Polarity? How can you tell?

[KeithS]

I have recently rescued a 1965 3.4 S type. It hasn't run for a while and the battery was completely flat. That battery (I'm 90% sure!) was connected up as a positive earth. The battery is U/S, when I went to fit another I discovered that the leads were in fact the other way round. I.E. although the earth lead had been connected to the positive side of the battery it was in fact a negative earth lead and likewise the otherside connected to the starter solenoid was a positive lead it had been connected to the negative side of the battery. Any signs indicating polarity on the car have long gone, the standard of work that I have seen on the car makes me think that the last owner hadn't got a clue what he was doing..... So is there any way that you can tell if it is positive or negative earth?
Thanks
Keith S

[PeteB]

If the car has been fitted with an alternator or a modern stereo system, it must have been converted to negative earth for these to work.
If it doesn't have either of these, you can choose which way to earth it.

Re-polarise the dynamo by briefly touching a feed wire from the battery to its field terminal (you'll probably get a small spark occur when you do this, which is normal.)
Depending which way its currently connected, you may have to reverse the wires on the ammeter if it reads backwards. The polarity of the car dictates which way round the coil is wired, and some fuel pumps/aftermarket electronic ignition systems are polarity sensitive. You might have to check any such equipment won't be damaged (or hasn't already been) by improper connection.
Bear in mind that If the car is fitted with a Kenlowe fan, it might operate backwards unless the wires are reversed. And, although they now seldom work, the time clock in the rev counter can be permanently damaged if it's connected the wrong way round and is best left disconnected.

[stuarthardy]

Hi

That's good, succinct advice.

I've been looking through various books and magazines and cannot find any reference to when S Type production switched from Positve to Negative Earth, bizarre. I have what I consider to be an excellent book by James Taylor (presumably, not he of mellow voice and enormous song writing talent) titled Jaguar S Type and 420 - The Complete Story. Clearly it can't be complete because it doesn't appear to tell us when the change to negative earth was made. It appears to list all the major and minor updates from January 1964 until December 1968 but, crucially, not that.

I know that all 420's were negative earth (production start 1966) and that E-Types changed from positive to negative earth when the 4.2 litre engine was introduced in 1964 but as for S-Types .....

Sorry that this doesn't move things on any further for you but at least I tried.

[smiffy]

Hi Keith
All Mk1 and Mk2 cars were positive earth, including 240 and 340 models, so your S-type is likely to have been positive earth originally. I believe that the Daimler versions did not change to negative earth until 1969 when they were fitted with alternators instead of dynamos. My 1966 Mk2 was posiitve earth until I converted it earlier this year.
Pete's advice is good, and you can probably choose whether to have positive or negative earth. All I would add is that to find out which way round it was wired:
1. Check the connections on the windscreen washer motor. The connector should have + and - marks on it.
2. If you have a radio, take it out and look for polarity markings (a radio of that vintage may even have a polarity reversal switch).
3. Remove the rev counter (a bit of a fiddle) and look at the clock, which will be marked with the car polarity.
4. The suppression fitted to the fuel pump may give you a clue - if it is fitted with a diode then this will indicate the car polarity, but other forms of suppression are not polarity sensitive.

There are a couple of other threads in this part of the forum that may also help you if you decide to change polarity. There is a bit more involved than just repolarizing the dynamo, as Pete has indicated.

Best wishes, David

[KeithS]

Hi Guys
Thanks for the advice, the car hasn't got any 'modern' upgrades and would appear to be pretty much as it left the factory (which includes no radio). From what is being said I guess it will start whichever way the battery is hooked up. I'll get out this weekend and give it a go and report back.

Thanks

Keith S

[smiffy]

Hi Keith
You are correct when you say that your car will start whichever way you hook up, but your dynamo will not charge the battery unless it is correctly polarised. Follow Pete's advice and repolarise the dynamo before you get serious with the engine.
Best wishes, David

[KeithS]

Ahhh
Thanks David, good job I checked the computer before I went out and started working on it!!