1975 R model, positive ground.

[Full Floater]

I'm starting work on resurrecting a 75 R model.  I believe 75 was one of the last years of positive ground on them.  Most of the electrical is functioning well aside from a few things that aren't working.   I was wondering what all I would have to do to convert to neg ground, just for the simplicity of things.  OR should I just leave it alone?  Is there truth to the myth of pos ground vehicles rusting/corroding less then neg ground?   This truck is pretty clean for rust and corrosion so im wondering if pos ground had something to do with it.

[73cf600]

I'm going through the same thing on a 73 CF600 I bought a month ago. I didn't know it was positive ground till I blew up a $300 fire radio I was installing. I have heard the same thing about less rust issues. I'd like to convert mine to negative ground as it sure would simplify things.

[67RModel]

On 8/22/2022 at 11:19 AM, Full Floater said:

Is there truth to the myth of pos ground vehicles rusting/corroding less then neg ground?   This truck is pretty clean for rust and corrosion so im wondering if pos ground had something to do with it.

Your thinking about that wrong. With either way you will have electrically motivated corrosion of one component or another. In a positive ground system the sacrificial anode is the vehicle itself (chassis and bodywork). In a negatively grounded system the wires, connections, and electrical components become the sacrificial anode(s). So in theory a positively grounded vehicle would be more likely to be "rotted out". However, with that being said the amount of chassis/body metal lost due to electrical corrosion in a positive grounded system would be miniscule whereas the same amount (mass of metal lost) of corrosion in a negative grounded system could have much more effect on the vehicle due to corroded connections and components as well as dissolved wires. With modern insulation materials and much more robust connections the "problems" associated with negative ground have become a moot point. There is still is a better theoretical justification for positive ground. When you consider the electromotive series of metals negative ground is incorrect. Why negative ground is the normal convention is a topic for a different discussion. 

[Full Floater]

On 8/29/2022 at 10:07 AM, 67RModel said:

Your thinking about that wrong. With either way you will have electrically motivated corrosion of one component or another. In a positive ground system the sacrificial anode is the vehicle itself (chassis and bodywork). In a negatively grounded system the wires, connections, and electrical components become the sacrificial anode(s). So in theory a positively grounded vehicle would be more likely to be "rotted out". However, with that being said the amount of chassis/body metal lost due to electrical corrosion in a positive grounded system would be miniscule whereas the same amount (mass of metal lost) of corrosion in a negative grounded system could have much more effect on the vehicle due to corroded connections and components as well as dissolved wires. With modern insulation materials and much more robust connections the "problems" associated with negative ground have become a moot point. There is still is a better theoretical justification for positive ground. When you consider the electromotive series of metals negative ground is incorrect. Why negative ground is the normal convention is a topic for a different discussion. 

Thank you for the detailed response.    Would YOU go thru a conversion from pos to neg ground, if you were in my shoes?

[doubleclutchinweasel]

It's not that hard to convert most of it to negative ground.  The amp meter (if it is a single post like mine was) will have to be changed out to a dual post so you can reverse the needle.

[67RModel]

1 hour ago, Full Floater said:

Would YOU go thru a conversion from pos to neg ground, if you were in my shoes?

That is a tough question to answer without any background information. If your goal with this truck is hobby/restoration then I see no need to change it. Personally I like all my antique/hobby trucks to be as original as possible and there is absolutely nothing "wrong" with positive ground. As stated previously it is actually a much more robust and theoretically better for a vehicle's electrical system. If your intentions are to use the truck in some capacity that will require the use of more modern electrical components for body upfitting, signaling, etc.; or you are doing a restoration and want to add modern accessories, then it would probably be wise to switch it to negative ground. If the truck is original the switch should be fairly simple as the a truck of that vintage would be pretty basic electrically speaking. The biggest thing is the starter. If its a Delco Remy 40MT or 50MT then it is a series wound motor and changing the polarity will have no effect on its direction of rotation. If for some reason it the starter is "something else" that has permanent magnets for the field then it will change rotation by switching the polarity. I guess it all just depends on what you are trying to accomplish and what you are comfortable with....Hope this helps.

[doubleclutchinweasel]

I think on mine the cables were swapped at the starter.

[Full Floater]

So far, everything has been trouble free in converting from positive to negative ground.  

