TR4A - Positive or Negative earth?

[BrianC]

I happened to be browsing the TR4A handbook on CDROM at work (lunchbreak of course!!) last week and came across the statement that the car is negative earth, and the wiring diagram confirms this. However, I am sure that my previous TR4A was positive earth, so when I got home I checked on Lynda's car. Sure enough it is positive earth, so I wondered if perhaps the earlier TR4As were postive and at some point Triumph switched over to negative earth. Both cars were built and registered in 1965.

 

The strange thing is that the wiring diagram in the TR4/4A workshop manual shows it as positive earth. Presumaby this is simply the TR4 wiring, as there is no separate diagram for the TR4A and I'm sure there are other differences. I do have copies of various wiring diagrams from the internet (icnluding this forum), but haven't got around to checking these yet.

 

Meanwhile, can anyone explain the apparent discrepencies?

Edited September 18, 2006 by BrianC

[stuart]

Brian as far as i know all TR4a were positive earth when they left the factory. Original workshop manual confirms this.I have always wondered about workshop manuals on cd rom because the old saying "garbage in garbage out" applies to a lot of re written stuff like that, ie you only get out what was put in by the person who wrote it!!

Regards

Stuart

[BrianC]

I have always wondered about workshop manuals on cd rom because the old saying "garbage in garbage out" applies to a lot of re written stuff like that, ie you only get out what was put in by the person who wrote it!!

Thanks Stuart.

I also have the Brooklands reprints of both the handbook and the workshop manual, and as far as I can tell they are the same as the Heritage CDROM versions (apart from copyright statements, etc) - certainly in respect of the earthing and wiring diagrams.

The CDROMs are simply scanned images (not text) copies of pages from either the original or reprint and are therefore uncontaminated by human error (or at least they should be!).

Main reason for having them is they are easy to search and I can print pages for use in the garage without getting oily fingerprints on the hard-copy versions. It's also, by far, the cheapest way to get all the handbooks, manuals and parts books.

I'm still intrigued as to why there is this discrepancy, unless cars for some overseas markets were built as negative earth and the handbooks adjusted accordingly - not very likely, I would have thought.

[graeme]

In this case it is the original workshop manual that is is incorrect as it is only a slightly modified TR4 manual!

 

The only correct wiring diagram that I have seen is in the TR4A 'glovebox' handbook.

 

As far as I am aware all TR4A should be Negative earth and this is confirmed in Bill Piggot's book 'Original TR4/4A/5/6'.

 

It would be relatively easy to change to positive earth but I can't imagine a reason for doing this.

 

Graeme

[stuart]

I think we may have opened a can of worms here. My previous TR4a was a 1967 car sold after cessation of production was positive earth also and that was an unmolested one owner car. My present one is a North American import and that was also positive earth although now converted to negative. Can other contributors shed some light on why this is so?

Stuart.

[BrianC]

I think we may have opened a can of worms here.

Don't I always!?

At least I'm not the only one confused this time.

 

Graham Robson may well know the answer on this one !!

Or Jon Marshall?

[meesonia]

Hello, as previously posted bought my 4A 12 months, got an MOT on it etc etc, but always knew it was +ve earth, previous PO had fitted a new battery but I can't remember if he said he'd changed the earthing. Anyhow everything runs fine and always thought it was +ve from factory, by the way it's a late 4A June 1967. BUT with exception the Ammeter has always gone -ve when engine is rev'd. Which I knew wasn't correct.

 

So started searching on forum and came across this post, and my glovebox handbook also, as I think Graeme has said states -ve earth. So has someone in a past life "messed" with it. I know I could swap the Ammeter wires over but I'd rather put it back to -ve earth. Is it as simple as swinging the battery round? I did read somewhere you have to "flash" the Voltage Regulator, and possibly swap a Distributor wire, is this correct? Thanks

 

Ian

[ianc]

Ammeter - Yes the leads will need to be reversed because it is a polarised device. If this is not done, the ammeter will read incorrectly. i.e. show discharge when charging and vice versa.

The fuel gauge is not polarised. These work on either a moving iron system, a hot wire system or in some situations by heating a bimetallic strip. The direction of current flow is not important - they merely register the current flow magnitude (same for temperature gauge).

