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Tractor Talk Discussion Forum

Positive Ground System Hooked Up Backwards

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T.C. in TX

09-04-2007 16:29:40

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I recently purchased an old Allis-Chalmers M-100 Motor Grader. Noticed the amp-meter reads discharge instead of charge when the engine is running. Checked the batteries--sure enough they are hooked up with a negative ground.

According to the manual this is supposed to be a positive ground system. Question is what do I need to do other than reverse the cables? It is a 24 volt system with 2-12 volt batteries connected in series. Looks like the batteries are fairly new and I do not know how long it has been running with the electric system reversed. The machine has a generator and voltage regulator which are likely original equipment.

Appreciate the advice.

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09-04-2007 23:03:03

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
It is most likely not charging at all, if wrong polarity, as the generator would need flashing to change it's polarity. The voltage reg might also be polarity conscious and may now have welded contacts.
Sooo... unlikely to be reverse charged, might get away with reversing the battery, might need a new voltage reg. If it has been changed over previously ( machines often were) and battery is in correct way for the set-up, all these problems will follow you if you change anything before checking out exactly what you have under there.
Regards, RAB

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09-04-2007 21:00:39

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
As previously mentioned. It is possible to charge batteries in reverse polarity. This usually happens when dead batteries are boosted - on + post & + on - post. Use a test meter to ensure that battery - post is really - and the battery + post is really + .

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Gerald J.

09-04-2007 19:52:01

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
You should check the ignition coil polarity. For positive ground the + primary terminal should go to the points. For negative ground the + primary terminal should go to the ignition switch and the - terminal should go to the points. You will get better spark that way.

The ammeter is telling you the batteries are reversed from the design polarity and its probable that the generator has been flashed to negative ground polarity, else it wouldn't charge (showing as discharge until you reverse the ammeter connections). It would have burned up the generator trying to charge with battery and generator of opposite polarities.

Gerald J.

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ken in texas

09-04-2007 19:00:42

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
The batteries will have to be discharged completely and recharged correctly,adifficult thing to do,otherwise it can burn up your charging system.Best to leave as is .My thinking.....................

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Bob M

09-04-2007 17:16:04

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
TC - If the generator is keeping the batteries charged you can leave your grader connected negative ground. All you need do is reverse the connections at the ammeter - it will then read correctly.

Alternatively you can swap the grader back to positive ground. To do this first swap the battery terminal connections. Then repolarize the generator (with the engine off, momentarily jumper the BAT and GEN terminals at the regulator).

My recommendation is leave your grader negative ground. Having it neg ground lessens the chance of a mistake should you ever need to jump start it from a negatively grounded vehicle (which virtually all modern vehicles are...)

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Jon Hagen

09-04-2007 17:00:13

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  
If the charging system is working, I would leave it neg ground. Just switch all the wires from side to side (post to post) on the ammeter so it reads correctly. I would not intentionally wire a machine with pos ground when 99% of what is out there today is negative ground. Pos ground just invites mistakes in connecting battery chargers and jumper cables which can lead to burned up electrical parts and personal injury from burns or exploding batteries.

If it is a collector toy that "must" be correct, then switch the battery leads for pos ground and polorize the generator BEFORE starting the engine.

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09-04-2007 19:13:22

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 Re: Positive Ground System Hooked Up Backwards in reply to Jon Hagen, 09-04-2007 17:00:13  
Jon is correct, the real test (not done yet) is to turn on the lights with the engine off. (or just the key if it is point and coil ignition. If the meter moves to charge, that is the answer. Change the wires from one side of the amp gauge to the other. If it still discharges when the lights are turned on, it needs to be fixed. JimN

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Kevin Schmidt

09-04-2007 16:54:25

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 Re: Positive Ground System Hooked Up Backwards in reply to T.C. in TX, 09-04-2007 16:29:40  

My guess is the system was converted to a positive ground a long time ago. The amp guage measures current, not polarity. The amp guage would still read discharge if you switch it around. Likely the amp guage itself is just hooked up backwards. Switch the wires on the amp guage posts. It also seems to me if the starter is meant for positive ground, it would not work on a negative ground, but I can't remember for sure.

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Jon Hagen

09-04-2007 17:10:03

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 Re: Positive Ground System Hooked Up Backwards in reply to Kevin Schmidt, 09-04-2007 16:54:25  
Kevin, an ammeter is very sensative to polarity, as it indicates the direction of current flow. Reversing the ground polarity reverses the direction of current flow through the ammeter and makes it indicate in the wrong direction (charge/discharge), thats why the need to switch the wires on the ammeter when switching ground polarity.

A starter with field coils does not care which way the power flows through it, it will spin correctly
Only a starter with permenent magnet fields (very late model car or small engine) will work only with one ground polarity

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