corveteer

i have a lab type scope that says voltage is positive.
this little snap-on tester (led type) says 'inverted'
i thought best polarity should be +
thanks for your reply........

Nathan Plemons

What's your state of reference?

The "ground" strap of the spark plug is attached to the cylinder heads which are bolted to the block which is connected to the frame. Therefore you could consider it the ground or "negative," which would dictate that the spark plug electrode, and thus the plug wires, coil, etc, be putting out a "positive" voltage.

corveteer

reference is engine ground

Nathan Plemons

If your ground state is engine ground, which is the same as the negative battery terminal, your spark plugs must be the same polarity as the positive battery terminal in order to work. If they were "negative" they would be the same as the reference, and thus no potential difference, and no spark.

Nathan Plemons

I can't help but feel like my explanation is going to leave somebody lacking so I'll try to clarify some terminology. Sorry if this is an insult to you, but SOMEBODY doesn't understand it and I hope this will clarify.

Voltage is considered a "potential difference" between two points. To measure it you use typically use a handy tool called a volt meter which has a "positive" and a "negative" lead attached to it. Really this is quite misleading, something like "probe" and "reference" would be more appropriate. The negative lead, or refrence is connected to one point and the positive lead is connected to some second point. If the second point has a higher potential than the reference, you will read a "positve" voltage. If the second point has a lower potential you will read a "negative" voltage. If you swap the leads on the volt meter you will reverse this relationship, which really makes sense. Think about it this way.

If 0 is your reference and the number 5 is your second point, the second point is 5 units HIGHER than the reference. Now if you swap them and make 5 the reference you'll see that 0 is 5 units LESS.

So if that's true, what's my point? Well the problem is that automotive terminology can be misleading. The "negative" battery terminal may or may not be negative, it depends on what you are comparing it to. Because the vehicle is an isolated system though and everything is connected to the negative terminal, we consider this to be a GROUND or ZERO state. The other terminal on the battery is positive, meaning that it has a higher potential. Therefore anything that is connected to this postive terminal is considered "positive" in relation to the ground state. There is nothing on the car to create a true "negative" voltage. The "negative" battery terminal is called negative only because it is at a lower potential than the positive terminal. It might make more sense if it was called the ZERO terminal or the GROUND terminal. In order to have a true "negative" terminal there would have to a point where the potential was less than the "zero" reference point. Since there are only two terminals on the battery, and the one of lowest potential is the reference point, everything measured relative to it will be positive.

Now assume you had a battery with three terminals and you had potentials of -12, 0, 12. If you let -12 be your "ground" state you'd have potential differences of 12 and 24 respectively at the other terminals. If you let 0 be your "ground" state you'd have potential differences of -12 and 12 respectively. And finally if you let 12 be your "ground" state you'd have potential differences of -24 and -12 respectively.

Really it's simple math and a knowledge of the terminology.

VenkmanP

I'm not sure if you are saying this is true of cars or electronics in general. I don't know if cars have any negative voltages in the electronics. If it's a statement about electronics in general it is false.

And then....

A quick google of "spark plugs negative voltage" indicates that the waste spark type ignition found on many cars including some C4s, has a negative voltage at half the plugs and positive at the other half.

I also found more than one reference indicating that with a negative ground automotive electrical system, the preferred polarity for the spark plug is negative, which agrees with the observation made by the OP with his LED tester.

Nathan Plemons

Since we're talking about cars, my statements refer to cars. If you stick with a simple DC circuit with a simple voltage source and you refer to the negative side of the voltage source as your reference, any other point in the circuit that you measure will be either positive or zero. Now if you use an AC circuit to start with you can get both positive and negative voltage when comapred to the ground or more appropriately the neutral.

Waste spark ignition systems are a little more interesting and were not mentioned anywhere in the original question. You can get as complicated as you would like or you can try to answer the quesiton as best you can provided the very limited information posed in the quesiton.

CentralCoaster

Before this turns into a college lecture, what are you trying to figure out? Which way the spark jumps?


As nathan said, the reference, zero voltage is battery/chassis/engine ground. None of that matter though, all that matters is the voltage difference. The plugs are positive voltage compared to the heads, as the coil boosts the battery voltage in a traditional system.




