Model T Buzz Coils

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

The way I do it is to just 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

The top and bottom post are primary, + and -, doesn't make any difference which way. The center button is secondary and be sure you have a wire with a 1/4 to 3/8" gap for the spark to jump. Letting it buzz without a place for the secondary to go is asking for trouble because it will find a path inside the coil and burn the insulation. 12 volts won't hurt it, but just don't leave it buzzing. When running it only has momentary contact. – JB

Mike, Does it hurt to run one with a 12 volt battery without putting a resistor in line? – Benny

Benny: I use a ohm meter and measure the resistance between the contacts +, - and spark terminals. Top(+) to bottom(-) should almost be a short, and top(+) or bottom(-) to the center (spark) should read a higher resistance around 33K ohms. If you get an open 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. 6-12 volts doesn't hurt mine. – Paul S.

Sorry if I implied that I would be letting the coil sit there and buzz. A quick touch of the wires to the terminals is enough. It may not be right, but I managed not to cook my coils using this "quick and dirty" method. Careful with that secondary terminal, I can testify to how much it steps up voltage from a 6 volt battery! – Mike

The secondary resistance should be 3-4000 ohms NOT 33K ohms. The other thing that is very important is the bridge contact on the armature, which moves. When the armature is pulled in, the contact is on a spring, which lets it travel 5 mils or so before breaking the contact with stationary point. This lets the field build up which collapses when contact broken giving a hot spark. Many times a coil will be considered weak when in fact, this bridge spring on the armature is not working. Also replace the condenser. I have rebuilt 60-80 of these things and I replace over half of them. Any old automotive condenser will work with 0.2 microfarads at 600 volts. NEVER run them without a spark load! – Paul G.

The T coils were designed for 6 volts. The buzz box takes that 6 volts and steps it up to maybe around 10,000 volts for the spark plug. If you run it on 12 volts, you will double the output voltage. The coils may break down and short or arc and the original condenser (if it still has it) may break down under double the voltage. I'm sure there are folks out there running these on 12V but I wouldn't - they weren't meant for it. -- Norm

I don't think that 12V will hurt a "T" buzz coil as long as there is a spark plug gap of .050 or less in the secondary circuit. Model T engines had an internal low-tension AC magneto that, at speed would put out around 30V. Doing the calculations gives an equivalent to approximately 20V DC. I agree that the condenser might go south at 12V on a coil that has been used a lot or has been sitting around for a long time but it probably would croak on 6V after a while anyway. -- Elden

The first thing to do is determine if it actually is a T coil. 6 volts will make a T coil spark, but performance will be poor unless you adjust the coil to draw extra current. If you have ever tried to run a T with a bad magneto on 6 volts you will find this out. If I forget to change my electric start T to mag it will act sluggish when I try to wind it up and when I change to mag it will take right off... Most guys that have T's with bad mags use 12 volts and get decent performance. I have seen a good T mag put out about 40 volts at 1600 rpm and the coils last just fine. – Ken

Benny if it doesn’t buzz try cleaning the points and tighten all the nuts that hold the points not the one that adjusts the points. --BobRR

Thanks to all that replied, it sure has helped an old man a lot. I also learned a lot by HARRY'S Ignition page. I have five coils that will be at my house next week and would just like to test them before building a buzz coil box. Thanks again and thanks HARRY for this site, if it was not for you I would not be in this expensive hobby. -- Benny

Hi Benny, I am a Model T restorer. I have three of the contrary things and I have rebuilt dozens of coils. I have a coil tester that will check the amps and the spark intensity. I can replace the capacitors (if that is the problem) and points with new ones if needed. Let me know if I can help. -- Larry

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, IN last week. I sold mine to a friend six months ago. A lot of people do not know what they look like. It is a hand cranked magneto with gauges designed to check magneto coils, horn and sparkplug. Early ford dealers had them in their shop. – Lonnie

I have one of those buzz coils but know nothing about them except from these threads. I would venture to say this one is probably quite typical as it's in a wood box with "FORD" on the front side. Some of the Re's make mention of the "top side" the terminal(s) on the bottom side, left side, right side... I would like also to test this one and can do so by following the suggestions in this thread. Where the problem is, I don't know what "top", "bottom", "right side", "left side" is. The measurements of this box are (measuring with it standing on end, contact points on top, "Ford" printing facing me) 5" tall, 3-1/4" in width & 2" thick. (depth) 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 + or - from battery and the other for the timer and is the lone connection on the bottom for the spark plug? Is the condenser on the inside of the wood box or is/was that mounted remotely? If 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, either positive or negative. The button in the middle (on the same surface as the previous button) goes to the spark plug. The button on the opposite end to the "buzzer" is the other end of the primary, the opposite polarity of the other end of the primary. It doesn't matter what polarity is connected to the primary. ALWAYS put a spark gap between the spark plug button and ground (battery -). 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 do not get a spark from the battery lead, either the buzzer points are set too "loose", 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. Ugly, but it works most times unless the internal condenser is shorted. To get at the internal condenser, one side panel of the coil will slide out. It may have a small nail or staple holding it in. The coil box is filled with tar and you will have to carefully cook the tar out to get at the old condenser and it's wires. After replacing the condenser, re-melt the tar that was removed and pour it back in. Hope that helped. -- Elden

Marty,look at the picture of a buzz coil on Harry's http://www.old-engine.com/magbuz.htm -- Benny

Elden & Benny, Thanks bunches as you have answered all my questions. I did find the sliding panel on the coil after looking a bit closer and Harrys ignition page is terrifically simple to understand for electrical beginners like me. If this coil is good or I can make it good, (of which I see no reason why not unless the coils are actually burned/shorted) this may very well become a handy source of ignition for those "hard to find" applications or strictly for testing purposes. I've seen an infinite variety of homemade timer trips being used so it would seem the sky is the limit as far as that goes. Thanks again guys. – Marty

Benny, I bet you long since tested your coil and now that I’ve got the answers on my coil, I just had to go and test mine. No I did not solder any leads to the post. I GENTLY clamped the coil in the vice and to hold both the spark plug and ground lead to the bottom terminal, primary side. I simply held everything in place using a rubber band. That allowed both my hands to be free, one to hold plug wire against the secondary lead and the other hand to touch the positive lead to the primary side. My coil throws one heck of a blue spark on 12V DC! This all took less than 60 seconds. – Marty

Hello, I've just picked up my first hit and miss engine a FM Z 1 1/2 hp. The coil has all screw terminals and are marked B, T and S. Can anyone tell me if this the correct coil, and if so how to wire it. Thanks Harry for this site and to all who use it. My wife thanks you also. – 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 timer on the engine, and B would connect to the positive terminal on the battery. The negative battery terminal would be grounded to the engine. – Tom G.

Guys, if you will switch the polarity, each time that you hook up any buzz coil to dc current. You 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 Ind.

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

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