S1/S2 Positive Earth Switch to Negative Earth

[TurbineHeli]

I'm getting ready to rewire and am curious if any of you have made the switch from + to - earth.
Are there any components of the S1/S2 electrical system that are not compatible with - earth?

Thanks,

AMA

[types26/36]

You will have to change the rev counter to neg earth, and if still using the generator & control box you will probably have to polarize them, that's all I did with mine (although I fitted an alternator)

[bob_rich]

Hi

Agree with Brian but just as a caution some of the motors used for the engine cooling fan, heater fan, and maybe screen washer may be permanent magnet type so might need to swop the leads over because they would run in the opposite direction.

many early motors were not permanent magnet type and so it would not matter but later replacements may be

best of luck

Bob

[Galwaylotus]

Like Brian, I converted to an alternator. My tachometer was very inaccurate so I took the opportunity to send it to Speedy Cables who completely refurbished it, converted it and calibrated it. I'm delighted with the result.

[Quart Meg Miles]

Galway, the tachometer should have worked provided that you re-threaded the wire leading to the coil. The sudden break of current must be in the correct direction. You didn't convert to electronic ignition at the same time did you?

[RichC]

Radio!

[Galwaylotus]

Galway, the tachometer should have worked provided that you re-threaded the wire leading to the coil. The sudden break of current must be in the correct direction. You didn't convert to electronic ignition at the same time did you?

I didn't even try to keep the tach as originally installed. As I said it was very inaccurate and well worth the full refurb and calibration. I haven't yet converted to electronic ignition but it has always been in the plan. Can't justify getting everything done at once due to the cost but a bit here and a bit there will get there in the end!

[ricarbo]

I always understood the change to negative earth was to reduce the effects of electolytic corrosion.
On what, I am unsure.

Was it to reduce battery terminal corrosion and shift it to the larger chassis, resulting in the terrible rust reputation of older cars, the other way round, or what? Or was it because most modern electronics are negative ground?

Regards
Richard

[Quart Meg Miles]

Corrosion was always quoted as the reason. But if the chassis is used as one half of the distribution system then changing polarity simply moves the corrosion from one end of the circuit to the other.

It may be a coincidence but positive earth suited the electronics of early transistors, the Germanium pnp types, whereas negative earth suits the later Silicon npn devices and integrated circuits (as it would the even earlier valves [tubes in USA]). The change-over was about the same time, early 1970s.

[nomad]

So radio tubes were called valve's in the UK??
Didn't know that but it really is more discriptive of what they do.

I always thought that battery polarity causing or preventing corrosion was so much hokem.

Kurt

26/3754

[SonnyS]

the Prince of Darknes (ykw ) had this to say about there history with polarty. hope it makes sense. enjoy - Sonny

?Electrical equipment will only function with current flow which requires a return path for the current that has passed through the component. Originally most vehicle manufacturers utilised either a special wire or the chassis for a return path. To distinguish between the two wiring systems, the type that used special wire return was designated ?insulated return or I.R?. and the chassis return as ?earth return or E.R?.
Earth return would be dangerous where high voltages are used, but earth return is quite safe for low voltages between 6 and 12 volts, but any leakage between wiring and chassis causes corrosion at the leaky point. Pre-war insulating material was usually basically wood, rubber, ebonite, glass or later bakelite. All but glass had tendencies to exhibit minute leakage, which over a period of time encouraged corrosion of the terminals. Battery connections were most prone to corrosion because of the added effect of the acid. Earth return is only effective if the battery has one terminal connected to the chassis, and for certain chemical reasons less corrosion occurs if the positive earth terminal is earthed.
It was because of the advantages of earth return that by about 1935 nearly all manufacturers were using this system and to reduce corrosion the positive battery terminal was earthed. An additional advantage of positive earth was that a sparking plug sparks better with the earth electrode positive. Since the war, plastic insulation has been used extensively, and because of this the leakage current no longer prevails. As there is no leakage to bother about, it does not matter which battery terminal is earthed from a corrosion point of view. Sparking plugs still operate more effectively with a positive earth electrode but now that ignition coils can be made which reverse the earthing potential of the power flowing through them, it is possible to use a negative earth battery and still obtain positive earth high tension. In theory the last few facts are a little more complicated than stated but in practice the effects are still the
same.
Post war negative earthing has many factors in its favour, and the most predominant are given below: -
Television interference is easier to suppress with negative earth, and of course this was no bother before the war. Electrical equipment is now being used as standard in more cars additional to the radio (probably the first electronic devise used in a British car)
It is characteristic of all electrical apparatus, as opposed to electric, to be polarity conscious, that is affected by which supply terminal is earthed. If a transistor is connected the wrong way round it will be immediately irreparably damaged. Universal negative earthling would ensure that all power supplies to electronic gear were to the same earth and therefore reversed connections impossible. Electrical components like bulbs etc are not affected by which terminal is earthed, therefore these can be unchanged and universal in their application.
To help avoid any confusion which may arise in the difficult in-between changeover period, when vehicles of both types of earthing are produced, components are being marked + and -. To consider an ignition coil, for instance with positive earth,+ is earth, so this is for distributor contacts and ? is live, so this is the ignition feed. The same coil when used with negative earthing would be earth at ? and so this is for the contact breaker, and live at +, making this the ignition feed.
Thus to summarise, Positive earth gives a better spark and less corrosion when insulators leak current. Negative earth interference is easier to suppress and electronic components are less likely damaged?.

[Quart Meg Miles]

Ironic that the Prince of Darkness (ykw = LKS?) is quoted in a forum devoted to the King of Lightness.

Battery terminal corrosion isn't related to poor insulation as it is due to metal mismatch (electro-chemical potential differences) between the materials creating local "batteries" in moist conditions.

TV interference isn't easier to reduce with negative earth as the suppresion is by inherently symmetrical passive components. But original (British) TVs changed from positive modulation to negative when they changed from the original Black&White 405 line system to the colour PAL 625 line, meaning interference made black spots instead of white which is much less noticeable. And the change over was post 1966, about when the negative earth bug bit. Can't speak for the LH countries with their NTSC (Never Twice the Same Colour) system.

[RichC]

happy to be a +ve earther ( not to be confused with a flat-earther now, by the look of it)