Inspired by a great commercial product and touched by the frugalness of the Dauntless Geezer, I decided to build a go-no-go coil tester to use at swap meets to determine if a coil is good or bad.
I hooked up a small meter, rheostat, push button switch and an AA battery. This is what I got
To use it I simply push the red button and adjust the meter to read full scale (just like you would with an analog OhmMeter), then release the button. All I have to do is touch the two contacts on the bottom or the tester to the high voltage contacts of a coil. A good coil will read half scale. Thatís it
The schematic is below.
Nice gizmo, Bob. John Regan also has a "pocket" tester available that beeps that I take along to swap meets.
That's what gave me the idea. This will fit nicely in my swap meet backpack.
To make it better you need to make it work on the Low voltage side of the coil(Primary) by adding a strip of brass to both contacts that will reach the contact on the end of coil and the bridge on the other end of coil. Now you will know if all coils are in working order. Most of mine have open primaries. That's what you get when you buy off of Ebay and they say untested
I have to say that I simply have found that testing the primaries is a total waste of time since in over 2500 coils I have never found an actual open primary. Nearly all anomalies that befall coils have to do with opens and shorts in the secondary windings and internal arcing of those windings. Coil point physical mounting geometry is also an issue for rebuilders but not for me since I am going to put good windings into all new boxes. I used to test the primaries and secondaries both at swap meets but not any more. To have primary winding continuity to measure with a meter you need to measure from the bottom contact to the top hardware where the points mount or have a known good set of points on the coil with points closed and making good contact to then measure from bottom contact to top side contact. I don't mess with that. If the High Voltage side is good - the coil will generally work and those that then flunk later is exceedingly small. A very few coils have leakage from the primary to the secondary but to check that at a swap meet would mean you had to take along a megger and that is just again a waste of time to perform on a swap meet coil.
Here is a slight improvement to the Analog Coil Tester. The additional resistor prevents excessive current through the meter.
Larry, I agree, I did pretty well on ebay coils till a few years ago. Guy sold me a dozen and said they had not been tested, when they got here all showed probe marks, 6 good 6 bad. At least the bad ones had good wood. Now I only buy complete coil boxes or coils at flea mkts that I can check and never pay more than 5 bucks. KB
You are one lucky guy on your coils. I have only 12 and 8 have open primarys. The other 4 have open secondarys. Not one good one in the batch. All from 3 different vendors on Ebay that said they have not been tested. Makes you wonder.. I need 4 good ones and maybe a spare for my TT. I now have a heat lamp and some chicken wire screen. I hope I can melt the tar out of one just to see if I can salvage them. I think chipping the tar out may break a wire on a winding so I'm going to try to melt it out.
Open primaries? Given the gauge of wire they're wound with, it's not going to happen unless the vibrator points are held shut and the coil connected across a battery for a very long time.
What will give the symptom of an open primary is a defective soldered joint at the back of the brass terminals. I've had an example of this in the last lot of coils I've just rebuilt.
I agree with John H.
The Model T ignition coil wood box has three-two part brass terminals. One is the outer terminal and the other is the inner thru wood wall coilbox terminal. These 90+ year old terminals commonly have corrosion between the two pieces if solder was not originally flowed between them when originally made.
All open primary windings I have seen are failures between these two terminal parts or poor solder connections of the wire in the center of the thru hole terminal. Never the winding heavy winding wire itself.
A careful look at the terminals with a Loupe and perhaps a fine pick will usually detect these faults.
Flow solder across the entire terminal and it usually solves the problem.
I rebuild about 1000 coils each year, check them carefully for common faults, guarantee them all and invariably get 1-3 coils back each year under warranty for something I have missed.
I fix it and return to the customer; life is simply too short to be arguing about a inoperative Model T coil. Grin
Ron the Coilman
I have to repeat it is never the primary winding itself unless there is tar bubbling out the top due to a stuck set of points or someone has deliberately jumpered the points and then had a timer wire get shorted. The points on a coil are its defense against shorts in the wiring. It with simply permanently BUZZ when the ignition is turned on to BAT but when you jumper the points you run a real risk of destroying the coil if there is a malfunction in the timer or timer wiring since the coil has no way of then limiting the duty cycle and reducing the coil primary current. Ron has a picture of what happens to a coil when the wiring to it is shorted and the points are stuck closed.
Here you go.
John I hope you are right ! All I have done is check for continuity from the leaded end of the box to the bridge brass on the other end of the box. Why I see opens I'll find out when I get the boxes apart. I have New points, Caps, and hardware to get 4 working again. I'm new at this kind of a rebuild. I have a trusty Fluke Meter that works good. All I was trying to do was add my 2 cents in and help make the coil tester a little bit better. With that all being said lets go get the Tar out!!!!
Please do share what you find. The wires soldered to the heads of the small carriage bolts that hold on the points can be broken off if the top of the coil gets soft and the bolts then spin in the wood. I melt out coils at a rate of about 75 per day until I get all of a current batch melted out and I just never find an open primary. It would not be a surprise to find a box of coils all with the same exact problem when someone has decided to modify some coils in some way since that has gone on from day one but then when the grand idea doesn't work - the coils are sold on ebay to the unsuspecting. I just didn't want to see folks thinking that checking the secondary only via the subject analog tester or my "beeper" tester was a flawed concept and likely to make one bring home a lot of bad coils. That simply would be a very inaccurate conclusion. Melting out coils is a messy smelly job whether you do one coil or 75. Best done outdoors but Ron pointed out a common problem with the coil contacts that may be fooling you into thinking there is a problem inside the coil when it might be the washer/rivet side or bottom contact that is the issue. Good luck with your project.
how do you melt out the tar?
I've bought a small tabletop owen on a swapmeet which only makes the tar softer but not as soft so it can just be poured out.
And the wood have a tendency to be burned.....
When I melt out the tar the box is going to be scrapped so I can really turn up the heat on the oven and don't care about the box which is already 1/2 disassembled (3 sides gone) when I melt out the tar. My method uses an oven of course but it would ruin your box so I wouldn't recommend it to you or post it here since others might just assume it might work for their coil rebuilding efforts and it would not. You must be careful never to heat the tar up too high with a lot of heat since the flash point of tar could be anywhere between 325 and 450 degrees F depending on which stuff is used in the coil you are melting or where the tar came from in the first place. The whole coil and oven melted tar drippings can suddenly burst into flames. I would strongly recommend you not attempt to melt out the tar but instead just dig it out cold and melt only replacement tar to pour back in which allows a much easier way to monitor the tar and control flashover. In any event make sure you have a good fire extinguisher handy even if you are only going to heat up a small amount of tar.