I recently acquired a '57 140. It has a brand new battery and brand new battery cables (and a brand new coil) wired negative ground. IF I convert it back to positive ground, do I need to do anything else, other than switching coil leads ? I will not have a radio or audio system.

 

Fuel pump and Clock!

 

The question is - why would you want to convert back to positive ground ?

 

Why - good question actually. That's the way Bill Lyon designed it; the clock and fuel gauge don't work; the car will live in a humid environment other negative ground vehicles are always having electrical connection problems. But then the change might become a can of worms, but then again the car is intended to be a driver, not a show car so originality doesn't matter. Maybe making the gas gauge work is the primary consideration, but the polarity switch may or may not be the answer to the problem Life is never simple !

Bruce

 

Reason I asked was because when I rebuilt a Mark 2 in the 80s, it was at a garage owned by an auto-electrician who worked at Lucas. It was he who advised me to convert to negative earth. Actually it wasn't William Lyons who designed it this way, it was engineers at Lucas, Jaguar just installed what they provided like most of the British car industry at the time. There was a reason for positive earth, but I've forgotten what it was.
Unfortunately it is so long ago I can't remember the details of the changeover to negative earth, but all the instruments worked afterwards.

 

I found this: -
https://mgaguru.com/mgtech/electric/et098.htm

 

According to Wikipedia, the way electrons flow, Positive Earth promotes corrosion of the bodywork, the 'earth' side; Negative Earth promotes corrosion of the electrical connections. 'Yo pays yo money and takes yo choice' since I see that the fuel guage is based on electrical wiring, not using tank body as one connection, the gauge readings should be independent of the 'earth', and besides I just tested the petrol gauge itself and it came out bad. I'm now strongly motivated to just leave the wiring alone.

Bruce

 

if you revert to positive earth you will need to replace the alternator as they only function with negative earth system.Originally positive earth systems were employed because the electron flow was believed to slow down the rusting out of the steel used in production.Why would you want to step backwards in respect to dynamos versus alternator?

 

Good points, but this car still has a generator, not an alternator, and I am more concerned about corrosion on electrical connections than bodywork. That being said, I think I am going to leave well enough alone and keep things the way they are. After EXTENSIVE work on the SUs it now starts and runs great. Hope to have new (vice 15 year old) tires on it by this weekend. Then I can really take it for a drive !

Bruce

 

Enjoy your ride !
What brakes are on it ? 120s were renowned for brake fade, and it wasn't until the 150 with discs that the problem was finally put to bed.

 

Hi Bruce
nice to hear you have it running ,enjoy.

 

Stock drums, not sure about the linings. In flat Kansas, USA, not yet seen any sign of fade.

 

Started, not well but OK this morning, at 50 degrees F, with cold-start carb still disconnected. Cold-Start problems clearly eliminated.

 

Modern brake linings may be a lot better, but having read the chapter on the XK sportscars in Paul Skilleters book on the Jaguar sports cars, it is clear that the fade came suddenly. One road tester admitted to having bad dreams afterwards when it happened to him ! Of course on the empty UK roads of the very late 40s and early 50s it was possible to drive very fast and get away with it on normal roads.