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Positive Ground Charging System

posted Jun 11, 2016 01:43:13 by Noah
We got an old Trojan 1900 front end loader at the yard with an air cooled Deutz engine. It's the one in my avatar pic. We've been getting it into operable shape so as to put the Caterpillar down and take care of some much deserved maintenance. The batteries were flat(24v system), so I sent one of my guys to change them out with a couple of big car batteries until we get out to buy some kickass loader batteries.
I walked by as he was making the connections and stopped him right away. He was hooking the short lead that goes right to the chassis to the positive terminal of the first battery! I asked him, "Are you sure you want to do that?".
He assured me that that was how he took it apart,and he's not a dummy, so I let him carry on. Sure enough, he was spot on. The old dinosaur fired right up.
But now I need to check the charging system and every where I hook up my meter, I'm getting negative voltage readings (obviously).
Anyone have any tips for checking a positive ground charging system?
The owner wants to change the alternator, and i'm willing to bet that it will fix it, but this is new ground for me (get it?), and I'd like to able to troubleshoot it with confidence in the future.
[Last edited Jun 11, 2016 01:45:44]
Massachusetts, USA
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10 replies
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AndyMacFadyen said Jun 11, 2016 05:04:51
The combination of a positive earth system and alternator is almost unkown so I would make 100% sure it is positive earth as apart from the charging system old school engines will start and run quite happily with the polarity reversed.

Not really any different to work on I haven't seen a postitive earth system since the 1970's they were phased out on cars around 1964 and positive earth alternator systems were practically unknown.
A lot of people converted their cars to from positive to negative earth so they could use more modern radios or fit alternators in place of dynamos those days when cars had no electronics it was real easy to do.
Not really any different from a negtive earth system apart from the polarity. Same goes for 24v, the voltages are just 2x and currents 1/2. Only system that was unusual were some miltary 24v systems with dynamos that used a carbon pile regulator to control the dynamo.
"Rust never sleeps"
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Noah said Jun 11, 2016 11:19:58
Alright, thanks for the reply Andy! I figured if it was reversed polarity we'd be welding the batteries togetheršŸ˜‰.
I'll see what we inherited as far as service info with the machine before I go any further with this and let you know what I find.
Massachusetts, USA
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George said Jun 11, 2016 23:19:45
Not really any different to work on I haven't seen a postitive earth system since the 1970's they were phased out on cars around 1964 and positive earth alternator systems were practically unknown.
A lot of people converted their cars to from positive to negative earth so they could use more modern radios or fit alternators in place of dynamos those days when cars had no electronics it was real easy to do.

Anyone have any tips for checking a positive ground charging system?

Check this video
https://www.youtube.com/watch?v=ZB6CrK3tdLw
[Last edited Jun 11, 2016 23:21:41]
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Noah said Jun 12, 2016 01:32:46
Thanks for the link George. I've watched a bunch of Ivan's videos, but never saw that one. One smart guy right there!
Massachusetts, USA
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George said Jun 12, 2016 23:55:47
One smart guy right there!

Agreed
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AndyMacFadyen said Jun 13, 2016 05:29:02
He is smart and not scared to tackle anything with an engine.
This particular video reminds me that 40+ years s back a local truck dealer asked my dad to look a at an army surplus Austin Champ a 1950's jeep style 4x4 (similar to this one https://www.flickr.com/photos/buzzer999/7598514370/in/photostream/). It had been sitting for well over a year because couldn't get to crank over although he had fitted new batteries. It turned out he hadn't realised it was 24v and had fitted 2x6 volt batteries. As soon as it got fresh 12v batteries the old Rolls-Royce engine in it fired up almost instantly.
[Last edited Jun 13, 2016 05:30:27]
"Rust never sleeps"
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Noah said Jun 14, 2016 01:52:02
So it's NOT positive ground. Either the people we bought it from were running it like that, or my minion was asleep at the wheel. The manual is in English, except for the alternator description which is French. The diagram clearly shows the negative terminal going to ground. So I corrected the polarity and now at least the fuel gauge isn't pegged at empty. Still not charging, but no time for trouble shooting that work horse today. WAY too many cars to murderize.
Massachusetts, USA
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AndyMacFadyen said Jun 14, 2016 10:59:55
Some ancient French alternators used a relay in the circuit ---- I can't remember how it was wired I presume it was in the field circuit but on French Chryslers of the 1970s we used to change them on a regular basis because the contacts burned out.
"Rust never sleeps"
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Noah said Jun 14, 2016 13:22:40
You are a well of information sir!
When I get some time to study the diagram, I'll be sure to let you know how the circuit works. Thank you very much!
It may just be a blown fuse, I know there's at least one burned. I gotta hunt through my Volvos and Mercedes for some of those old plastic cylindrical fuses with the exposed conductor.
Massachusetts, USA
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AndyMacFadyen said Jun 14, 2016 16:18:27
I always took the the view that the words French and car electrical should never be used in the same sentence.
"Rust never sleeps"
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