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> Negative Earth
Martin Hamilton
post 17 Aug 2017, 14:37
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So, the chap I bought my Traveller from told me that it had been converted to negative earth. As he has disappeared of the face of the earth (ill health, I think) I can't press him for more details.

After installing an alternator and electronic ignition, I now notice the ammeter is always negative and wonder also if this is why the fuel guage doesn't work (because its trying to go backwards?).

Is there anything else I ought to be paying attention to, like e.g. trafficators or windscreen wipers? And is it simply reversing the wires to any accessory that is required?

Any thoughts?
Cheers!

Martin
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ozieagle
post 17 Aug 2017, 20:59
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Hi Martin,

Yes, the ammeter will need to be reversed, as the electrons are running the opposite way.
I doubt that the fuel gauge will need reversing, from what I know it and its sensor are resistive, no polarity sensitivity there.
With motors, it depends on whether they have a wound field winding or a permanent magnet one. The permanent magnet ones will need to be reversed, the others not.

Other items that you may need to consider are an electronic tacho, radio electronic ignition and LED lights, if any of these are fitted.

Herb

I thought I should add a bit more about the tacho, it is not simply a case of swapping the wires, as the case is at ground (+ve). You will need to open it up and swap two connections inside. The following article tells you how to go about it.

http://www.mgexp.com/article/neg-convert.html

When done make sure that you get and use stickers that specifically say that the car is neg earth. Also, with the tacho dial, black out the word "positive" and with Letraset over write it with -ve.

This post has been edited by ozieagle: 17 Aug 2017, 21:09
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Penguin45
post 18 Aug 2017, 18:08
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Might be worth getting the test meter out and just checking that the dynamo is actually charging.

P45.
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ozieagle
post 18 Aug 2017, 21:30
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Hi again,

Re-reading your post I've had some more thoughts.

If the ammeter read correctly before you fitted the alternator, then the car most definitely was NOT converted to neg earth. Common alternators are negative earth beasties. You can get expensive positive earth dynamo look alikes.

I also note that you have fitted electronic ignition, don't try going back to the dynamo, or you might fry the ignition module. Double check which way the battery is connected and if the polarity is correct. Maybe when the PO "converted" he also charged the battery backwards. I've seen it done.

Maybe the PO simple switched the battery terminals and called it a neg earth car, then wondered why some things didn't work too well.

Herb

The penny has just dropped as to the meaning of P45's post. What he is suggesting is that the ammeter may be reading backwards because the alternator is not charging and the power the car is using comes from the battery. An issue I hadn't thought of, assuming that the alternator was in fact working.

Herb

This post has been edited by ozieagle: 18 Aug 2017, 21:38
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ozieagle
post 18 Aug 2017, 22:04
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Rather than another edit, I have added another post.

A quick way to determine if the alternator is in fact charging:-

Does the IGN light come on when the key is turned and goes out once the engine is running? If so it indicates that the charging circuit is working. If it doesn't go out the alternator is not charging. It may be an wiring error when the alternator was fitted. If it doesn't come on when the key is turned then the alternator may not be charging. The IGN light circuit energises the alternator field until the alternator starts generating, then it becomes self energising, and the light goes out. The alternator may self energise, without the IGN light, but won't kick in till the engine revs to 2000 or more.

This link explains how to convert to alternator. Even though it is an MGB site the changes should be similar.

http://www.mgb-stuff.org.uk/electricstext1.htm#convert

Herb

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enigmas
post 18 Aug 2017, 22:53
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QUOTE (Martin Hamilton @ 18 Aug 2017, 00:37 ) *
So, the chap I bought my Traveller from told me that it had been converted to negative earth. As he has disappeared of the face of the earth (ill health, I think) I can't press him for more details.

After installing an alternator and electronic ignition, I now notice the ammeter is always negative and wonder also if this is why the fuel guage doesn't work (because its trying to go backwards?).

Is there anything else I ought to be paying attention to, like e.g. trafficators or windscreen wipers? And is it simply reversing the wires to any accessory that is required?

Any thoughts?
Cheers!

Martin



Here's some very comprehensive links to the MGA guru.
Choose your topic and follow the comprehensive instructions. All the electrical info will apply to your car.

http://mgaguru.com/mgtech/electric/electr_2.htm.

