Model T Ford Coil Tester


(Click on the image for high resolution)

Since I was a kid, I've wanted to buy the Allen Electric Manufacturing Company Model T Ford coil tester that resided in the local auto repair shop.  The owner, Bill Herdt, a longtime friend, kept telling me that he would sell it when he thought he wouldn't be needing it any longer.  The tester was bought new sometime during the Model T era and used for many years before being retired to the loft of the shop.

A Ford buzz coil is inserted into the slot on the tester and the crank is turned to generate an AC voltage from a Model T Ford "magneto".  The coil vibrator was adjusted while turning the crank until the proper voltage and current were displayed.  The tester was also used to test the magneto in the car.

Earlier this year, Bill retired from running his business fulltime after attaining his mid nineties.  Since he figured he wouldn't be needing it, I finally was able to purchase it from him.  I'd only been wanting it for about sixty years!


29 August 2011:

The first thing on the agenda was to disassemble and clean the parts.  One of the things I discovered after cleaning off all the dirt is that the color is a very dark blue.  So much for easy paint acquisition.

The meters were first tested and they were good.  I then disassembled and cleaned them, prior to painting them flat black, the original color.


Ammeter, as removed.                                                                       Ammeter, disassembled.

Data and and instruction plate.

The data plate, although slightly damaged only needs to be cleaned and the black background redone.  Since the plate is made of thin aluminum and the text is only raised a few thousandths of an inch, I'm unsure how to do this but I'll ask an expert or two.


30 August 2011:

The meters are back together and tested for accuracy.


     Voltmeter under test.                                                                                Ammeter under test.

I got out my trusty variac and a 120 volt to 24 volt step down transformer for the tests.  I figure 60 Hz is as good a test frequency as any other and I found that the voltmeter is within an easy 5% tolerance at the 5 volt point. 

I put my DVM in series with the ammeter and an auto taillight bulb  The ammeter, on the other hand, is just within 10% and is probably about as close as it will get unless I want to get into the trimming resistor inside the meter.  The ammeter is odd because it has two coils (three posts on the meter).  I checked both meter posts against ground and the one that read most accurately was marked.  The other one is way off and, until I get a wiring diagram, I don't know if the meter's out of whack or there was some reason to have a different calibration for the second post.

Now, I've got to figure out if I want to paint the magnets or leave them al natural.  As far as I can tell, they either had a very bad Japanned coating that virtually fell off or they were never painted.

A friend led me to a couple of URL's that had instructions for charging the magnets in a Model T magneto while in the engine.  That will simplify the matter as the magnets are pretty weak and not of the same strength, running from near zero to 300 Gauss.

I found these instructions while browsing the Model T Ford Club of America (MTFCA.COM) and thank the club for making them available.


Model T Ford in-the-car magneto magnet charging.

(As with all the other images, click on the image for a higher resolution version)


31 August 2011:

Today, I cleaned bolts and flywheel and sorted, measured, milled and cleaned the magnets


Determining polarity of magnets.                                                             Milling to match height.

After cleaning the magnets, I decided that, the same as the flywheel, they were never painted.  Then, I made the mistake of checking the height of the magnets.  They were made in a press and there is a stamped area where the pole pieces go so I assumed they were all made the same.  I don't know if these were within Ford's tolerance range but I measured from 0.742" to 0.764".  The aluminum spools that the magnets sit on at the poles were all within 0.001" of each other and the pole pieces were all within about 0.006" of each other.  That makes the worst pairings give heights in a range of over 0.025".

I thought that was a bit much so I milled the pole surfaces to the shortest height.  Because of the hardness of the magnets, I still had a range of about 0.010" so I chose pole pieces to help even out the difference.


Magnets and pole pieces sorted according to finished height.

The magnets and pole pieces were then grouped for best fit.  The polarity was again checked and marked where the markings were milled off and assembly of the magnets on the flywheel commenced.

Flywheel and magnets partially assembled.

When the magnets were removed from the flywheel, six of the brass flathead screws broke at the head.  They looked like 12-24s'.  No problemo, I'll just order some more.  Woah!  After checking the bolt size, I find that they are 14-24's!  Although listed on a lot of thread charts and I've got a 14-24 tap and die, the bolts appear to be nowhere to be found.

Does anyone out there have a spare Model T Ford flywheel laying around that you could steal six of the magnet pole piece bolts from?  If you have a selection of 14-24 X 2-1/2" flat head slotted brass machine screws, please contact me.  I'd be eternally grateful to get a half dozen of them.  I guess if worse comes to worse, I could buy some brass bar stock and make them but it seems like a lot of trouble and a pain in the kazootie to turn something that small in diameter and long.

