By Staff
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Schematic for the Ford Model T buzz coil.
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The following comes from a recent topic on SmokStak, which can
be found on the Internet at: smokstak.cgi. As
ever, various individuals started, commented on and concluded the
following bulletin board thread.

Could someone tell me how to check a Ford Model T coil before I
go through all the trouble of soldering some clips on the post? –

I hook a 6- or 9-volt battery to the positive and negative
terminals of the coil and see if I can make it buzz. Nothing fancy,
but it works for me. -Mike

I use an ohmmeter and measure the resistance between the
contacts positive, negative and spark terminals. Top (positive) to
bottom (negative) should almost be a short, and top (positive) or
bottom (negative) to the center (spark) should read a higher
resistance. If you get an open circuit then you need to work on it.
Usually just cleaning the points and all the connections on the top
of the coil does it. Then you may need to adjust the tension on the
bottom set of points by bending them at the coil connections just a
bit up or down to get the proper BUZZZZZ.

You really need a meter to measure the current draw, which
should be around 1.5 amps. More than that has a tendency to burn
the points and shorten battery life. Operating the coil without a
spark plug or tester connected can cause the secondary to short out
and then it’s junk. – Paul S.

The top and bottom post are the primary positive and negative.
It doesn’t make any difference which one you choose. The center
button is secondary, but be sure you have a wire with a 1/4- to
3/8-inch gap for the spark to jump. Letting it buzz without a place
for the secondary spark to go is asking for trouble because it will
find a path inside the coil and burn the insulation. Twelve volts
won’t hurt it, just don’t leave it buzzing. – J.B.

Can I run a coil with a 12-volt battery without putting a
resistor in the line? – Benny

The secondary resistance should be 3,000-4,000 ohms. Also, watch
out for the moving bridge contact on the armature. When the
armature is pulled in, the contact is on a spring, which lets it
travel 5 millimeters or so before breaking the contact with the
stationary point. This lets the field build up, which collapses
when contact is broken, rendering a hot spark. Many times a coil is
considered weak when the bridge spring on the armature is not
working. You might also replace the condenser. I have rebuilt 60 to
80 of these things, and I’ve replaced more than half of the
condensers. Any old automotive condenser with 0.2 microfarads at
600 volts will work. Don’t ever run them without a spark load.
– Paul G.

The ‘T’ coils were designed for 6 volts. The buzz box
takes those 6 volts and steps them up to about 10,000 volts for the
spark plug. Twelve volts will double the output voltage, but the
coils may break down and possibly arc. The original condenser (if
it’s still there) may break down under the double voltage.
I’m sure many folks run on 12 volts, but I wouldn’t –
‘T’ coils weren’t meant for it. – Norm

I don’t think 12 volts will hurt a ‘T’ buzz coil as
long as the spark plug gap has 0.050-inches or less in the
secondary circuit. Model T engines had an internal low-tension AC
magneto that, at speed, would produce about 30 volts. Calculations
estimate 20 volts DC. I agree that the condenser may go south at 12
volts on a coil that has been heavily used or has been sitting
around for a long time, but it probably would die on 6 volts after
a while anyway. – Elden

Thanks to everyone who replied, it sure helped. I also learned a
lot by Harry’s ignition page ( magbuz.htm). I
have five coils that will be at my house next week and will test
them before building a buzz coil box. – Benny

Benny, take it to the shows that you attend. I’m beginning
to see a lot of Model T coil checkers at shows. I saw one in
Boonville, Ind., last week. A lot of people don’t know what
they look like. It’s a hand-cranked magneto with gauges
designed to check magneto coils, horns and spark plugs. Early Ford
dealers had them in their shops. – Lonnie

I have a buzz coil but know nothing about them except what I
read in these threads. Mine is probably quite typical since
it’s in a wooden box with ‘Ford’ on the front side.
Some threads have mentioned the ‘top side’ terminal(s), but
the problem is I don’t how to tell which is the ‘top,’
‘bottom,’ ‘right,’ or ‘left side.’ The
measurements of this box are (measurement standing on end, contact
points on top, ‘Ford’ logo facing me) 5 inches tall, 3-1/4
inches wide and 2 inches deep. There are two contact points (solder
connections) on the left side and one on the bottom side when
sitting in said position.

Am I correct in assuming that one of the two solder points on
the left side is for positive or negative and the other for the
timer? Is the lone connection on the bottom for the spark plug? Is
the condenser on the inside of the wood box mounted remotely? If
it’s mounted in the wood box, how does one go about getting the
box open without cracking or destroying it? – Marty

The terminals on a Model T coil are as follows: The button
closest to the ‘buzzer’ is one end of the primary and can
be either positive or negative. The button in the middle (on the
same surface as the previous button) connects to the spark plug.
The button on the opposite end of the ‘buzzer’ is the other
end of the primary, the opposite polarity of the other primary end.
It doesn’t matter what polarity is connected to the primary.
ALWAYS put a spark gap between the spark plug button and ground
(battery negative).

If the buzzer doesn’t work when you hook up the battery (but
you get a spark from the battery lead), the buzzer points may be
stuck or set too close. If the buzzer doesn’t work when you
hook up the battery and you don’t get a spark from the battery
lead, either the buzzer points are set too loose and are dirty or
the connections from the buzzer aren’t making good contact. If
the coil buzzes but you get a very puny spark from the plug contact
and the points are arcing a lot, the condenser is bad.

There are two ways to fix this. One is to simply get an old
automobile condenser and wire it across the points of the buzzer.
It’s ugly, but it works most times unless the internal
condenser is shorted. To get at the internal condenser, slide out
one side panel of the coil. It may have a small nail or staple
holding it in. The coil box is filled with tar, and you’ll have
to carefully cook the tar out to get at the old condenser and its
wires. After replacing the condenser, re-melt the tar that was
removed and pour it back in. – Elden

Elden and Benny, many thanks since you have answered all my
questions. I did find the sliding panel on the coil after looking a
bit closer. If this coil is good, or I can make it good (I see no
reason why not, unless the coils are actually burned or shorted),
this may very well become a handy source of ignition for those
hard-to-find applications or for testing purposes. Thanks again. –

I’ve just picked up my first hit-and-miss engine, a
Fairbanks-Morse Z 1-1/2 HP. The coil has screw terminals that are
marked B, T and S. Can anyone tell me if this is the correct coil,
and if so, how to wire it? -Richard

I would say that S = Spark Plug, T = Timer and B = Battery. So,
S would connect to the spark plug, T would connect to the engine
timer, and B would connect to the positive terminal on the battery.
The negative battery terminal would be grounded to the engine. –
Tom G.

If you switch the polarity each time you hook up any buzz coil
to DC current, it will eliminate the wear on the points. The
transfer of metal from one point to the other is switched each time
that you switch polarity. I use a Chrysler resistor with a 12-volt
battery to cut down the voltage some. My coils have run for many
years without any trouble. -Dick in Indiana

Six-volt lantern batteries are cheap, will work for more than
one show season and do the job just fine. – Harry

SmokStak is an engine conversation bulletin board with over
50,000 messages on file and is part of the Old Engine series of Web
sites that started in 1995 as ‘Harry’s Old Engine.’
Harry Matthews is a retired electronic engineer and gas engine
collector from Oswego, N.Y., now residing in Sarasota,

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