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Allis Chalmers Discussion Forum

Positive or Negative Ground - Does it matter?

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09-16-2015 02:12:59

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Looking at the specs on the D series Allis tractors, it says that some (or all) of them are a positive ground. I know a lot about electrical stuff, but this has me puzzled. First off, WHY would they wire it that way? Negative ground is just the norm. If they may have made some very old cars with POS ground, I have not heard of it. Every car and truck I have ever owned has had a NEG ground.
When I think what's connected, most of the stuff should not matter. Light bulbs wont matter, and neither will a horn. Yes, electronics, such as a car radio would not work, but that's not a common item on an older tractor. An electronic ignition, WOULD matter, but for a system with Points, that should not matter

For an older tractor, all that remains, is the Starter and Generator or Alternator.

This is where I get puzzled. Will a starter run backwards if the battery polarity is reversed? I really dont know, for a brush motor, which seems to be the type of motor used on tractors (and I think all vehicles)?????

That leaves the generator and I know those older D series Allis tractors had generators, not alternators. It's been at least 40 years since I last worked on a car with a generator, so I'm fuzzy on the circuitry. I only recall the voltage regulators had 3 wire terminals and were a pain to trace and repair if they failed. So, whether the polarity matters, I can only guess, and my guess would probably be answered with a YES. (just a guess).

Besides wanting to better understand this, I'm also trying to learn what would happen if I wanted to get rid of the generator and replace it with a modern alternator. For example, my Farmall M, came from the factory with a generator and a 6 volt battery. was it a POS or NEG ground originally? I dont know? But now it's a NEG ground, and I've worked on several other Farmalls (M and H) and all of them have been converted to 12V with a modern alternator with the regulator built into it. (GM type).
So, lets say I was to buy an Allis D series with a generator, and wanted to replace the generator with a modern alternator.

As I said earlier, the lights will work fine, and if the ignition has points, it too should work. I have never heard of an alternator with a POS housing. All of them are NEG ground, and the housing is part of the circuit, so it would have to be a NEG grounded system, in order to use an alternator.

This just leaves the starter. If it runs backwards with the polarity reversed, that's a big problem. If not, I cant see how the polarity would matter. Once a modern alternator was installed, the tractor would become NEG ground.

Feedback appreciated!


By the way, I was talking to someone about this, and the guy insisted that when a tractor has POS ground, you put the red clip from a battery charger on the NEG battery terminal and the black clip on the POS terminal. I told him NO NO NO..... That's not going to work. But he insisted he's right, even though I know he's wrong and all he will do is ruin his battery charger if it dont have built in protection.

This post was edited by chappie at 02:21:36 09/16/15.

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09-16-2015 16:23:13

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  

Most cars before 1955 were positive ground as were the tractors. Many converted to negative ground when they changed from 6 volt systems to 12 volt systems.

Your buddy is wrong on the charger... red goes to + terminal, no matter what it is connected to.

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09-16-2015 12:23:22

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  
there were plenty of positive ground setups.. series field starters will turn the same direction, either polarity. ford alone, on tractors used it thru 1964 at least.

Why? PHYSICS!! scarry huh.. actually using science to base something on...

Edison effect

for those of you wanting a bit more reading thermionic emission

( this will also help you understand vacume tubes too! )

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09-16-2015 07:13:03

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  
There is nothing mysterious about positive ground. Positive and negative ground is like right hand and left hand threads on a bolt. With bolts the nuts screw on in opposite directions, with positive and negative ground current flows through the circuit in opposite directions. Some electrical devices are polarity sensitive, meaning that current flows through them in a certain direction such as ignition coils, ammeters, charging systems, and some gauges.

Coils and ammeters can work either polarity, but coil is more efficient if it matches battery polarity and ammeter will indicate backwards if reverse connected. Generators can be polarized to work either polarity. Some generator regulators are marked “negative ground” some are marked “positive ground” and some are marked “works on positive or negative ground.”

Modern alternators, unless specially modified are 12 volt negative ground.

Starters turn the same direction regardless of battery polarity.

Points, condensers, switches, and lights work either polarity

Positive ground does not seem right to us because manufacturers have not used it in over 50 years.

The output of a battery charger has 2 cables. A Positive cable (usually red) and a Negative cable (usually black). The battery charger output does not have a ground cable as the charger only knows negative and positive, it knows nothing about ground. Battery charger actually sees the vehicle frame that we call “ground” merely as an extension cord from the corresponding battery post. Simple rule to connect battery charger or jumper cables: "Positive to Positive, Negative to Negative, forget about 'ground'".

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John T

09-16-2015 06:04:13

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  
1)Before NEG ground became the decided upon industry standard, some old cars circa around 20's thru 50's were POS Ground, others NEG ground. GM Ford and Chrysler had different ideas and standardized at NEG ground at different times.

2) Some old 6 volt tractors (like IHC) were POS ground but when they used 12 volt they went NEG ground.

3) ALL two cylinder Deeres were POS ground, BOTH 6 and 12 volt.

4) The typical old tractor electromagnetic field tractor starter (unlike a permanent magnet starter) will still turn same direction regardless if polarity were reversed.

5) A battery powered coil ignition is polarity sensitive and the coil polarity should match what tractor polarity is used for most efficient operation.

6) A typical old tractor and car genny will work REGARDLESS of polarity provided properly polarized.

7) A battery don't care which of its terminals happens to be tied to an old rusty hunk of iron (POs or Neg ground), however the genny polarization and ammeter and coil etc all matter and need to be matched up so it works correct

8) They manufacture a POS ground Alternator for special applications far as I know

HOPE THIS HELPS and all above is true far as I know even though many folks have never heard of it

John T Retired Electrical Engineer

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09-16-2015 03:58:02

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  
No the starter will run the same way with polarity reversed. The reason these and most every old truck car and tractor were originally positive ground is the original theory on how electrons moved. Originally they were believed to move from neg to positive. Later they changed their minds and came out with neg ground. Now were back to neg to pos theory but non one wants to go back to positive ground. If you really want to get confused on this a 730 john deere with electric start had 4 6v batteries two on each side. One side is positive ground one side is negative ground the starter is 24v the lights are 12v and if you have the cig lighter its 6v

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09-16-2015 12:29:47

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 Re: Positive or Negative Ground - Does it matter? in reply to jr1983, 09-16-2015 03:58:02  
series field starters will turn the same direction, either polarity.

Your facts are a tad backwards...

It was originally thought that current flowed positive to negative. you can thank the kite and string for that flub...

Once we got out of the stone age and actually started practicing observed science. we figured out that electrons do in fact move neg to positive. it's a mass thing.. and electrons are quite shy of mass.

well, this f&^%ed up, and threw a wrench into the gearing of common thinking on electricity....

to salvage that cluster %^$k they had to invent extra theory to explain charge carrier movement.

that broke down electron theory ( real ) and added hole current theory ( complete BS strung up to support the house of cards that people were operating under ).. to save face they made conventional current and hole current theory the same, relegating electron current to the back pages of those who really knew how things work... ;)

ps.. google edison effect and/or thermionic emission if you want to find out why positive ground...
This post was edited by souNdguy at 12:31:39 09/16/15 2 times.

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David R Inman

09-16-2015 03:43:07

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 Re: Positive or Negative Ground - Does it matter? in reply to chappie, 09-16-2015 02:12:59  
Never been sure why, but lots of tractors pre 70 where Originally positive ground, even your Super M. Lots too have been converted to negative ground.

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