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POS/NEG ground and COIL wiring


deerio

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So in my attempt to convert my 320 to NEG ground, I did a bunch of reading (most MG related, some VW) and decided to finally tackle it. Unfortunately, I've hit a wall and now I'm more confused than before. 


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-My truck has been wired from the day I bought it: black wire from the body to the NEG battery post and the red wire from the starter solenoid to the POS battery post.

-The coil has been wired: NEG coil post to the distributor and the POS coil post to the ignition side of the fuse block.
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With that being said, it SEEMS as if it was wired NEG ground and already been converted right? Well I decided to try out a couple things. Now be aware, I have NO electrical knowledge whatsoever.

So I read about doing a pencil test that involved using a #2 lead pencil and sticking it in the path of spark between the plug wire and spark plug. In theory, I should get a flare between the pencil and the spark plug if the coil was wired correctly. Well it was way too difficult for me to tell what I was seeing, even in the dark.

So I tried using an analog voltmeter. The idea is that with the NEG lead of the meter touching the plug wire, and the POS lead of the meter touching the block, I should ideally see an upward (positive) swing of the meter when the engine is cranked.  A swing going the opposite direction (negative) should mean that the spark is not as strong as it should be and that the coil is wired incorrectly and should be switched.

 

However I checked readings with the engine running instead of during cranking (would this make a difference?) With the truck wired as I bought it, the needle was swinging downward to the negative side of the range. This means the spark was not as strong as it could be right? Well when I switched the coil wiring (so now POS coil post to the distributor instead) the voltmeter jumped up and bounced to the positive side of the range. So this is good right?

 

 

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Well my question is: Why is the spark "better" when wired with POS side of the coil to the distributor but the battery is still wired as NEG ground? Has my 320 been converted before or not?

FYI most of what I've been doing has been based off of things similar to this site: http://cny.mgcarclub.com/electrical_systems.htm

 

By the way, I tried switching the battery cables so that it was POS ground.  In this setup, the spark was reading "better" when the NEG side of the coil was connected to the distributor.

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Yes, You truck was already converted to negative ground.

 

This is correct for negative ground

-My truck has been wired from the day I bought it: black wire from the body to the NEG battery post and the red wire from the starter solenoid to the POS battery post.

 

-The coil has been wired: NEG coil post to the distributor and the POS coil post to the ignition side of the fuse block.

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Yes, You truck was already converted to negative ground.

 

This is correct for negative ground

If this was truly the case, then how can I make sure?

 

I thought that's what I was doing by using the voltmeter.  However, it sounds like a NEG ground converted system "SHOULD" show the voltmeter swinging positively when the engine was cranking, indicative of a properly wired coil and stronger spark.  But this is not the case with my truck which shows this positive swing only when the POS side of the coil is connected to the distributor (while the battery is NEG ground).

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Although the wiring is correct, I thought conversion to NEG ground involved also messing with the Field wire on the generator or regulator?  How would I know this was completed?

 

EDIT:

Oh and btw, you're correct about the wrong coil being used.  One of the previous owners replaced the coil with something else, and I have no clue what's being used at the moment.

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You can also check out forums about old tractors.....a lot of pos grd to neg grd conversions on those as well.   My understanding is that cars switched from pos ground to neg for better wear/life on the spark plugs because of anode/cathode wear. 

 

On the 320, when you switch between pos grd to neg grd, the only thing that has to be done is to disconnect the factory radio if it has one.  The antenna connection is a direct short to ground if left connected and the radio will fry if you turn it on with the polarity switched.  Technically, the generator should be re-polarized, but I'm not sure that's a "must do" to make it run.  The lights, wipers, starter will all be just fine with the swap.