I have only the signal lights left to get working.    Im getting nothing from them, nor any power from the rectangular plug that hangs down and plugs into the signal-stat pigtail.   The relay and all other relays on the relay board, are functioning.  Changed flashers, to no avail.

I'm suspect of this unit, mounted above the drivers side kick panel area.  (this is a picture of 2 separate ones, one off of a parts truck).   Anyone know if they have anything to do with the signal lights? 

Im at a loss as to where to look next, the harness is in tact and im hoping to leave it that way so I figured i'd ask here before I pull that harness all apart and make a mess, whilst trying to solve this problem.    HOWEVER, I do not know if the signal lights were working in this truck, before biting the bullet and converting to negative ground and replacing LOTS of wiring, grounds and various things along the way in part of the restoration.   So I may have had a problem to begin with, that I do not know about.

Edited July 4, 2023 by Full Floater

[Geoff Weeks]

Those look like horn relay or buzzer Buzzer often only has two terminals. 

 One word of caution on changing polarity. SOME but not all  Gauges will not work on reverse polarity, others will not care. I have run into it in Autocar stuff.

 As to the rust issue, most rusting is between the truck and the surrounding air and in that case polarity of the electrical system has no effect. 

 Both my old IHC's are + ground, but I have no feeling one is better than other. If you want to go adding stuff, the world is mostly - ground. There are solutions to have + ground and still use - ground stuff.  Whether you want to take the risk that you will not forget and plug in something modern or you want to convert, is up to you.

 I use modern "electro-mechanical"  12 volt flasher on a 6 volt + ground truck without problems ( have to open and stretch the return spring on the relay, that was all that was required to use on 6 volt) and reverse the power in and out wires the other way around.

Edited July 4, 2023 by Geoff Weeks

[Full Floater]

For sure.  I had to switch voltmeters, but everything else worked in regards to gauges and interior lights.   Heater motor needed the wires reversed.  Not sure about low air buzzer yet, but will probably need the wires reversed. 

 

I would have left it + ground, but I installed Jacobs 675 jake heads on it and they are grounded through the casing so I figured this is my calling to convert.  Really wasn't a probably, just stumped on this signal issue.  But it could very well be another issue

[doubleclutchinweasel]

8 hours ago, Full Floater said:

For sure.  I had to switch voltmeters, but everything else worked in regards to gauges and interior lights.   Heater motor needed the wires reversed.  Not sure about low air buzzer yet, but will probably need the wires reversed. 

 

I would have left it + ground, but I installed Jacobs 675 jake heads on it and they are grounded through the casing so I figured this is my calling to convert.  Really wasn't a probably, just stumped on this signal issue.  But it could very well be another issue

I don't remember the flasher caring about polarity.  But, I have been wrong at least twice today already.

[Geoff Weeks]

1 hour ago, doubleclutchinweasel said:

I don't remember the flasher caring about polarity.  But, I have been wrong at least twice today already.

Might depend on the flasher, some of the new electronic ones might care.

 Volt meter will not care unless it is grounded through the case.

 The problem I remember was the temp and oil pressure  gauges wanted to read backwards and swapping the wires wouldn't solve that.  Others Like my K's couldn't care less, either polarity is fine. If I'm not mistaken it was on a Autocar gauge that there was a problem.

[doubleclutchinweasel]

1 hour ago, Geoff Weeks said:

Might depend on the flasher, some of the new electronic ones might care.

 Volt meter will not care unless it is grounded through the case.

 The problem I remember was the temp and oil pressure  gauges wanted to read backwards and swapping the wires wouldn't solve that.  Others Like my K's couldn't care less, either polarity is fine. If I'm not mistaken it was on a Autocar gauge that there was a problem.

Volt meter on my R600 was grounded through the case (1-wire gauge).  So, I had to get a different, 2-wire gauge so I could reverse the polarity at the gauge.  It had insulaotr bushings to separate the metal case from the dash.  My fuel gauge did not work.  But, I don't know if that was related to the system being changed or not.  I just checked it with a dipstick!  Oil pressure gauge was a manual gauge (copper tube), so that was not an issue.  Temp gauge worked, but I can't say what all had been changed there.

Did I mention already how happy I am that you are here now?!

[dogg rescue]

My gauges were easy to identify with the small plus and minus sign.