PS. If your car was first converted to negative earth but retained the original generator it would have required 'repolarisation'...big word but a very simple straight forward task done in seconds.

http://mgaguru.com/mgtech/electric/et202.htm

Note.
Ensure you fit a heavier gauge wire from the alternator output terminal (B+) to wherever it terminates and provides power to the system.
If you fit an alternator of greater output than the capability of the ammeter in your car (if fitted) it may fail and cause an electrical fire, so either bypass it and fit a voltage gauge or simply fit a lower output alternator. Consider that if you make changes from an 18 amp generator to an 80 amp alternator you need to ensure the wiring gauge is up to the task.

This post has been edited by enigmas: 18 Aug 2017, 22:55
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Penguin45
post 18 Aug 2017, 23:53
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Oops - alternator not dynamo. Good points from both of you.

P45.
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ozieagle
post 19 Aug 2017, 03:28
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QUOTE (enigmas @ 19 Aug 2017, 08:53 ) *
Note.
Ensure you fit a heavier gauge wire from the alternator output terminal (B+) to wherever it terminates and provides power to the system.
If you fit an alternator of greater output than the capability of the ammeter in your car (if fitted) it may fail and cause an electrical fire, so either bypass it and fit a voltage gauge or simply fit a lower output alternator. Consider that if you make changes from an 18 amp generator to an 80 amp alternator you need to ensure the wiring gauge is up to the task.


Good point about the heavier wire, but it doesn't matter how many amps the alternator can provide, the amps purely depend on the car's electrical draw. I would fit a 40 amp fuse inline with the power lead going to the rest of the car.

Herb
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enigmas
post 19 Aug 2017, 04:10
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Fair enough Herb, but if someone's going to fit an alternator, they'll probably upgrade the headlights and perhaps several other accessories. Both my classic cars have 35 - 45 amp alternators (old style and outdated today, but put out more than enough current to run H4 halogen headlights, all the original accessories and the electronic/programmable ignition modules.

Interestingly with the development of LED headlight bulbs, indicator lights and flasher cans the electrical draw can be significantly reduced on the car's original generator system. The main benefit of any alternator IMHO, is that it charges virtually at idle speeds...and they're also very durable. To use an analogy, it's the like the difference between a electical vacuum tube valve and a solid state component of similar rating.

This post has been edited by enigmas: 19 Aug 2017, 04:16
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webmonster
post 19 Aug 2017, 04:44
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I've changed my Cowley, Oxford and Isis all to -ve earth. All have dynamos still.
Things I know that matter:
- swap wires to ammeter
- something inside the clock needs changing

All other 'leccy bits work fine if original fitment.
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webmonster
post 19 Aug 2017, 04:49
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QUOTE (enigmas @ 19 Aug 2017, 16:10 ) *
Interestingly with the development of LED headlight bulbs, indicator lights and flasher cans the electrical draw can be significantly reduced on the car's original generator system.

Yes! I'm experimenting with this right now on my cars. I still need to see if I can aim the LED headlamps properly. Amazing how low the current draw can be. The windscreen wiper motor and heater blower are now the most power-hungry devices.
I'd like to be able to fit an electric radiator fan to replace the original one too.

QUOTE
The main benefit of any alternator IMHO, is that it charges virtually at idle speeds...and they're also very durable. To use an analogy, it's the like the difference between a electical vacuum tube valve and a solid state component of similar rating.

I think valve technology is fascinating (IMG:style_emoticons/default/smile.gif)
Certainly not efficient, though.



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Martin Hamilton
post 19 Aug 2017, 05:27
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Thanks guys for all the advice, its extremely very useful !!

On the matter of current draw, I'm planning to install LED for all the lights where possible - including the trafficators and indicators - which should reduce the load "somewhat".