 

I'm familiar with the pencil trick, but have never tried it.  I'm not familiar with the meter test on the spark plug lead at all.  The coil is an auto transformer.....2 coils with one common side, which is connected to the neg term on the coil.  Whether you run pos grd or neg ground, the coil should be wired the same way.  The neg side goes to the points which simply turn the current flow through the primary winding on and off.  It doesn't care which direction the current is flowing, just that the primary is turned on/off by the points.  The secondary coil creates the spark by discharging(field collapsing) through the spark plugs.  It's a really high voltage so it jumps the gap in the plugs.  Changing the polarity changes the direction of electron flow between the ground electrode and the center electrode.

 

I left a lot of details out, but that should cover at least the basics.  Also......you have to have neg ground if you want to run an alternator instead of a generator. 

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A coil is a coil for the most part.  There are some differences that will affect performance, but I don't know of anything that would stop any coil from at least running the motor.  A coil designed for points uses an external resistor which is taken out of the ignition circuit while cranking.....so that the coil gets as much juice as possible while the starter is loading the system heavily.  When the motor starts and the ignition switch springs back to run, the resistor is put back in the circuit to cut the voltage back down to approx 6 volts.   A coil designed for electronic ignition has the resistance built in and is always in the circuit.  The electronic ign can provide a lot more amps than the points, but it's still just a switch.  The fact that it provides a lot more amps is the reason that the spark is so much stronger.  They also have better insulation these days, so coils can get some extra windings on the secondary circuit to up the volts.

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Is a POS Ground coil different from a NEG Ground coil?

 

There are lots of different coils. Common types include:

* 6V

* 12V points type with external resistor

* 12V points type with internal resistor (e.g. hard to start British cars)

* 12V low resistance for electronic ignition. That's where the amps come from (low resistance). Points burn up when switching high amps. So do 1975-1977 Datsun Electronic Ignition. Starting in 1978, redesigned Hitachi module could handle high amp switching.

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It's a really high voltage so it jumps the gap in the plugs.  Changing the polarity changes the direction of electron flow between the ground electrode and the center electrode.

 

I left a lot of details out, but that should cover at least the basics.  Also......you have to have neg ground if you want to run an alternator instead of a generator. 

 

That's pretty much a consistent point in the readings I've done.  And supposedly what the voltmeter test determines? I'm uncertain.

 

"Why then do we worry about coil polarity? Because the spark plugs do care which way the electrons are flowing in the high tension circuit. The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going. So the spark plug prefers to see a voltage potential that is negative on the center electrode and positive on the end electrode for the very first hop of the spark. Oddly enough, this has nothing to do with polarity of the vehicle electrical system, but it is influenced by the common connection inside the ignition coil. " http://mgaguru.com/mgtech/ignition/ig104.htm

 

I do plan to ultimately change to an alternator down the line so I was hoping to do some of the baby steps to get there along with other plans.

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They're built the same and work the same, but the markings can be different on the terminals.  

 

I just went and read deerio's link.  Not everything I said is wrong, but just forget it all and go with what his link and this one have to say....

 

http://mgaguru.com/mgtech/ignition/ig104.htm

 

I'm back up to speed now......I blame nodding off at the computer while typing....lol!!  :)   

 

The original owner may have replaced the coil with a positive ground coil.....a lot of coils have it painted on the body whether they're pos or neg ground. 

 

The tests deerio is talking about are the only way to be sure how the wiring of the coil is.  You'd probably never notice the difference...unless maybe it got really cold or your battery was discharged.

 

Anyway.....I'm once again reminded why I don't post these days! LOL!!!  I just don't do it(work on cars) enough to remember the info well enough.  Time for a nap! :)

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Is a POS Ground coil different from a NEG Ground coil?

 

The original owner may have replaced the coil with a positive ground coil.....a lot of coils have it painted on the body whether they're pos or neg ground. 

 

 

I had no clue there was even such a thing as a POS ground coil.  Mine has no markings whatsoever other than a "+" and "-"

 

I appreciate all the info.  I suppose I can get a replacement coil and try it out to see if it makes a difference?  I believe the Standard Motor Products UF2 is listed as a replacement, does anyone have any info on this?