According to Tony Tranter in Haynes "Automobile Electrical Manual", ignition systems are designed to have negative polarity at the centre electrode and if reversed can be up to 40% less efficient.

Coils for both Positive and Negative earth vehicles were produced, but who knows if anyone has changed a coil for the wrong type in the history of older vehicles, so it is wise to check polarity of the spark (see below for testing polarity using a lead pencil).

With early coils, swapping the CB and SW leads will switch polarity of the spark. Later coils use + and - but the requirements are the same.

To test the spark direction with a lead pencil, interpose the writing tip of a lead pencil between the spark plug top stud and the HT lead, leaving a small gap either side of the pencil tip. If the polarity is correct, a flare between the pencil and the spark plug will be seen (if the flare is between the pencil tip and the HT lead, then the polarity is reversed).

Since all modern vehicles use a negative earth system, almost all replacement coils will expect the vehicle to be negative earth. Almost all in-car entertainment systems are designed for negative earth, as are almost all in-car electronic devices.

 

Ian Cornish

[Lebro]

Yes, flash the regulator, reverse the wires on the coil (assuming they are correct for +ve earth at the moment).

It would be advisable to check the regulator settings, as these can need a tweak when changing polarity.

 

Bob.

[peejay4A]

I think you flash the field coils of the dynamo rather than the regulator, although I've never done it myself.

[stuart]

I think you flash the field coils of the dynamo rather than the regulator, although I've never done it myself.

Correct.

Stuart.

[AlanT]

Until quite recently I thought that all the original TR stuff was polarity insensitive.

I my mind only "electronic" items would mind a polarity switch.

 

Electrical stuff like motors does not care, nor do thermal gauges.

 

But it turns out the sparking plugs work better with one polarity thn the other and so you need to swap the coil connections.

[Alec Pringle]

As a teenager in 1968-70 I used to help out in Uncle Tug's repair garage, where he looked after a lot of Triumphs, and I also did a bit for a local hifi shop who fitted car radios, cassettes and 8-tracks as well as domestic audio.

 

Triumphs required special attention, as one common optional extra was alternator fitment - available on most models, not just TRs . . . . . if the car was dynamo equipped ex-factory it would be positive earth, whereas if it had the alternator upgrade it would be negative earth.

 

That wasn't the end of it though. Early alternators were unreliable at best, and jolly expensive - failure usually required a genuine Lucas exchange unit, as repair kits were not easily available at the time. You could buy three reconditioned dynamos from the local supplier for the price of one Lucas alternator. Not to mention the possible wait, a week or two, for a replacement alternator . . . . if one wasn't on the shelf at the Triumph dealer or the Lucas agent. Dynamos were an instant over the counter swap at the local reconditioner.

 

Often enough the customer had stretched himself to buy the car, and subsequent generator failure was a big hit - alternator and dynamo alike were costly components, in real terms hugely more expensive than they are today.

 

So the exotic alternator went pop, and it would be replaced by a dynamo, which merely needed 'flashing' to reverse the polarity - although the local rebuilders did that for you, you specified pos or neg earth as required.

 

The car radio, if there was one, almost certainly offered the potential for switching to pos earth if it had been fitted to an OE neg earth car, although polarity switching was not necessarily an option if your car had been built as positive and the radio factory or dealer fitted.

 

Hence the confusion years later, as dynamo-equipped 4As might be pos or neg earth . . . .

 

Cheers

 

Alec

[Lebro]

Just to clarify, "flashing the regulator" means pushing the "cut in" contact down , & then pulling it pack up again.

This has the effect of temporarily joining the battery to the dynamo armature. This in turn will connect to the field winding via the regulators field contacts which are closed until the dynamo starts to O/P a voltage higher than 15V.

 

So, yes, it is the dynamo which needs attention to reverse the residual magnetism in the field winding cores, but "flashing the regulator" is the easyest way to do it.

 

Bob.

[meesonia]

Thanks guys or the all the thoughts and suggestions, nothing in the Triumph world is ever straight forward. But to summarise, mainly for myself, car still has dynamo fitted BTW, and Coil +ve wire goes to the Dizzy, -ve wire goes into the loom. So I guess that means the Coil is +ve earth and the wires need swapping when I swing the battery round, then flash the Reg as described by Lebro?

