Hi im hoping someone can help. I'm confused at the negative or positive energy. So I'm to believe my 1966 jaguar 3.8s is positive energy but when I started the car the amp went far right. I turned it off took the keys out and the ignition light stayed on and there was a bit of smoke from the engine. Can someone advise me on what way round the battery should be. Thank you

 

A 1966 car would originally have been positive earth; that is the positive battery terminal is connected to the car body. I think Jaguar started to build some models negative earth (or ground) from 1967 or 1968. However, many older cars have been converted to negative earth in order to facilitate the use of more modern radios, fuel pumps, and other electronics and also because an alternator has been installed in place of the dynamo. An alternator can produce a charging voltage at lower engine speed.

 

I didn't say how you can easily tell if your car has been converted to negative earth if there's no (correctly connected) battery in place. If the car has an alternator instead of a dynamo, it is negative earth. An alternator is generally shorter and wider than a dynamo. Apart from that, there's the coil of the ignition system. Assuming it's connected correctly, the negative side goes to the distributor in negative earth and the positive side goes to the distributor in positive earth.

There's probably something simpler/easier that I've not thought of, but others will chip in.

 

All S type Jaguars were built Positive earth from factory with a Dynamo or Generator. The 420 which was built at the same time as the S type but production started later were fitted with an alternator and were all negative earth.
So unless your car has been converted by a previous own it should still be positive earth. Problem here is that you can still convert the old generator to run negative or positive earth just by tapping the connectors to earth and changing some wiring around then connecting the battery up to negative earth.
Ways to tell as Peter has said are to check if the car has a modern alternator rather than the old fashion block of iron which is the Generator. One of the problems you might find now is that there are companies that produce an Alternator that looks identical to the old generator. I have one fitted which stills powers the PAS off the back, looks identical to the generator but has a sneaky alternator inside to run Negative earth.
If in doubt get a qualified auto electrician to check for you as connecting the battery up the wrong way can fry your wiring loom (hence the smoke). Lastly if your car has been converted to an alternator the chances are that the RB340 (Voltage regulator which is a black plastic box on the left side inner wing just behind the windscreen water bottle) has either been removed as in my case or disconnected as it is no longer required with an alternator. The alternators voltage regulator is all built in.

 

His wiring shouldn't be smoking if he did connect it the wrong way.
The only thing that will get damaged is the positive ground radio.

If the AMP meter went far right into the "Discharge", I think you have a short somewhere.

The AMP meter should stay in the middle with nothing switched on whether it's connected wrong or not.
It will move into the "C" (or positive side) if the battery is connected wrong with the head lights on _ as an example.

And if it was dead short, what ever was getting hot enough to smoke, would get red hot in an instant.
Look for frayed wires that are partially shorted to the body somewhere, a partial short in the regulator if you're running a generator.
It's fairly common to find shorts in the fuse box under the hood (bonnet), there are only two fuses there and the whole car pretty much runs through them.
The rubber grommet that protects the wiring is usually gone and the wires are hard and can crack.

Shorts can happen in the door light switches from rust, the short happens through the rust causing a partial short.

 

A fault at the Regulator or shorted Ammeter can cause smoke from the generator. Usually a bad sign requiring a Generator rebuild, regulator change & another ammeter. Don't ask me how I know. Bad experience with a buggered ammeter. Fortunately had multiple spare ammeters. Generator worked for a while & then fell over. (it had just been rebuilt) Glass on bad Ammeter immediately went white from overheating, Another ammeter & all worked well until the Genny let go. Then it simply did not charge but the car would run fine on the fully charged battery. Still positive earthed.

 

It is a good idea to put a short fusible link between both of the two brown wires connected to the starter solenoid. On my Mk2-340 the solenoid is on the firewall (scuttle) to the left of and at the same height as the battery. I use 10 amp fusible links with soldered on eyes to fasten to the posts on the solenoid. Those two brown wires feed all of the circuits that are not fused. I hope this information is correct... Use it at your own risk. I have only had the car for four months. Working on it is almost as much fun as driving it. Best Wishes to All, Morgan

 

it is a lot smarter to change to Negative Ground, and simple to do.. Why "smarter"? Because nowadays all cars are negative ground and if you don't tell a mechanic "the electric system is positive ground", chances are they will assume it is Negative Ground and the rest is history.
Unless it is a very smart mechanic...

I changed my 1965 3.8 S to Negative Ground and I would never go back to Positive Ground.