 

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When I do decide to start the alternator project, what would happen if I were to install it assuming my system has already been converted to NEG ground but truly has not?

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Standard Motor Products UF2 is a generic 12V, 1.5 ohm (points) coil for use with external resistor. So it will work.

 

BTW, I believe there is no difference between POS ground and NEG ground coils, at least coils marked + and - should work with either system.

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From what I read, the coils used back when pos grd was prevelant were marked with letters to indicate the switched side and the power side. 

 

deerio.......I wouldn't change coils unless that one has an internal resistor.  Use your test and hook up the coil accordingly.  Usually, the internal resistor coils are noticeably bigger and need a different bracket to hold them.  As Dave put in his last post... 1.5 ohm points coil.....if yours measures higher than that, it's probably an internal ballast(resistor) coil.  Keep it until you upgrade ignition to EI  

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The polarity of the spark at the plug is really not that important.   A good strong sparks is.  Is spark plug life shorter with the wrong polarity?  Possibly, but the spark plugs should be changed with new ones before that is an issue.

 

Generators use residual magnetism in the field coil winding iron core to energize them selves.  The magnetism is left from ythe lat time the generator was ran.  You do need to polarize the generator the first time a new one is installed, or when the polarity of the vehicle is changed, but after that, you do not need to polarize the generator again.

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Thanks for all the input everybody. I just did a quick check on the generator and I'm pretty certain it's not working properly. So I think I'm gonna end up using an alternator earlier than planned.

 

Still gotta do a lot more reading on converting to EI but it's on the checklist.

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  • 7 years later...

Auto electrical makes my head hurt, but I MUST understand it better to move on so... 1. Am I to understand that a Pos. ground system that would have come factory in a 320 pickup would have the Pos. battery post connected to ground (like frame) and the Neg. connect to starter? I've never actually seen this so wonder if all my 320's have been changed to Neg. ground all ready. From my readings in this and other posts, all you have to do is swap leads on the battery and coil? And then it seems that it might not be necessary to swap coil? (To clarify I mean necessary to make it run not to be correct).  2. If converting the system to Neg. ground then should the Pos. side of the coil run to the fuse block and neg. to the dizzy?  

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11 hours ago, Beer Snob said:

Auto electrical makes my head hurt, but I MUST understand it better to move on so... 1. Am I to understand that a Pos. ground system that would have come factory in a 320 pickup would have the Pos. battery post connected to ground (like frame) and the Neg. connect to starter? I've never actually seen this so wonder if all my 320's have been changed to Neg. ground all ready. From my readings in this and other posts, all you have to do is swap leads on the battery and coil? And then it seems that it might not be necessary to swap coil? (To clarify I mean necessary to make it run not to be correct).  2. If converting the system to Neg. ground then should the Pos. side of the coil run to the fuse block and neg. to the dizzy?  

 

To be wired positive ground the positive battery cable goes to the engine block near the thermostat housing, the negative cable goes to the starter relay.

 

The coil has the fused ignition wire(keyed) going to the negative side of the coil, the positive side goes to the distributor.

 

To convert to negative ground, you need to use an Alternator as the stock generator will not work as negative ground unless it has internally been changed by an electrician.

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Hey thanks Whano! Long time no see (or talk)! I'm cleaning up the wiring mess in my 320 with a J15. It has an alternator, and everything worked prior but there was just too much use of wire nuts and tape for my liking. I'm in the process of building a proper harness end to go to the alternator as well. Does this look correct? IclLorEx.jpgWhite/Red tracer back to the harness end I'm 99% sure of.......

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I do not remember where the alternator wires go anymore(which goes where), it has been a while since I did any wiring except for aftermarket gauge lights.

 

As I recall the main wire goes to the battery, one goes to the dash light, one goes to a 12V keyed source, and one goes to ground.

 

I thought about stopping by a month ago or so, but I had another job to do elsewhere, I hope you are well.

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