17.1 - Positive to Negative Polarity Swap ( Frank Benschop,  )

The following are instructions on changing to negative ground for a positive ground saloon:
* Turn the battery around, or preferably, find one with the poles reversed (limits the possibility of shorting on the back of the open bonnet).
* Reverse the wires on the ignition coil.
* Inspect your fuel pump for a capacitor (common on later Mk 2/240/340/S-Type). If there is no capacitor, go on to the next step. The capacitor in the fuel pump is a one lead axial type. The lead is the negative pole. The positive pole is the housing, which is earthed by the attachment clip. There is no simple way to 'reverse' it. Replace the capacitor with a 50uF electrolytic capacitor of at least 20Volts with about the same physical dimensions. Solder the positive pole to where the old lead was attached. Solder the negative pole to the attachment clip or other suitable earth.
* Run a wire from the positive post of the battery and briefly "zap" the field terminal of the generator (reverses the residual field).
* Reverse the wires on the ammeter (hard to get to on a Mk 1).
* For Mk 2 and later cars, the clock in the rev counter should be disconnected. Otherwise it gets fried. No easy way to convert it. This will probably not be a problem in most cases as the clock in most cars is already broken.
* If the car has an original radio, you will need to either disconnect it, or electrically isolate it from the rest of the chassis (including the antenna) - Mark Stephenson.
* Start it up and go for a drive.
There is no need to fiddle with the wires to the starter, fan motors, etc. They seem to sort things out on their own. There is some indication that the washer bottle mechanism in the Mk 2 and later Saloons is polarity sensitive.