 

Picking up on Ian C idea of testing the polarity using a lead pencil, between plug & HT lead, can I be sure with todays pencils from the likes of Staples do actually contain lead, in todays lead free society I would have thought not. Do I have to find a Joiner from the early 1970's with his collection of old pencils still intact :-)

 

Appreciate confirmation I've got it right, many thanks

 

Ian

[LGFromage]

The original owners handbook for my late 1967 export TR4A states "A 12 volt negative earth system is employed in all circuits". The publication is part no 512916 and it is the 3rd edition apparently dated 2/66. Of course it could have been a USA edition as was the accompanying Servicing Schedules manual.

 

However it is what happened in practice that counts and in my car's case it definitely was negative earth when new.

 

Tim

[peejay4A]

Some electric motors are polarity sensitive, permanent magnet ones for example, not the ones on TRs though.

[meesonia]

Found this article on the VTR website which explains succinctly the change procedure

 

Ian

 

http://www.vtr.org/maintain/negative-ground.shtml

[mike3md]

............and I think it is the graphite in the pencil lead that provides the conductivity?

 

 

Mike

[Lebro]

Yes it is

[ianc]

The reference to "lead" in pencils harks back to the Romans. In modern times, pencil 'lead' consists of graphite mixed with some sort of clay - and graphite is a form of carbon and a good conductor, which is why the brushes in motors, dynamos etc are made of carbon.

Ian Cornish

[cbxman]

Hi Folks,

 

I'm a new boy here and have been following just about every thread I can find, so I can 'gen' up on all things TR4A. However, I couldn't help but mention that ignition coils are effectively an "Auto Transformer" and together with the capacitor (across the breakers) form a resonant circuit which provides the spark plug with a burst of sparks at the resonant frequency of the circuit. If you just reverse the connections to the coil then you destroy the resonant circuit, so the spark efficiency is severely reduced. By leaving the coil connected correctly it is irrelevant as to whether it is neg or pos earth as it is a transformer. As the contact breaker opens, a series of sparks which alternate +ve then -ve sparks with respect to earth are created. The spark plug does not care which way the current actually flows. There may be some esoteric arguments regarding the polarity of sparks, but they are largely irrelevant in this application. The drop in performance associated with changing the car's polarity is likely due to the mistaken practice of crossing of connections on the coil.

 

I'll get me tin hat and run.........

[stuart]

Years ago I was always taught to re polarise a new or rebuilt dynamo before fitting to make sure that it worked the right way round and that also applied to a s/hand one that had been sat on the shelf for long periods of time. It only takes 30 seconds to flash it up.

Stuart.

[Lebro]

I also was of the opinion that the coil should always be the same way round - if you look at the internal wiring, one end of the HT winding goes to the HT output terminal at the top, but the other end instead of going to ground as you might expect - goes instead to the "CB" terminal - the terminal which connects to the contact breaker. As you say efectively an autotransformer with the bottom of the combined winding going to the IGN terminal ie. +12V from the ignition switch

 

I held this view right up to last year, when I was shown a very long article explaning the requirement to have the correct polarity of spark. the article was very strongly convincing, & I weakened.

Am now having second thoughts ! (or is that third thoughts)

 

Bob (ex electronics engineer)

[cbxman]

Bob,

 

I have seen some articles too regarding the polarity of sparks. I believe the more dangerous lightning strikes are ones which emerge from the ground rather than those from the clouds, or maybe the other way round, so there is some merit in the argument about their polarity. BUT we are talking transformers here, so there is no control over the direction. A negative earth vehicle will always produce the initial spark in one direction. The same coil on positive earth vehicle will produce the initial spark in the opposite direction. So if you want the initial and strongest spark to be in a particular direction, then you need to wind the secondary (HT) coil in the opposite direction.

 

I'm not sure whether coils can be purchased specifically for +ve and -ve vehicles. That is only way you can change the initial spark polarity. Just changing an ignition coil connections over will not work as you spoil the Auto Transformer action.

 

Cheers, Retired electronics engineer