Classic cars forum & vehicle restoration.

MikeG · Posted: Mon Jul 10, 2017 9:31 am Post subject: Positive or Negative Earth

I have a problem.I have been advised that my 1936 Triumph Gloria is Negative Earth.A second person has told me that his car is Positive Earth.This is supported by the live wire Battery Terminal(which looks original,but could have been changed) being marked "N"
Please can anyone tell me how I confirm one of the conflicting ideas.?

MikeEdwards · Joined: 25 May 2011 Posts: 2467 Location: South Cheshire

I know nothing of cars of this age, but surely if you connect a volt meter across something like a lamp terminal and turn on the lights, you'll be able to tell based on whether the reading shows negative or positive? That is, if you put the meter + wire to the lamp supply, and the meter - wire to the body, if it's negative earth you'd see roughly 12v, and if it's positive earth, you'd see roughly -12v?

Or is it in the process of being rebuilt and you need to know in order to put it back correctly?

MikeG · Posted: Mon Jul 10, 2017 11:25 am Post subject:

Mike.Thanks for your reply.
I am in the process of rebuilding a car that has been stored for over 50 years.
I have had the Engine reconditioned and wish to time it and run it.
I have no lights connected,as the wings have been refurbished and are at the painting stage.
I do not want to reverse the polarity and risk damaging the Dynamo or Voltage Regulator.
I do have a meter,but my understanding is poor.
Thanks for any help.
Mike

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

Hi Mike,

Assuming that you haven't added any nasty modern accessories to your car then none of your components will suffer any damage regardless of which way you choose to connect the battery. The only component that you might choose to reverse connections on is your ammeter otherwise its indications of charge and discharge could be the reverse of what it is showing you. All other components will operate normally.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

Peter_L · Posted: Mon Jul 10, 2017 5:10 pm Post subject:

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

MikeG · Posted: Tue Jul 11, 2017 8:33 am Post subject:

Hi Guys
Thanks for your suggestions.
The first thing I would like to do is to establish the original Earth polarity.
If I diisconnect the new Electronic Pump and the new Coil which I was adviced to buy,both negative earth.
Then connect the battery positive terminal to earth.
What next?
Mike

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

Hi Mike,

Sorry, I don't know the earth sense of the Gloria. Most pre-war cars were positive earth.

Modern coils usually have their terminals marked + and - so if you plan to have positive earth then connect the + terminal to your contact breaker.

Is your fuel pump reversible? Show us a photo of its innards.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

MikeG · Posted: Wed Jul 12, 2017 8:40 am Post subject:

Hi Peter
The new coil polarity can be changed and I have the instuction for doing so.
If I can confirm that the car should be Positive Earth,then I will contact SU and see if they will change the Pump.
I am still trying to find how to test for the polaity of the Earth Strap.I will see if I can get a reading on my meter across the Stater with the Battery connected.
Thanks for your help
Mike

jp928 · Joined: 07 Jun 2016 Posts: 249 Location: Australia

FWIW my 1926 Rover 9 is neg earth. Despite the fact that the wiring diagram shows 3 wires out of the dynamo, I have 2 dynamos off 9s with only one output lead, and its positive. If you have a dynamo, and know how its leads were connected, the wire colour might help. The test looking at the ammeter should help, but its only going to work if the battery is already connected, in which case you should know which side is earth - if there is an earth lead at the battery, the size of the pole clamp is usually smaller on the negative lead. Whichever test of meter from one battery pole to chassis gives a reading will show you the polarity, if the battery is connected.
jp 26 Rover 9

MikeG · Posted: Wed Jul 12, 2017 10:50 am Post subject:

The feed cable from the battery has a clamp with an N moulded into it, which would indicate Positve Earth.
A member of the Pre 1940 Triumph Club with a number of cars ranging from 1934 to1937 says that all his cars are Negative Earth.
A member who has a car like mine 1936 Short Saloon and is also undergoing restoration thinks that it is Positive Earth.We have two out of three ONLY of these cars the other one is in bits. So may be the Positve Earth is on this model only.
I will try your suggestions over the next few days and post results
Mike

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

If this is your car it does rather look as if it has negative earth.
Not the clearest of diagrams but it looks like the starter motor connects to the positive of the battery. I don't understand what is between the battery negative and ground but perhaps a switch???

Peter

_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

MikeG · Posted: Thu Jul 13, 2017 9:29 am Post subject:

Hi Peter
Were did you find diagram? Is it of a 1936 four cylinder? If it is then problem solved.
There was a Battery Isolater Switch fitted originaly.
I do have a diagrame showing Negative Earth on a 1937 car.
I did have a short test yesterday,without any result.
More to follow
Mike

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

Hi Mike,

I got it from a Google image search. It is an item that was sold on eBay some years back. If you increase the image contrast and sharpen it it becomes almost useful.

See: http://tinyurl.com/y8ks98xm
The title reads "Wiring Diagram for 1935-36 Triumph Gloria" Looking at the illustration of the distributor it does appear to be for the 6 cylinder car but is that car very different to yours? Is the dashboard layout the same?

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon