fuel pump.

Now the problem with switching wires to reverse the polarity is that one of the wires (original black wire) is grounded to the body of the fuel pump. The early pumps which had bare contact points or a capacitor across the points are not polarity sensitive. For those units, when switching from positive earth to negative earth you don't have to do anything to the fuel pump. For the later fuel pumps with a diode across the points, you have to reverse the diode when changing polarity, and keep the external wires connected as original. If you switch the external wires you end up with the body of the fuel pump being electrically "hot", and any short to the chassis is bad news and can burn the power feed wire.

Barney Gaylord
1958 MGA with an attitude

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