Changing polarity from Positive to Negative

I have an S-type 1968 and wondered how I can change the polarity so I can use my Sat nav. If I do change it will it affect the fuel pumps and other items please?

By 1968 Jaguar had changed the polarity of the E-Type to negative ground and had installed an alternator. I don’t know about the S-Type so that is something for you to investigate. By 1968 the clock was polarity sensitive, and by “sensitive” I mean that it will burn up if you reverse the polarity, so disconnect it first. The fuel pump is another item that could be polarity sensitive, so you need to see if there are any electronics involved with it. I believe the windshield washer pump motor will run backwards with reverse polarity.

Look at your fuel pumps part number. I typed in mine and realized they were not polarity sensitive. This is not the case for all fuel pumps, but I got lucky. I had no problem with my wipers or gauges (except for the ammetre which was backwards) it was a much more simple task than I expected. Here is a link to the posting.


Thank you I will check this out. Glynis

An option to consider is the reversal of your sat-nav power plug wiring. If the external elements of the sat-nav unit are all non-conducting material, or the sat-nav case and outward touching elements other than the plug will never contact vehicle ground, that could be a simple solution.

Another option to consider is using a map/traffic function on a smartphone. In the US and Europe, all my family members have put away the sat-nav units in favor of the phones.

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If it’s only for a sat nav or phone, consider a rechargeable battery pack.

If the electrical system is otherwise fine, and all you want is to be able to plug a modern device in every now and then, then that is easier, not to mention cheaper.

If you’re doing a rebuild… fine, change the car over.

Thank you for your help.

I never use my standalone GPS anymore: it’s ALLLL Waze!

I run a Garman designed for negative earth, in my two positive earth Morris Minors and an MGTD using a pig tail lighter plug making the outside of the plug hot and the center contact ground. To prevent shorts I wrapped the entire plug in duct tape and for added security fitted an inline fuse. The lighter plug was easily available at an auto supply store.

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Dear John,

Thank you for this, very useful.

Best wishes


Thank you for your help. Glynis

If your jaguar has a generator not an alternator, polarity changers are very simple. I have changer XK’s E types, MGs and Triumphs.
there are just 5 units that are polarity sensitive on your car. The clock, the windshield wash pump, fuel pump,and Amp gauge. Clocks are usual dead by now and need to replaced any way. Washer pump depenses on the number of wire leads, if two just reveres them. SU fuel pump will need to be changed if equipped, if aftermarket two wire pump just reverse leads. Amp gauge reveres the two wire or the amp gauge will read backwards. Some radios had a polarity switch in them. if not the radio will need to be changed.
The starters aren’t sensitive. The Generator only has to be re polarized with voltage regulator.
To polarize a generator: with the engine off take a length wire and momentaly spark the F connector and the Battery connection on the voltage regulator… Spark it again and you are done. hope the is is help fun.
Merced, CA

Point distributors aren’t polarity sensitive but electronic distributors are. When I change 6 cylinder Jaguar polarity I install a 1980 -85 XJ6 distributor with clamp, controller (control box) and coil. I have done numerous XK’s and E types. this is the similar housing of that of the Protronic.

Many decades ago when I changed my positive ground generator equipped 1965 3.8S I did nothing to the two fuel pumps and they have worked fine ever since. Did nothing to the three wire washer pump connections and it works as well, albeit pretty anemically but it always did.

Some Lucas coils are polarity sensitive - if the connections are marked + and - , observe those signs. Sold a 40s Rover (+vs earth) once. Buyer rang me a few weeks later - its not running well! What have you done? Nothing…well nearly nothing - changed it to -ve earth so could fit modern radio. Swap the coil low tension leads. Fixed.