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Author Topic: Positive ground system  (Read 3603 times)
kyle4501
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« Reply #15 on: April 26, 2007, 05:26:53 AM »

The starter does not care about pos or neg ground. The direction of rotation is determined by the physical construction of the starter, not the polarity of the DC voltage.

I have an old (1951 I think) Farmall Cub tractor with a magneto ignition, so it only needs a battery for starting (the lights were long gone when I got it). It doesn't matter wich way the battery is connected, it starts just fine. The 12 volts really spin the motor over a lot faster too! Grin

I did a web search a while back & the only thing that needed attention was the generator charging the battery.

ymmv
Good luck!
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DrivingMissLazy
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« Reply #16 on: April 26, 2007, 05:41:49 AM »

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The 12 volts really spin the motor over a lot faster too! Grin

That really works great on the old VW sand rails and dune buggys I used to build. We always put a six volt starter on them with the remainder of the electrical system being 12 volts. In the many years that I did this I never had a starter burn out, and it really cranked the engine good for a fast start.
Richard
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Don4107
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« Reply #17 on: April 26, 2007, 02:58:05 PM »

Any DC motor that is mechanical, ie has brushes, not electronically controlled, will run the opposite direction if you reverse polarity.  A quick chat with a good heavy duty starter rebuild shop will clear this up.  If you have starter in hand they will be able to tell you if they can convert it for you.  If not they should be able to supply (trade) for one that will turn the correct direction. 

If you have any DC motors laying about, starter, HVAC fan removed for conversion, coolant boost pump, drivers compartment fan, toy DC motor, ect, try reversing the polarity and see what happens.  This is why you can reverse a DC motor that has only two leads.  Reverse polarity and it turns the other way.  It may not have the same power in both directions because timing can be set to give optimum performance in one direction.   

Same thing would apply to any DC motor still used in the bus, blower (heater and AC), fans ect.  May be as simple as reversing the leads, may not if the motor has one lead and the case is tied to ground like the starter.
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Don 4107 Eastern Washington
1975 MCI 5B
1966 GM PD 4107 for sale
1968 GMC Carpenter
Len Silva
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« Reply #18 on: April 26, 2007, 03:33:52 PM »

Don, I think you are wrong about that.  Series wound DC motors only care about the relationship of the field winding to the armature.  The direction of the starter, blowers etc. will not be affected by the polarity change.

Only permanent magnet DC motors such as used in window operators and some RV fans can be reversed by changing polarity.

My 4104 has all the original starter, blower and fan motors.

The only thing that needs changing is the alarm rectifier and the temperature gauge, possibly the electric oil pressure gauge.

Len
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kyle4501
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« Reply #19 on: April 26, 2007, 03:58:07 PM »

Any DC motor that is mechanical, ie has brushes, not electronically controlled, will run the opposite direction if you reverse polarity.  A quick chat with a good heavy duty starter rebuild shop will clear this up.  If you have starter in hand they will be able to tell you if they can convert it for you.  If not they should be able to supply (trade) for one that will turn the correct direction. 


There is something amis here. Stop by & I'll demonstrate that the starter on my cub doesn't care what the polarity is. Maybe I've missed something in the past.  .  .  OK, OK, I know I missed a LOT of things... Grin

The heavy duty rebuild shop that rebuilt my stuff said polarity didn't matter if I wasn't changing the field windings.

If it mattered, wouldn't the starter have a label indicating pos or neg ground only?
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Lee Bradley
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« Reply #20 on: April 26, 2007, 04:28:32 PM »

Starter rotate the same direction on either polarity because the elector-magnet (field windings) are reversed when the polarity is reversed. Same thing as a generator will work on a positive or negative ground system; you just have to flash (polarize) the field windings when you hook it up.
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Don4107
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« Reply #21 on: April 26, 2007, 04:41:16 PM »

Ah, there is were I went wrong.  I fool around most with permanent magnet motors.  I stand corrected. Shocked
Guess I should stick to playing with toys!
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Don 4107 Eastern Washington
1975 MCI 5B
1966 GM PD 4107 for sale
1968 GMC Carpenter
Hartley
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« Reply #22 on: April 26, 2007, 09:03:09 PM »

Some of the big older starters have a + and - Terminal, The (-) goes to Ground and the (+) goes to battery power.

Only way to tell is look and see if there are markings for polarity. and adjust the cables accordingly.

Just Guessing based on what I saw on and old bus I had. ( GM )... They had moved the wires to change from postive to negative ground...
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gus
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« Reply #23 on: April 29, 2007, 05:04:27 PM »

No, the starter does not have to be changed.

Although I haven't changed over a bus I've changed a bunch of antique trucks, cars and tractors including my British TR4.

So, starters don't care about polarity.
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gus
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« Reply #24 on: April 29, 2007, 05:11:02 PM »

Len,

The only electrical gage affected is the amp/volt gages and they will only read backwards until the gage connections on the back are reversed.

The temp and oil gages are just the ground path for the bus power and don't care if it is pos or neg ground.

Rectifiers I agree on because they are the old time diodes and will pass current only one direction. I'm not certain that the leads on those can't be reversed and work ok too, but don't know for sure.
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Len Silva
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« Reply #25 on: April 29, 2007, 05:35:58 PM »

Len,

The only electrical gage affected is the amp/volt gages and they will only read backwards until the gage connections on the back are reversed.

The temp and oil gages are just the ground path for the bus power and don't care if it is pos or neg ground.

Rectifiers I agree on because they are the old time diodes and will pass current only one direction. I'm not certain that the leads on those can't be reversed and work ok too, but don't know for sure.


Gus,

Mine was converted before I got it so I didn't actually make the change.  I was relying on instructions from Deans Coach - http://coachinfo.com/Manuals/Coach/GMC/4104.html

They definitely say that the original temp gauge and possibly the oil pressure gauge must be replaced.

Len
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