Having just acquired a 31 saloon, I found the battery is connected as positive earth - which I believe is incorrect as cars were negative earth until the rubies?? My diagnosis is based on the fact that the ammeter shows a charge instead of discharge when the the lights are switched on. In any event, the charging circuit is not functioning for some reason which I have yet to find out.

I'm not sure whether I should switch over to Neg earth - are there advantages one way or the other?

Do I need to 're polarise' the dynamo if I switch back to Neg earth?

It should be Negative earth. No problem. Just change over the leads . The battery will repolarise itself.I am sure you are right the dynamo problem is another issue, not effected by the "polarity". Mac

Apparently the change from Neg to Pos earth occured on a lot of cars around the mid thirties. There are technical implications to do with corrosion, and I believe that introduction of alternators on more modern cars prompted the change back to Neg earth which moderns use.

Sorry to hear about the dynamo I can synpathise with you!!

Have you seen the article Naomi refers to on the Cornwall club site?

Steve

Given that your ammeter indicates charge when it should indicate discharge, then it does appear as though your battery connections have been reversed. As I think your car has the SM5 Switchbox/ammeter set up, it would be difficult to reverse the ammeter connections. While there are literally reams of paper which have been written on what needs to be done to reverse the polarity of a dynamo system, including a momentary "flash" of current through the field coils to repolarise the dynamo, I have always managed to ignore the instructions and have simply reversed the battery connections.

Joseph Lucas led the charge (no pun intended) towards positive earthing in the late 1930's, on the grounds of some arcane considerations related to corrosion which I have never quite properly understood. When Silicon Transistor technology( Silicon Transistors require a Positive power rail) was introduced to Radio and other automotive accessories, circa 1960's or so, the convenience of Negative earth was recognised, and the rest is history.

A quick way of seeing if the dynamo is charging is to connect the D terminal and the F terminal together. Don't run the engine too fast as the dynamo , if it is working, will be putting out maximum voltage. If your ammeter then shows a charge you will know that you have something wrong in the external circuits to the field. If you still don't have a charge, and have a voltmeter, you could check to see if you have a voltage at the D terminal Something better than 6 volts, ( probably a lot higher if the dynamo is running open circuit)all is well will the Dynamo, something less than 6 volts, start massaging your wallet for Dynamo surgery.

If there is reasonable output volts from Dynamo, and you still don't show a charge on ammeter, and you are very brave, you could try momentarily bypassing the cut-out contacts and see if that produces a charge current on your ammeter. If it does then you know where to make further enquiries.

The above really is a two person operation as you need to be in two place at once, watching the ammeter and fiddling under the bonnet. An assistant is also a convenient person to "blame" if something goes wrong!

REMEMBER NOT TO RUN THE ENGINE TOO FAST DURING TESTS AS THE OPEN CIRCUIT VOLTAGE FROM EVEN A 6V DYNAMO CAN REACH FRIGHTENING LEVELS.

On the subject of "polarity" there is a right and wrong way to have the ignition coil low tension connected. The theory goes something like this: It is easier to get electrons to jump (spark) from a hot to a cold surface, rather than vice versa. So, we need to have the centre electrode of the plug having a negative HT pulse supplied by the coil.

If you have a coil which was designed for a positive earthed system, then simply ensuring that the CB connection goes to the contact breaker and the "SW" or whatever the other LT terminal on the coil is marked is appropriately connected, everything should work out OK.

But if you have a coil so marked which is designed to work in a Negative earthed system and you have it installed in a Positive earthed setup then you will end up with a Positive HT pulse on the cntre electrode of the Plugs.

Later coils, simply marked with a + and a - sign supposedly make things easier, in that if you have a negative earthed system then the "-" terminal goes to the contact breaker and vice versa.

According to theory, there can be as much as a 20% reduction in spark efficiency if we get it the wrong way around.

I don't think you would notice the difference on an engine in good fettle and with good plugs. But it could be of some importance in a marginal situation.

There is plenty of info on the web about this "phenomena" and various means of checking the actual electrical "direction" of spark pulses are suggested. I have a simple little uncalibrated "centre zero" voltmeter which I use to see whether the the pulse goes plus or minus.

Believe it or not, my son was playing around with a disgusting old 1960's "bomb" here a few weekends ago, which had a terribly lumpy idle and which was obviously on its last legs. It definiteley idled better with the coil connected the "right way round!"

Barry R
Tasmania
The Land of ABC

The 'Companion' refers to the "advantages" of positive earth without actually explaining what those advantages were. I presume that they were referring to the corrosion issue?

Strangely enough when I switched over the battery leads back to Neg earth to get the ammeter to read in the correct sense, (and to see if it made any difference to the dynamo - which it didnt!), the engine didnt run as sweetly.

norman , If you have a coil that is marked + and- the contact breaker connection must be the same polarity as earth connect of the battery. YOu may, possibly,notice a perceptible difference to the running if was wrong in your case.In my experience I have come across many connected "the wrong way round" that seem to be o.k. but I always correct them.

Norman,
I understand that the theory behind "Positive Earth" preference went something like:
If there are leakage currents caused by poor insulation, moisture, dirt etc etc, then in a Positive earth system the 'corrosion deposits' arising out of these stray currents would appear on the positive (anode) side of affair. That is the chassis and other 'substantial' parts of the mechanism, rather than on the "finer" electrically "alive" parts of the electrical components. What I have never been able to come to grips with, is, that if that is the case, then the "finer" parts must act as sacrificial elements and be eaten away. Like I said in my earlier posting, I have never really understood the arguments.

Getting back to your problem of "no charge" you really have to establish whether the dynamo is actually functioning, and/or whether you have a problem in either wiring or maybe the cut out box. All in all not difficult if you have access to a voltmeter. Does the cut out try and operate? Should latch a bit above idle speed, asy 800 to 1000 rpm. If it is coming in, are the contacts closing and are they clean?

I presume you have a wiring diagram.

Regards
Barry R.

Norman,
I understand that the theory behind "Positive Earth" preference went something like:
If there are leakage currents caused by poor insulation, moisture, dirt etc etc, then in a Positive earth system the 'corrosion deposits' arising out of these stray currents would appear on the positive (anode) side of affair. That is the chassis and other 'substantial' parts of the mechanism, rather than on the "finer" electrically "alive" parts of the electrical components. What I have never been able to come to grips with, is, that if that is the case, then the "finer" parts must act as sacrificial elements and be eaten away. Like I said in my earlier posting, I have never really understood the arguments.

Millions upon millions of cars produced since the 1960's seem to indicate that negative earthed vehicles work satisfactorily anyway!

Getting back to your problem of "no charge" you really have to establish whether the dynamo is actually functioning, and/or whether you have a problem in either wiring or maybe the cut out box. All in all not difficult if you have access to a voltmeter.

Does the cut out try and operate? Should latch a bit above idle speed, say 800 to 1000 rpm.(or maybe a bit more) Idea is that cut out prevents battery discharging iself through the dynamo, and only connects the dynamo output when it is above nominal battery voltage. If it is coming in, are the contacts closing and are they clean?

The SM5 switchbox is also capable of creating problems if either actual contacts are not giving good connections and/or there are burnt out tracks.

I presume you have a wiring diagram.

Regards
Barry R.

Thank you for the advice everyone. Results of the post mortem on the dreaded dynamo any day now. Significant voltage drop out of the SM 5 means I will have to take that to bits as well

All changed over and running like a dream. Even the dynamo has sprung into a new lease of life (once I drained all the oil out of it!) Thanks for the advice everyone