But I wanted to be sure that the charging system could cope with heater, wipers, lights, foglights and also with radio, stanav, heated rear window (maybe!); electric fan (maybe!)..... all operating on a cold*, possibly wet*, maybe foggy* evening stuck in a traffic jam**, just in case I hadn't gotten round to the LED-ing of the lighting...... the law of sod and best intentions never to drive the car in such circumstances almost certainly means that I will end up in them sooner rather than later, so I thought a good starting point ought to be to have an alternator ragardless of future planned enhancements to the lighting (etc.).
[* normal year-round weather in the UK, (IMG:style_emoticons/default/wink.gif) ].
[** normal year-round traffic conditions for the UK (IMG:style_emoticons/default/sad.gif) ]
I also plan to add a second (original type) 8-fuse fusebox to spilt out the circuits and have smaller fuses for the acessories / ancillaries that need them and to cope with the addition of aforementioned goodies - Oh plus a couple of 12v charging sockets (one for satnav, one to charge phones & tablets). At least then a fuse blowing will not wipe out most of the electrics and may still leave the car largely operational and legally driveable!

Good points all and ta for the suggestion of adding a 40a fuse in the main supply wire "to the rest of the car" - from the alternator? After LED-ing the lights there really shouldn't be much draw anyway.



Cheers!
Martin

This post has been edited by Martin Hamilton: 19 Aug 2017, 05:36
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geeksteve
post 19 Aug 2017, 08:04
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QUOTE (webmonster @ 19 Aug 2017, 05:44 ) *
- something inside the clock needs changing


I tried to repair an Oxford clock once.. *looks over at box of bits*

Never again...

Steve
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MYKE
post 19 Aug 2017, 08:35
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The first excuse for changing to negative earth was because radio manufacturers stopped producing positive earth models although some were fitted with a switch so that you could change polarity to whatever your car was. Once alternators became the norm, everything was designed to be negative earth and classic car owners started converting to keep up with convention.

Of course if you fit an alternator it will only charge negative earth, so you have to change the polarity which means that you need to change the battery terminals around for a start Then you have to swap the connections on the coil over. If you have an ammeter fitted, that needs changing too, and the control regulator needs by passing. There is a diode built into an alternator which controls output, which is why they will only work on negative earth.

The only other thing that needs changing are the two wires that feed the heater motor. Otherwise it will suck rather than blow and you will spend several months wondering why you can't get the demister to work in winter.

My own view is that it's best to keep things original. The whole point of running a classic car is to experience what they were like to drive and when you start altering things, you get away from the whole point.

The advantage of a dynamo is that if it goes wrong, you can take it to bits and fix it. A duff alternator is scrap unless it's only a matter of changing the brushes, and a lot of folk can't even be bothered doing that.

You only need to produce enough amps to replace what goes out of your battery so, so long as your battery remains fully charged you are putting enough power through the system. You don't need any more.

With the advent of leds we don't need as much power and sat nav.s work on a much lower voltage. (They can be used in a car with a six volt system). so maybe there is a case for going back to the standard specification. Not only does it look right, it can be repaired. Adding modern bits may seem like a good idea at the time, but can you guarantee that you will be able to replace THEM when they need it?

P.S. I just had a look at the MG instructions and noticed that the diagram shows the alternator to be earthed. It can only be earthed through the engine so you must make sure you have a good earth between the engine and chassis. Probably not a problem because if there is a bad earth, the starter motor won't work, but worth checking.

This post has been edited by MYKE: 19 Aug 2017, 08:49
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Martin Hamilton
post 19 Aug 2017, 11:31
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QUOTE (MYKE @ 19 Aug 2017, 08:35 ) *
If you have an ammeter fitted, that needs changing too, and the control regulator needs by passing. There is a diode built into an alternator which controls output, which is why they will only work on negative earth.

The only other thing that needs changing are the two wires that feed the heater motor. Otherwise it will suck rather than blow and you will spend several months wondering why you can't get the demister to work in winter.


Thanks Myke
I understand your POV. My car isn't intended to be concors, but very usable on a daily basis and I wanted to ensure that every electrical enhancement would have access to sufficient amperage so as not to overload the battery. The alternator plus electronic ignition are the first major departure for me from "stock" and my thinking was "what if you don't install LEDs but also add all the extra electical bits". So, having incandescent headlights (and all the rest) plus (maybe) HRW and radiator fan could well tip the balance, power-wise. But with an alternator I don't have to worry. I'll probably buy a spare electronic ignition module to cover off a failure, yes I know its much more expensive than points!

I want to car to be as reliable and safe as possible for a classic so I almost certainly will go LED to see and be seen, but until then I won't have to worry about the electrics.

