Positive or negative earth?

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andrewh
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#1 Positive or negative earth?

Post by andrewh » Fri Dec 04, 2015 10:09 pm

I have a car, a 3.8 which is minus its regulator and the battery leads are not marked. Is there an easy way to find out whether she is still positive earth or negative before I cook something wiring it up for a start? I am sure there is but my brain is tired tonight
1962 3.8 Series One FHC

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Heuer
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#2

Post by Heuer » Fri Dec 04, 2015 11:11 pm

You can polarise the generator one way or the other so you know what you are dealing with. The rest of the stuff (with the exception of the radio) you can ignore. Details of how to do it are in the Forum KB and very simple to do. Decide whether you want to be positing or negative earth and touch the dynamo cables to the relevant battery posts - this only works with a dynamo though, not an alternator.
David Jones
S1 OTS OSB; S1 FHC ODB

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mooney1el
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#3

Post by mooney1el » Sat Dec 05, 2015 12:42 am

Wouldn't a simple way to note a potential change to + earth be to look at the coil wiring?
Richard
1964 FHC 890248, owned since 1970

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cactusman
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#4 Polarity

Post by cactusman » Sat Dec 05, 2015 7:58 am

Coil will not help. The coil has a battery terminal and a points terminal on the LT side. It is important they are correct but the actual battery polarity does not matter. If you don't know then I would go negative earth and polarise your dynamo appropriately. You then have the freedom to install a modern radio, led bulbs etc should you wish. Nothing else cares except the clock (it has a diode inside that will burn out if connected the wrong way around) so make sure your clock is either suitable or get it rebuilt...there are a couple of places that will convert to electronic operation removing the unreliable points and diode but retaining the mechanism and tick :D
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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cactusman
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#5 Polarity

Post by cactusman » Sat Dec 05, 2015 8:05 am

BTW I rebuilt my clock using a module from www.clocks4classics.com

They do modules for both positive and negative earth. Diy fitting is a delicate operation but I managed fine. Or they will do the conversion for you I believe. They are UK based.

Six months on the clock works like a dream!
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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Heuer
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#6

Post by Heuer » Sat Dec 05, 2015 8:50 am

Actually coils are polarity sensitive and they come in both flavours - +ve or -ve earth. With the incorrect polarity you will still get a spark but about 10% weaker which may (or may not) cause misfiring under heavy load. This is an extract from our post on coils in the Factory Fit thread and I am grateful to the author (Me) for allowing me to use it here:

Coil Polarity
The polarity of the coil should match that of the battery - coil +ve to battery +ve, coil -ve to battery -ve. No matter whether the car is +ve or -ve earth or which way the coil is installed (regardless of coil markings) it will still output voltage. Spark plugs are however more sensitive when it comes to polarity and the coil should be connected to provide negative polarity to the spark plugs centre electrode. It takes 10% less voltage for the plugs to arc if the hotter centre electrode is -ve, and the cooler (by comparison) earthed electrode is +ve. With the terminals reversed and the centre electrode is +ve, the car will still run with but with a 10% voltage handicap and in low temperatures, driving fully loaded and accelerating hard up a hill may misfire. If the coil is modern and has - & + markings connect accordingly. With CB and SW markings there is no way to tell whether the coil is positive or negative earth other than looking up the Lucas serial number. The coil can be tested for polarity using a voltmeter set to its highest range, with the positive (usually red) lead connected to a cylinder head bolt and the negative (black) lead connected to a plug cap - this applies regardless of whether the car is positive or negative earth. Cranking the engine will cause the needle (or digital readout) to swing upwards if the coil is connected correctly. If the needle swings down off the scale (or the digital readout goes negative) the coil is connected wrongly, and the coil leads should be reversed, regardless of markings.

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David Jones
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andrewh
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#7

Post by andrewh » Sat Dec 05, 2015 7:13 pm

Thank you chaps. Actually the coil is missing anyway. However a bigger problem is that on close inspection of the wires that connect to the missing regulator one of them Has burnt. I guess that is why the regulator is missing. This is a car I plan to fully restore but fancied starting it up to see how it ran. I am now down to thinking it's not worth the risk of setting the loom alight. Cannot think why the wire has shorted but when I fit a new loom it will not matter. I am down to thinking of a basic hot wire to start the motor as the only safe way to proceed. Ah problems problems
1962 3.8 Series One FHC

http://etype860897.blogspot.com/

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cactusman
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#8 Positive or negative

Post by cactusman » Sun Dec 06, 2015 9:49 pm

Well I learned something today. One lives and learns. Best make sure you get the right coil depending on your choice. Happy wiring. J
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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andrewh
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#9

Post by andrewh » Sun Dec 06, 2015 9:52 pm

Ok so using the old grey matter today I spotted that the car has an alternator fitted. Now I reckon that MUST be negative earth. It's perhaps the reason why there is no regulator connected? If it has an integral control then that would explain the lack of regulator. I reckon the wire that has burnt is the wire to the voltmeter. If the wires had been left un taped could it short back from the voltmeter back to earth? It's possibly not worth messing with this but want to start it if possible. What would y do with the wires to the regulator if integral control box in the alternator?
1962 3.8 Series One FHC

http://etype860897.blogspot.com/

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#10

Post by paydase » Mon Dec 14, 2015 3:16 pm

David,

The info you quote on the 10% less voltage need for a negative spark is very interesting and intriguing.
Is that the reason that pushed car makers (at least Jaguar) to shift from positve ground to negative ground?
And what is the physical reason explaining why less voltage is required?
Serge
S1 FHC 889725

