Positive ground system

keithshotrodshop · 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?

gus · 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.

NJT 5573 · 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!!!

DrivingMissLazy · 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

keithshotrodshop · 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?

JackConrad · 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

TomC · 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

gus · 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.

DrivingMissLazy · April 23, 2007, 04:37:52 AM

QuotePlease 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

Len Silva · 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

DrivingMissLazy · 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

gus · 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.

keithshotrodshop · 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

Don4107 · 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.

DrivingMissLazy · April 26, 2007, 04:50:54 AM

Quote from: Don4107 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.

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