I think it is, at least somewhat, common knowledge that you shouldn't try to make a T coil jump a gap larger than about 1/4" due to it tending to make the park find a path of lesser resistance in the form of breaking down the interwinding insulation and beginning to arc internally. This makes sense to me and I can understand it. However, I have heard that it is bad to actually ground the spark plug wire and not have any gap at all. I know on more modern vehicles, lot's of manuals say to pull the coil wire from the distributor and ground it when you want to crank the engine over but don't want it to start (Like, say when you are doing a compression test). Being more mechanically minded than electrical minded, I really don't see how grounding the spark plug wire would hurt a coil, but I know I don't really know enough to say. Looks like the current would just easily flow round and round. Is that the problem? Not enough resistance causing the amperage to be too high for the tiny secondary wire? Or is it just an old wive's tale and there is no problem?
If you are doing a compression test in the Model T unlike more modern cars, the key is off and there is no power to the coils same with many early cars with a foot starter switch. On modern cars I just lay the plugs with the wires attached on the engine to keep them in order.
Do not understand the question!
If the question is grounding the wire end that goes to the spark plug instead of the wire being connected to the spark plug, I think that would be acceptable if you are looking to preserve the coil. In a modern vehicle the better choice would be to eliminate the power to the coil in order to prevent internal breaking down of the insulation. In other words eliminate the problem at the source
I am a mechanical guy so take this accordingly!
Also please remember that 1/4" in air is not the same as a 1/4" gap when the spark plug is on the compression stroke installed in the engine as the voltage required to jump the gap while in compressed air is greater, so do not run your spark plugs with 1/4" gap in the model T!
Short circuiting the output of a T coil to prevent insulation failure is not harmful. The output impedance is high enough for sufficient current limiting. A test on a typical coil adjusted for 1.3A at 6V showed an increase in current to 2.5A when the output was shorted. Quite harmless for short term testing.
Thanks JohnH. I suspected as much, but wanted to verify.
The question was not about a specific situation per se. Just one of my many mental musings. The Model T powered chronograph got me to thinking about it. With the two pipes 1/2" apart, it would not be good for the coil. I was wondering about maybe shorting them with a piece of foil, or something like that, that the bullet would break when passing, so the coil would not be arcing a 1/2" gap the whole time you were getting set up. I still probably won't build the thing, but I have to admit, the seed was planted.
The only thing I have ever been told about Model T coils is don't leave the switch in battery with a coil buzzing it will melt the tar right out of them! I think Ron the coil man has posted a couple of pic's in the past showing tar oozing out of the coil
What happens when tar oozes out the top is that the tar quickly builds up under the vibrator and blocks the lower point into the closed position. Since the coil is still then connected to battery the coil has no defense against the high DC current that will then flow so the result is thermal runaway and the very high DC current (>12A on 6V and >24A on 12V) will melt more tar and the process will intensify until either the battery is dead or the coil primary burns open. This event in a wooden coil box can result in a catastrophe for more than the T coil. I have personally witnessed the results of a coil, coil box, and wooden dash fire that started this way. Coil buzzing is the coils defense against a shorted timer wire or timer and alerts the T owner that something is wrong.
Shorting the coil secondary to ground significantly reduces the primary inductance to just the leakage inductance. Consequently, the primary coil current increases to significantly higher peak values than normal (much higher than 2.5A). The higher primary current produces much higher reverse voltage when the points open creating conditions that result in excessive point arcing. This condition is more stressful on the coil and points so I prefer holding the points open or lifting the coil up to check if the cylinder is firing as opposed to shorting the secondary winding to ground.
Mike has it right. Here is a 'scope trace if you'd like to see what it looks like. The top trace is of a coil buzzing away with the secondary shorted. The bottom trace is the exact same coil jumping a .070" or so free air gap.
I appreciate the additional information. However, I must admit I don't quite understand all of it. I understand that excessive arcing at the points will burn and pit them, but what other failure modes should one expect to see if a coil were allowed operate with the secondary grounded? Bear in mind electrical theory gets over my head quickly.
I must admit I didn't do the short circuit test on my CRO based electronic tester so wasn't aware of the higher peak current. 2.5A was just the average current. I was thinking more along the lines of excessive heating within the coil and if would there be enough current to burn out a winding.
I wouldn't run a coil like that all day, but for a few seconds for setting timing or whatever isn't going to harm it, not with a 6V supply anyway. The coil just isn't going to have time to get hot enough to either melt the tar or burn the points.
When I look at the adjustment of some of the coils I've been given to rebuild, it is amazing the points have lasted as long as they have - run off 12V with a setting of several amps. Often with an incorrect capacitor that furthers the arcing. .022uF was one value I saw screwed under the point mounting nuts. Electrolytics usually short circuit or go very leaky and save the points from further destruction
Ron How long would it take to melt a coil like that if you were dumb enough to leave one coil buzzing on battery ?
I had one start to show some melting on a 12 volt battery in less than one minute. I don't leave a single coil buzzing while I loiter prior to pulling the crank any more than absolutely necessary now.
You are correct that it would take some time to heat up the coil by simply momentarily shorting the secondary to see which cylinder is not producing power. I have now opened up over 2000 coils and a bad or open primary is rare but carbon tracked secondaries are much more common failure mode. We test windings thoroughly for any internal leakage or arcing. Having a coil not be able to stand 1/4" gap is by far a bigger problem than anything else but that would change if the points were not allowed to buzz since that is what limits the current via the duty cycle of turning off and on. If the points short across then the coil gets immediate dose of high current when timer wires short and then all bets are off but that too is not all that common.
Can you please explain what the red and blue trace are measuring? I assume the blue trace is voltage, and is the red trace amps?
Also on the first graph it appears to me that the 1st, 5th, 8th and 9th red trace appear to have a lot of "ups and downs" while the other traces appear to have a slightly slopping up configuration. Why?
Yes Arnie, the red is current and the blue is voltage. The amperage scale is divided by ten, so .4 = 4 and so on. I don't know why the upper trace does what it is doing. I would assume that it is caused by point bounce and arcing likely from lousy point adjustment.
Thanks for your rapid reply!
If I am reading the traces correctly when a coil is firing with it shorted to ground a peak amperage of about 6 amps is possible.
That same coil firing in air with jumping about a .070" gap results in a peak amperage of about 2 amps. Is this correct?
My next question is do I see about a -10 amps also on the trace of the coil firing in air. If so what is this telling me?
Still so much for me to learn about the "T"!
A1. About 2 amps is correct.
A2. That is a transient spike from the current induced into the primary from the the back emf of the spark in the secondary.
A3. That there is much more back emf because the secondary was not shorted, which caused a lot more voltage to be built up in the secondary.
FWIW, I played with a Model T coil I found in my grampa's junk box in the basement as a kid.
Used it to make a Jacob's ladder, and often tried to see how wide a gap I could get it to jump (about 1/2 to 5/8" if I remember). Hours and days of this kind of playing and it worked until I lost interest.
Not very scientific, but I guess they will take a certain amount of abuse.
You were lucky it didn't arc inside the coil wood box since once that begins, the coil will spark there again as it builds up a carbon track. Eventually it will not spark where you want it to at the plug and then the coil is essentially no good even though its winding may be OK. It just depends on where the breakdown is. If the breakdown is between windings then the winding is toast and so is the coil.