Now, for fun, I could park my vette next to yours and run a battery cable from my ground post to your 12V post, and it wouldn't even arc. And both cars would run perfectly normal. Now if I hooked a VOM across your ground to my 12v post, it'd read 24V.

Redeasysport

I think he is looking at a DIS.

VenkmanP

So does this one - the spark plug polarity is negative in most cars, at least according to the several web pages I read during a quick search.

I also expect that there are negative voltages in the electronics, but the spark polarity seemed more to the point since it addresses the OP's question.


Incorrect.


We don't need to bring AC into it.


Then we can leave them out. By the way - how much of your lecture above was mentioned in the original question?


Indeed. Which post above is the overcomplicated one?

Nathan Plemons

Take a battery and connect it any load. Connect the negative lead of your volt meter to the negative terminal of your battery. Then find anywhere in the circuit that you can find a "negative" voltage, it's not gonna happen.

kalister1

I have read alot of technical information here, but now answer to the question. What is best polarity?

Nathan Plemons

I think the answer is going to be that it doesn't matter. In a waste spark ignition system, half of the plugs fire one direciton, half of them fire the other. If there was any significant difference then manufacturers would not use such systems. It doesn't matter if the spark goes from right to left, left to right, up to down, down to up, south by down east or north by up west. The spark still requires the same amount of energy to jump the gap as long as the conditions inside the cylinder are the same.

brookman

You guys are assuming that the spark system follows conventional DC rules. to create the spark a voltage is placed across the primary side of a coil. There is ground(car body potential) on one side and +12V (or higher depending on your charging system). When that voltage is removed from the primary side of the coil a voltage is induced magnetically into the secondary of the coil. Now here is the variable. Depending on which way the coil is wound or whether the top or the bottom of the coil is referenced(connected) to the car ground will determine what the polarity of the pulse is.

Nathan Plemons

Agreed. I looked up the wiring diagram and thought about it as I went to sleep last night. So, you can get a "negative" value depending on how the coil is configured. In a traditional ignition system all of the plugs will be the same, in a waste spark style DIS you have both. As to which you have? Probably depends on who designed the car. Which works better? I'd bet there's not a bit of difference.

96_LT4_FE1

I agree, there is not much difference. You can have a "negative" ground car or a "positive" ground car, it'll work both ways if its designed that way, but it would be weird to work on "positive" ground car. I've read that having a positive ground car can actually reduce corrosion cause it won't attract salt as much.

anyway, I didn't see anyone mention the fact that the electrons actually travel from the "negative" side to the "positive" side, cause if positrons are moving, you must have done the Mr. Fusion conversion on your Delorean. So, in a "negative" block situation, the spark is starting from the prong closest to the center of the combustion chamber, then traveling to the center eletrode. Of course in the other situation, it is exactly the opposite. So, you could theoretically say that having the spark start closest to the center of the combustion chamber is better.
»keith

CentralCoaster

Seems the plug designers intend for the spark to jump to the ground strap. One single point of voltage, and a big, or multiple ground straps for it to jump to. And maybe electrode erosion is less for + polarity plugs?

rocco16

That's misleading, at the very least.
A 'waste spark' system simply means that the system will fire a spark in a cylinder that is not at (or near) TDC on the compression stroke. In other words, it fires during an exhaust stroke in that cylinder and does nothing...hence, the "wasted" aspect. The 'polarity' is the same on all plugs, believe me.
At least that's the way wasted spark systems work on motorcycles and have for decades.

Larry
code5coupe

Nathan Plemons

The waste spark system on my old cutlass was wired in this manner. Each spark plug was connected to a coil pack, two per pack.

The complete circuit consits of current flowing through one coil terminal, through the plug wire and into the electrode of the first plug. From there it would jump the gap to the ground strap, continue through the cylinder head to the ground strap of the other plug. From here it would jump from the ground strap too the electrode of the second spark plug and then through the wire back to the other terminal of the coil. No matter which direction you run the current in this system one plug is firing electrode to ground while the other is flowing ground to electrode.

Redeasysport

From what I have read if it is positive the plug will wear much more than negative.If you pull the plugs you will see hardly and wear on the negative plugs.I still have a problem trying to visualize how you can have a plug fire with negative voltage in a negative ground system.Could someone enlighten me?