Converting Positive Ground to Negative Ground

The reason to do this conversion is convenience, not reliability – it allows you to use modern electrical accesories, especially radios.

There is an article in the April 1993 issue of British Car summarizing this conversion – the following is a summary of the important points, with additions based on the experience of VTR members.

  1. Remove the battery
  2. If you have a radio, remove it.
  3. Remove the central panel in dash and reverse the wires conected to the Ammeter. Replace panel.
  4. If you have an electrical fuel pump (non-original on TR-4), and it’s a late model SU with a diode across the points rather than a condensor, reverse the connections to the diode.
  5. Connect the (-) post of the coil to the wire going to the distributor, the (+) post to the wire going to the ignition switch.
  6. Replace the battery, with the terminals reversed. If you are using the car as a daily driver, I’d strongly reccomend that you replace both the ground strap and the positive lead going to the starter solenoid with new cables. It’s cheap, and will give you a better connection, which means more reliable starting.
  7. Disconnect the two leads to the generator. Temporarily connect one end of a length of wire to the positive terminal of the battery. Touch the other end of the wire to the F terminal on the generator (the one the smaller lead is connected to, wire is brown with green stripe) several times briefly. This repolarises the field windings so you get the proper output.
  8. Reconnect the two leads to the generator.

That’s it – all the electrical devices on the car will now work like normal. You can install a new negative-ground radio, or other electrical accesories. I put in an electric windshield washer pump – much better than pushing that stupid button! Feel free to ask the VTR mailing list if you have any questions.

By the way, if you have starting problems, check that the starter motor bolts are tight and that all connections are clean. Most electrical problems on my car have been caused by corrosion and bad grounds rather than component failure.

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