UPDATE 26 February 2012.  I eventually found the replacement screws (14-24 or 1/4-24) brass screws at the following site: 


1 September 2011:

Parts got cleaned and marked for painting today.  Also, I cleaned-up the Model T coil I recently got from a friend.  It was really grungy.  I found that I could get most of the nasty off of it (including paint spatters and tar) using a cloth dampened with lacquer thinner.  

"Restored" Model T buzz coil.

After the wood dried thoroughly, I brushed on a coat of LPS-3, a wax based metal protectant.  The vibrator parts and hardware were cleaned-up and it was put back together.  Looks good and works good, too.

I'm hot on the trail of a set of pole piece screws.   As soon as I get them and get the paint, I'll be about ready to charge the magnets and start re-assembling the tester.


13 September 2011:

After a pause to finish The One Valve Engine, I'm back to the Model T Coil Tester.  In the meantime, I ordered and received the pole piece bolts - the actual #14-24 bolts!  I also found an automotive paint shop that could use a digital scanning process to match the paint color and got a pint of acrylic enamel.

After painting the parts and waiting for them to dry, I recharged the magnets.

Using the old nuts with the new screws, the flywheel/magnet assembly was reassembled.  Then, I started on the magnet recharging procedure.  After discussing the issue with my magnet Guru, we decided it would be just about as effective to charge a lot of capacitors up to a voltage then dump the charge into the coil ring to charge the magnets.  This was decided on because my power supply can't put out the 75 to 100 amps needed to do the job.

In accordance with "the plan", the first thing I did was to hang all of my big electrolytic capacitors in parallel.  Including the filters in my big moose of a power supply, I had a total of just over 100,000 microfarads that I could charge up to 35 volts.  Since the magneto coil resistance is on the order of 1/2 Ohm, the instantaneous current in the coils would be about 70 Amps and would quickly fade down to zero as the capacitors disharged.

Magnet charging setup.

I then checked the coils, first to determine for sure what the polarity was and also, using the Gaussmeter, to make sure all the coils were good.


Setup for checking the magneto coils.                                                              Charging the magnets.       

To check the magneto coils and determine magnetic polarity, I used the setup in the photo above, left.  The light bulbs were used to limit the current into the magneto.  All of the coils had the same strength and the magnetic polarity correctly alternated between North and South poles.  Incidentally, with about 5 Amps going through the coils, each one read about 100 Gauss.

In order to see just how well this recharging method would work, I marked four random pole pieces and carefully measured their strength in Gauss before charging.  To charge the magnets, I set the coils over the flywheel with the South poles of the coils against the North poles of the magnets.  The capacitors were charged up to 35 volts and the power supply was turned off before touching the positive lead to the magneto hot contact as shown in the right photo above.  I turned off the supply after charging the capacitors because, after the capacitors discharged, the power supply would have to supply about 70 Amps.  Since the supply will only do about 30 Amps, this was to keep the magic smoke from leaking out of it.

Magnet Charging Chart

       Magnet Number  Strength Before Charging  Strength After Charge #1  Strength After Charge #2  Strength After Charge #3
                  1            360 Gauss             740 Gauss              870 Gauss             880 Gauss
                  2            500 Gauss             780 Gauss              810 Gauss             830 Gauss
                  3            480 Gauss             780 Gauss              780 Gauss             840 Gauss
                  4            500 Gauss             820 Gauss              820 Gauss             840 Gauss

All of the readings are of the South poles.  As you can see by the chart, there is a marked improvement in strength after the first charging shot.  Subsequent shots increase the strength measurably but, after the fourth shot, no noticeable improvement was noted.

Things that may make this work better would be charging the capacitor bank to a higher voltage but, due to the 25 Volt rating of some of the capacitors in the bank, I didn't think it would be advisable to go over about 35 volts.  I believe that, if a suitable bank of 180 volt capacitors can be ganged together and charged through a light bulb from the AC line, which would give about 180 volts, the charger may work better.

As it is, I'm satisfied that the magneto will work fine. 


14 September 2011:

Today, it's finished and tested.



The finished restoration.

There were a few issues during assembly.  First was that, in spite of selectively fitting the magnets and pole pieces, there was still a pretty wide margin of magnet to coil spacing.  I ended-up putting the flywheel in the mill and taking a skimming cut on the sixteen pole pieces to get them all on the same plane.  There's still about 0.020" runout in the flywheel, most likely from sometime in it's life being dropped, slightly bending the shaft.  I didn't feel it was necessary to face the shaft at this time.

The tester works fine and I am happy to have it finished.  Now, where do I put it?

Here's the YouTube video of the coil tester in action.


BOY!  This is Fun!


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