How do I know if my Jag has positive ground or negative ground from the factory?

 

From the factory it was positive earth. A WSM will show this in the wire diagram.

 

Blow up the High Res pdf. Laminate it & stick it on you garage wall. The positive terminal of your battery will be connected to the car body as seen at top left of diagram.

 

As Coventry has stated all S Type Jaguars 1963 to 1968 were Positive earth from the factory with a generator not an alternator. The Jaguar 420 1966 to 1969 was negative earth with an alternator.
Many S Types including mine have been changed to run Negative earth and been retro fitted with an alternator to be able to run modern electronic ignition, more powerful lights and modern radios. Changing from Positive to Negative earth is simple and not expensive.

 

there is no cost whatsoever to convert to negative ground.

you need to Polarize the Generator and invert the Ammeter wires behind the gauge. That is all, other than inverting the battery cables so that Battery Ground Cable goes to car body, and Positive Cable goes to Positive Terminal on battery. That is it.

you posted this question in another forum and I gave you a link for instructions on how to do it. It is very easy.

read instructions here:
https://starautoelectric.com/instruction-for-polarizing-generator/

 

There are some minor points, probably covered in Jose's link. Your clock, if it works, needs modification. So does the radio, if you have one. And, strictly, if you still have OE ignition, you should swap leads on the coil otherwise you'll have a less energetic spark. However, it's possible the car already has the coil connections swapped (due to mechanic's error). If the car isn't all OE, the fuel pump may need a switch throwing or other modification. The same applies to electronic ignition and non-OE instruments.

If you eventually replace the dynamo with an alternator, you may want an ammeter that reads more amps or to replace it with a voltmeter.
​​​

 

Beat me to it Peter

 

yes I forgot the radio if the car has one.

The clock never works anyway. Send it to Mike Eck at www.jaguarclock.com for conversion.

 

The following worked well for me
- remove the instruments (noting the connections) and dissemble
- clean up inside and repaint the reflecting internal surfaces using a white but not too glossy white paint
- leave the blue plastic bulb dome filters
- install warm white LEDS (provide significantly more light lumens)
A too glossy or reflective paint leads to uneven illumination. I didn’t try removing the filters but this would . The switch dim function no longer makes a significant difference but no big sacrifice.
It’s worth the effort and results in a much improved illumination but still a classic look.