I also plan to have 3 point seat belts fitted by welding captive nuts just below the trafficator housing (and abviously elsewhere) if at all possible, but that's another story.
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enigmas
post 19 Aug 2017, 12:44
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The very reason I fitted an alternator to both my classics was due to the combined current draw of headlights, heater fan, radio and wipers, many, many years ago whilst stuck in traffic on a cold, wet and wintery night slowly watching the battery discharge. Frankly, I don't care which way the electrons run, but since automotive electronics are now standardized negative earth/ground for virtually all electronic devices why fight the inevitable. Nothing wrong with a satnav nor a virtually maintenance free electronic ignition but I still carry a Melways and a pocket compass.

PS. Martin I've run an electronic Piranha ignition system in my Rover P5 coupe for over 24 years and over 300,000 kms (was my daily driver). My ZB MG Magnette (now coupe project) has an electronic points fired module I built from a kit in my 30s...still in the car. Recently (2 years ago) I built a programmable ignition system for my P5 Rover ('Silicon Chip' kit) to control some intermittent pinging under load of the hi comp V8 fitted to the car. It uses a MAP sensor to monitor load. Problem solved. I can remove a degree or two of ignition timing anywhere in the engine's rpm/load range with a push of a button on a hand controller.

If you're still worried about reliabilty carry an old points distributor in the boot as insurance. The main thing to watch when fitting an electronic module to your car is to use the correct ohm coil and ensure the module is not being powered by too high a voltage output...so check this with the engine running at charging speed. Too high a voltage will eventually burn out the module usually after intermittent failures.

Not trying to overstate this, but most of the issues regarding unreliable operation of electronic modules is due to fitting 'sports coils' of low ohm rating (the standard points coil of 3.4 ohms is generally fine) and ensuring the voltage to the module is about 13.5 - 14 volts no higher...but check the literature that comes with the kit.

This post has been edited by enigmas: 19 Aug 2017, 13:23
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Martin Hamilton
post 19 Aug 2017, 13:28
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QUOTE (enigmas @ 19 Aug 2017, 12:44 ) *
but I still carry a Melways and a pocket compass.


I'm with you on that, although I had to google "Melways" to find out what one was!

In Europe I use the Maps.Me app on my Android phone and tablet. It may not be as detailed as google maps, but its free (or a few £ to remove ads). The bonus is that you can download regional or country maps and store them on your device's data card so it doesn't rely on a nearby cellphone tower (I bonus, I imagine, in the deepest outback). It uses satellites and will do directions as well. And its regularly updated. The only issue I've ever had was needing to re-download maps when my Samsung S7 had a major upgrade. But that's the first and only time.
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ozieagle
post 19 Aug 2017, 20:24
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QUOTE (MYKE @ 19 Aug 2017, 18:35 ) *
The advantage of a dynamo is that if it goes wrong, you can take it to bits and fix it. A duff alternator is scrap unless it's only a matter of changing the brushes, and a lot of folk can't even be bothered doing that.


P.S. I just had a look at the MG instructions and noticed that the diagram shows the alternator to be earthed. It can only be earthed through the engine so you must make sure you have a good earth between the engine and chassis. Probably not a problem because if there is a bad earth, the starter motor won't work, but worth checking.


Alternators are repairable! Diode packs and regulators are readily available, and to get at them requires the alternator to be opened and three wires unsoldered.

Yes the alternator is and needs to be earthed, but then so does a dynamo.

Herb
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enigmas
post 20 Aug 2017, 00:07
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I'm currently rebuilding the auto trans in my P5 Rover coupe. I stripped the alternator down to check the bearings, brushes and slip rings after 24 years and 300,000 plus kms of service. Cleaned up the slip rings on my lath, fitted a new set of brushes and just regreased the 2 bearings. No other issues and yes it's still working fine. That's good reliability in anyone's terms.

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Danny
post 20 Aug 2017, 05:33
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QUOTE
The only other thing that needs changing are the two wires that feed the heater motor. Otherwise it will suck rather than blow and you will spend several months wondering why you can't get the demister to work in winter.


Would this actually be the case with a 1950s Smiths blower? This would imply that reversing the polarity would reverse the direction of rotation and I don't believe it would on a field wound motor of this type.

We've been here before LINK

Danny
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