When I bought my T three years ago, it came with three plastic coils and one wood one, plus a plastic spare. It ran fine on battery (12V) or mag, so I never worried about them. A coil failed right before the winter so I decided to take a closer look. Kind of surprised at what I found. Testing done on a Strobo-Spark.
Plastic coils are the type with the X in the case, which are rumored o be better than some other plastic coils.
cap 0.15 (saw somewhere this might be common for plastic coils, but wrong)
point gap .028
cushion gap .005
#2 newer K-W points
point gap .019
cushion gap 0
point gap .027
cushion gap .012
#4 wood - Ford script
point gap .068
cushion gap .002
point gap <.005 (!)
cushion gap .005
Obviously, these shouldn't go back in the car the way they are. Should I rebuild them, have somebody else rebuild them, or go looking for some "new" ones? I'm sure there are many opinions! :-)
Looks like you need to adjust 3 and rebuild one. You have the Strobo Spark unit so have at it.
Why is the top terminal on the side marked positive? That terminal should go to negative through the timer. I know that polarity really doesn't matter, but the model T is usually chassis negative.
The power for the coil enters the coil thru the bottom contact. The top side contact is grounded by the commutator when a spark is needed.
I read somewhere that the plastic case coils cannot be repaired. Number 2 needs new points, currently it will probably produce a double spark of low energy.
Hi Chris and others,
I seem to recall something about the plastic coils with polarity labels being intended for use with stationary engines, or maybe Fairmont speeders. I have heard that capacitor values can be inconsistent and many times are of a too low voltage rating. One advantage of the plastic case is that it is resistant to moisture and the resultant warping. They can be easier to slide into and out of the coil box.
I have rebuilt four plastic coils, for someone else. Removing the plastic lid without breaking it or the case itself was an interesting challenge. Removing the old cap and soldering in a new one was much the same as with a wooden case coil. Refilling the first one with hot tar taught me that it needed to poured in slowly and in small amounts to prevent warping the case from the heat.
Replacing the points is straightforward. Adjusting them can be as easy or as problematic as on a wooden coil.
A plastic coil rebuilt with the correct value capacitor and properly adjusted points seems as good as a wooden coil. Good luck with your project. Bill
Those black coils were never built or tested for use in a Model T engine. They were built for 12 volts DC and other higher voltages. The capacitor is usually much smaller.
Some actually seem to work OK, but they heat up more.
The best thing to do with them is throw them away and buy a good set of coils.
I rebuilt a set back when I was first learning. They warped up and while they still fit, they were a real pain to get the points situated so they could be adjusted, probably due to the case warping when I refilled it with tar. I later learned that some folks make a wooden form to hold the case to shape while refilling and it looks like Bill has had luck with just filling it slowly. I've not tried either and while I might rebuild one for a paying customer, there's no way I would ever buy any to rebuild and try to sell outright. I thought I was gonna have to keep those things forever, before someone finally bought them from me at a reduced price.
with regard to coil #3. The total absence of current could be that the points are not physically closed. Look at the points while the coil is not under power and see if you can see daylight between the point contacts. If they are closed and even remotely correct then push down on the lower point and watch the upper point. If the upper point follows the lower point down for a small amount then the points are closing physically but maybe not electrically. Try this trick only if you can tell that the points are in fact closing and the upper contact is following the lower contact in which case insert the coil back into the SS. Put the mag switch into the hi position and the other switch in the coil current position and push the button. While holding the button down, use your finger of the other hand and simply give the lower vibrator a downward "flick" with your finger to force it to bounce a bit. Many times this will cause the coil to start sparking since you can thus make the points rub against each other and the coil will start working with the points now at least functional. There is an oxide that often forms on tungsten points that can be burned off this way and even new points often need a "flick" when first installed to get rid of the oxide on there surface if they have been sitting around for a while. The main purpose here is to see if the coil itself is OK. The "hit and miss" motor crowd are very interested in functioning T coils and they don't use them inside of a coil box nor do they care about matching up a few of them.
Well, I took the easy way out and found a set of rebuilt wood Ford coils on ebay at what I thought was a reasonable price. If they test good, I'll just drop them in. I'm still going to try an tune up the ones I have as a learning experience. If they come out good, I'll have a bunch of spares . . .
John, interesting detail about oxide formation on tungsten points. Even new points seem to have this oxide on it that is not washed away when cleaned with lacquer thinner as per Ron's recommendations. I have had to use 320 grit sand paper in order to establish good electrical contact between new point contacts. I thought the manufacturer was putting some kind of protective film on the new tungsten contacts but just may be this oxide you describe.