Again, very simple to do. Instructions are here:
https://starautoelectric.com/instruction-for-polarizing-generator/

Only issue will be the Radio if the car has one. You will need to convert the radio to Negative Ground or replace it with a newer radio.

Then you need to invert the two big wires at the Ammeter, simple to do.
And of course, invert the cables at the Battery.
Then last proceed with the Polarizing of the Generator and Voltage Regulator, the famous RB340. I have a new one if you fried yours.

Did I forget anything?

 

A study somewhere showed that a 12 Volt positive ground provided less potential for corrosion. I don't recall the logic. One thing is for sure, having parts of the car negative ground and parts negative is a recipe for trouble. I am not sure of the logic of converting a car from positive ground to negative ground. Repair for positive ground radios is available. Most other do-dads such as GPS have internal power systems. Is keeping a car original as important? Regards, Morgan

 

That corrosion thing...
The British finally figured out you can't rubber mount things on their own _ ground straps went a long way.
In the end it didn't matter except in one very important area, ignition spark.

The spark jumps much better from the "HOT" centre of the plug to the cold edge - Negative Ground
Then from the cold edge to the hot electrode - Positive Ground

You get a hotter more positive spark with a Negative ground car.

 

I wonder how much difference there is. The spark jumps due to the potential difference between the tip and the body. The potential difference is the same either way. the potential ionizes the air between, and the electrons arc across. the ionization drops off rapidly until the next surge from the coil. Bluntly, the direction does not matter. However, my understanding is subject to change based on new and superior information. Respectfully submitted, Morgan

 

I don't think the polarity of the car matters much to the breakdown of the spark gap. What helps is to make the hotter central electrode negative relative to the other. It's easier, that is it takes less voltage difference, to pull electrons out of hot metal and breakdown the gap initiating the spark. It also helps to have a sharp edge and, if there are deposits on the electrode, they should be ones that help breakdown and electron emission. Surprisingly enough, chalk is good.

So here I am giving opinions as if I know what I'm talking about (I ought to or I used to ... but that's another story). When I was looking over my Mk2 earlier today trying to find something to help the OP's original question, I noticed that the coil is the wrong way round. Most likely, it has been like that at least since the last time a mechanic worked on it, sometime before about 1975 and for all the two decades that I was using it ... It ran OK in spite of that. Though it went better if I checked the plugs every 3000 miles. Especially when I tried Bosch plugs (around 1979 or so) - they were particularly bad. Perhaps, this is why. As Jose wrote, it's easier for mechanics if you have the same system as the majority.

 

How much hotter is the electrode than the side of the plug? A sharp point of fouling on either one will have more influence on the arc... Regards, Morgan

 

Standard Jaguar Fitment. On Battery lid or more often strap.

 

The whole issue of 'hot' and 'cool' plugs relates to the temperature of the central electrode. High temperatures help electron emission directly and also by cleaning deposits. Good deposit clearing requires a temperature of a few hundred degrees C. Modern spark plugs use materials that can better resist high temperatures and erosion. Basically, it's desirable to make the central electrode as hot as possible within its material constraints and the possibility of autoignition. Sharp edges are good, but they erode quite quickly. That's why conventional plugs (the original ones in our cars) have thicker electrodes. Modern materials can tolerate a smaller diameter.

 

Lead in fuel was one cause of plug erosion (NGK produced a whole white paper on the subject). No longer an issue. The 100,000 Km plug is now a reality as long as operated in the self cleaning range adequately/sufficiently. A modern Bosch plug will easily do 100,000 Km in a Mercedes or whatever. The recommended change interval.

In days of yore the Golden Lodge ringfire plug was a good solution for Jags & Alfas that tended to use oil. Modern vehicles don't use oil in any appreciable quantity unless worn at very high mileage. My present & previous Mercs use/used zero oil between services. (I insist on 15,000Km servicing & not 30,000Km) I have my Assyst system set at 15,000 Km with a Star computer & only use genuine MB Mann + Hummel "Fleece" filters. (Spun Polyester.) These are large sump vehicles so the oil charge is not stressed & using Merc approved Synthetic oils.

 

I have changed quite a few Jaguars to negative earth. As Jose points out it is pretty simple. This then lets most modern electronics such as portable GPS and solid state radios to be driven by the vehicle electrics..
However if you have any small electric motors such as in the windscreen washer pump they have permanent magnets for the fields and will require the leads to be reversed.
Cheers