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Author Topic: Positive ground system  (Read 3608 times)
keithshotrodshop
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« on: April 21, 2007, 05:51:38 PM »

My 1948 Silversides is equipped with a positive ground system. Can I wire up modern appliances such as TV's and radios simply by switching the positive and negative wires? Or should I consider switching the whole coach over to a negative ground system?
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gus
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« Reply #1 on: April 21, 2007, 06:53:49 PM »

Keith,

If you just switch the wires to appliances you will be directly shorting the whole electrical system and will see plenty of sparks!!

There is no real reason to keep a pos ground system and all kinds of reasons not to.

All it entails is a neg ground alternator and reg, switching the battery cables and switching the ammeter wires. If you have a bunch of diodes in the alarm system you may have to change them also but most of those are long gone from older GMs.

If you have an original generator you need to flash the field of the gen and regulator while hooked together.
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« Reply #2 on: April 21, 2007, 11:03:04 PM »

Keith, Freightliner had a positive ground for years and we had to live with it. The wires would switch and everything was ok until the unit was accidently touched to ground in any way. Then smoke! If you can isolate the electronics, (case, speakers, wiring, etc), from ground it will work ok. If anything touches the frame, SMOKE!!!
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« Reply #3 on: April 22, 2007, 04:28:17 AM »

I switched my 4104 to negative ground about 25 years ago and as I recall it was extremely easy. Replaced a defective selenium rectifier in the safety circuit with a silicon diode wired in the reverse direction, reversing the battery terminals and then flashing the generator field. I did not change the generator or the regulator or anything else except for the rectifier. Did not really have to replace it either, since it was not working anyhow. LOL
Richard
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keithshotrodshop
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« Reply #4 on: April 22, 2007, 08:05:21 AM »

So what about all the lights, gauges, radio, etc. Don't all those need to be taken apart and wired accordingly?
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« Reply #5 on: April 22, 2007, 09:18:43 AM »

     Not sure about gauges, lights do not care which way the power goes through them. I have never seen a positive ground radio. At the time they changed to negative ground, I think the radios were still the old tube type. Your radio is probably a negative ground that is installed so that the radio is completely isolated, electricaly, from the bus chassis such as mounted in a wood box. Jack
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« Reply #6 on: April 22, 2007, 11:38:16 AM »

Positive ground was used as an effective way to discourage electrolysis in aluminum vehicles before modern insulation methods were used.  Especially in old outboard motors on boats, positive ground was very effective at preventing electrolysis in the lower unit-too bad it isn't still used today instead of the complicated bonding systems in use. Good Luck, TomC
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« Reply #7 on: April 22, 2007, 09:11:04 PM »

The only gage affected is the volt or amp gage and it will just show the reverse, no harm done.

Lights don't care which direction.


Please note that if you have an alternator it will have to be replaced or rebuilt with new diodes. If hooked up in reverse it will go up in smoke.

Generators can be flashed.
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« Reply #8 on: April 23, 2007, 04:37:52 AM »

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Please note that if you have an alternator it will have to be replaced or rebuilt with new diodes. If hooked up in reverse it will go up in smoke.
Gus is correct so be sure of what you have.

The odds are that if you a have positive ground system it is a generator, since positive ground systems were generally abandoned when the manufacturers switched from generators to alternators. Sometime in the late 50's or early 60's as I remember.

Richard
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« Reply #9 on: April 23, 2007, 06:06:42 AM »

I do believe that the electric oil and temp gauges will have to be replaced as well.

Len
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« Reply #10 on: April 23, 2007, 07:17:05 AM »

I do not remember replacing any guages when I converted from positive to negative ground, but I really do not even remember what guages were on the 4104.
Richard
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« Reply #11 on: April 23, 2007, 08:18:48 PM »

No gages will need to be replaced. Most work on pressure and/or some type of electrical resistance so polarity does not matter as long as all are hooked up the same.

There are pos ground alternators, I have one on my '59 American LaFrance fire truck. I will change this one of these days as it is a pain. Nothing works with pos ground anymore.
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keithshotrodshop
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« Reply #12 on: April 25, 2007, 06:03:46 PM »

My bus has an alternator, but it was smoked when I got it. I am assuming the guy hooked it up wrong. New alternator can be run either way, but I would prefer to go with a neg ground system
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Don4107
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« Reply #13 on: April 25, 2007, 10:51:22 PM »

The starter is going to need to be either changed or retimed to turn the right direction too.
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Don 4107 Eastern Washington
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« Reply #14 on: April 26, 2007, 04:50:54 AM »

The starter is going to need to be either changed or retimed to turn the right direction too.

I do not recall doing anything to the starter in my 4104 when I converted from positive to negative ground. As I recall it is very difficult to change the rotation direction of a starter. You have to actually get inside and change the relationship of the field to the armature if memory serves correctly. Changing from positive to negative ground does not change this relationship.
Richard
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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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« 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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« 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
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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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« 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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« 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
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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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« 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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« 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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« 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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