How to tell whether a car is pos or neg earth?

: Hi. How can I tell whether my car is wired positive or negative earth? : There is no battery, and the new battery would easily fit either way : round - the leads are long enough. The car left the factory positive : earth, but could have been changed. No, no radio either. : : Presumably the car would run with the battery wired either way? The : only damage would be to the battery? There must be a way to tell which : way it should go?

Do the battery leads have proper positive and negative clamps on the end? It's a reasonable indication, but not certain. If it has any electronics fitted it's probably negative earth.

Wot I'd do, guv, is attach a voltmeter to the dynamo output and earth, give it a spin in the normal direction of rotation, and see whether the output is positive or negative.

Ian

The coil if a replacement might be marked +/- rather than sw/cb?

However, if it's a dynamo car with no electronics just try the battery either way. You'll do no damage. It simply won't charge if the wrong way round.

In message , Ian Johnston writes

A '68 Traveller, before electronics were invented :-) Battery terminals are both pole, or circular, type.

That is so obvious! Thank you.

In message , "Dave Plowman (News)" writes

Ah! Yes - but which way should it be, i.e. does + or - go to the distributor?

Thanks Dave. I'm reassured.

: In message : , Ian : Johnston writes

: >Do the battery leads have proper positive and negative clamps on the : >end? It's a reasonable indication, but not certain. If it has any : >electronics fitted it's probably negative earth. : : A '68 Traveller, before electronics were invented :-) : Battery terminals are both pole, or circular, type.

One should be bigger than the other. Buggered if I can remember which is the big one, though.

: >Wot I'd do, guv, is attach a voltmeter to the dynamo output and earth, : >give it a spin in the normal direction of rotation, and see whether : >the output is positive or negative. : >

: That is so obvious! Thank you.

Ay thang yew.

Ian

: >The coil if a replacement might be marked +/- rather than sw/cb? : : Ah! Yes - but which way should it be, i.e. does + or - go to the : distributor?

On a positive earth car, sw goes to the switch and cb to the distributor (contact breaker). On a negative earth car + goes to the switch and - to the contact breaker. The high tension is earthed to the contact breaker side (not the casing) and is wound in opposite directions in each case, so the spark goes the same way across the plug gaps regardless of LT polarity.

In theory it therefore matters that the coil polarity is correct. In practice, for things like Minors, Heralds, Minis, Beetles and the like it doesn't matter a hoot.

Ian

The contact breaker goes to earth so this would be connected to the positive of the coil on a positive earth car.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Ian Johnston" saying something like:

The Pos is the bigger one.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember News saying something like:

Make it neg earth anyway. Disconnect the dynamo, connect up the battery as neg earth and polarise the dynamo by stroking the field terminal with a pos lead a few times. Reconnect and start up.

All MM's started life with positive earths, so unless the polarity has been reversed by a previous owner..... Mike.