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#11

Post by Heuer » Mon Dec 14, 2015 4:24 pm

The 4.2 cars had -ve earth coils to overcome the problem with the switch to negative earth systems. The effect is only a problem nowadays if you run a positive earth car with a modern negative earth coil, in which case you should reverse the connections or perform the above test. Car will still run with the connections either way but you may experience misfire under load if it is wrong.
David Jones
S1 OTS OSB; S1 FHC ODB

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#12

Post by mgcjag » Mon Dec 14, 2015 4:51 pm

Hi Serge....have a look at this link....it gives a detailed explenation....basically down to electron flow (which is from -ve to +ve) in a circuit from the center electrode of the spark plug to earth.....the center electrode is a hotter surface and its easier for the electrons to flow from a hot to cold surface so needs less voltage to drive it.....this is for the HT side of the coil http://mgaguru.com/mgtech/ignition/ig104.htm
Last edited by mgcjag on Mon Dec 14, 2015 4:54 pm, edited 1 time in total.
Steve
1969 S2 2+2 & Building a C type replica

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#13

Post by paydase » Mon Dec 14, 2015 4:53 pm

mgcjag wrote:Hi Serge....have a look at this link....it gives a detailed explenation....basically down to electron flow (which is from -ve to +ve) in a circuit from the center electrode of the spark plug to earth.....the center electrode is a hotter surface and its easier for the electrons to flow from a hot to cold surface so needs less voltage to drive it.....this is for the HT side of the coil
No link but now I get it.
Smart
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#14

Post by Heuer » Mon Dec 14, 2015 6:19 pm

I think the link only works for UK and USA for some reason. Here you go:
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David Jones
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#15 Re: Positive or negative earth?

Post by rfs1957 » Sun Jan 19, 2020 6:18 pm

Sunday evening in front of the fire, okay okay I admit it ....... guaranteed to throw up lazy questions.

The link to the JCNA guide to positive-to-negative-conversion shown in the Knowledge Base doesn’t work any more, for me anyway ; and looking via the JCNA « search » function throws nothing useful up .......

Is there a trick please ?

Not looking for a step by step guide, reckon I grasp the principles involved, but would have liked to see an overview to check there’s nothing odd that might have escaped me.

I’m moving to an alternator at the same time, will keep the RB340 for decorative purposes, have a quartz clock from Mike Eck which is indifferent to polarity, am making a new solid-state instrument voltage regulator anyway, know that my USB port will need switching, that I don’t have any radio issues, that I will have to change my indicator beepers, that my Lambda probes and meter will require a flip, understand the coil polarity issue, is there anything else ?

Or are there no complications save the ones I’ve introduced by myself !?
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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johnetype
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#16 Re: Positive or negative earth?

Post by johnetype » Sun Jan 19, 2020 8:54 pm

The biggest challenge in a polarity conversion is reflashing the dynamo field but you're sidestepping that issue by fitting an alternator.

On a standard car, early enough to have no electronics and motors that all have field windings rather than magnets that's it. I believe you have a cam driven tacho generator so that's not an issue.

Depending on style you might need to change the battery leads as the positive lead may now not reach/fit the terminal post and vice versa.

I don't see why you need to change your indicator units unless you've retrofitted electronic ones. I'll take Mick E's word that he's made his clock polarity neutral - probably by fitting a bridge rectifier at its input. Any LED lights which currently would have to be +ve earth specials, would need changing.

I'm assuming you've still got mechanical points, in which case no issue.

If you've fitted an aftermarket radiator fan - you know that company in Worcester you love - it'll need rewiring so it runs the right way to blow not suck.

A question I can't answer if you've got one of the in tank fuel pumps is if they are polarity sensitive - no doubt someone else will help. The fuel gauge will be fine if on a thermal voltage regulator ie standard, but you've said you're changing that to an electronic one so that's all fine too.
John

1969 Series 2 FHC

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Geoff Allam
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#17 Re: Positive or negative earth?

Post by Geoff Allam » Mon Jan 20, 2020 5:06 am

I am pretty sure the in tank fuel pumps are polarity sensitive. When a mechanic converted my 3.8 from positive to negative ground he neglected to reverse the leads on my fuel pump. The car made it half a block till the fuel bowl were dry and then started up and ran fine once I reversed the leads after a bit of head scratching.
Geoff Allam
67 series1 ots under restoration
64 series 1 fhc burning lots of oil
53 xk 120 fhc sitting in garage waiting for attention

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cactusman
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#18 Re: Positive or negative earth?

Post by cactusman » Mon Jan 20, 2020 7:30 am

The in tank pump is polarity sensitive so will need to have its wires swapped over at the joint block in the wheel well.
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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rfs1957
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#19 Re: Positive or negative earth?

Post by rfs1957 » Mon Jan 20, 2020 2:39 pm

Mine is electronic SU from Burlen, out of tank.

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Suspect the guts may need to be changed, and not a simple wire-swap, as they do list both -VE and +VE as different references.

The "indicator bleepers" I referred to are the things that go beep after a certain number of flashes, for when I forget to turn them off - paranoia for us motorcyclists. The system I use (on all my vehicles) requires two units for positive earth, hence the plural.

The JCNA document is now here, many thanks to Ralph for passing it my way.

https://www.jcna.com/positive-negative- ... nversion-0
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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#20 Re: Positive or negative earth?

Post by rfs1957 » Mon Jan 20, 2020 8:40 pm

Indeed, this type of pump can't just be flipped with the wiring, according to Burlen, so I'll have to buy a negative-earth one after all.

Anyone needing a perfect +ve earth electronic points pump, as a spare maybe, step this way.

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Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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