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Replacing master vibrator for Model T and TT Ford

12 Feb 2015  | Craig D. Merz

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I am quite keen on keeping old cars in their original condition as much as possible. There is often some difficulty in making and holding the contact point adjustment on each of the four model T coils. In addition, the internal capacitor may have failed due to age. This article describes a possible solution that does not require any permanent modification to the Model T.

Described here are some revisions to make the circuit easier to build and more efficient compared to a version published in Model T Times (July-August 2012). As with the original, use of the circuit requires that each of the four coil contacts be removed from the circuit using four short jumper wires with fork type lugs attached (the fork type lugs make installation and removal easier as well as preserving the contact adjustment and allowing for easy return to the original Ford system). The master vibrator circuit board is then connected in series between the wire from the key switch to the power input for all four coils. A wire is also needed to connect the circuit board to chassis ground.

There is some difficulty in making the master vibrator circuit accommodate both the 6V DC battery supply and the higher output from the AC magneto (alternator). Therefore, it might make sense to separate the rectifier/filter function from the master vibrator portion of the circuit.

Figure 1: The master vibrator part of the circuit.

If you only run your model T on the 6V battery supply (or have a car modified for 12V), you need only build the master vibrator part of the circuit (figure 1). However, you must ensure that the magneto output may never be inadvertently connected to this board as it will destroy the circuitry. The advantage of doing it this way is that the forward drop of the rectifier diode is not subtracted from the 6V supply, allowing a hotter spark when cranking. In addition, a much smaller filter capacitor is needed.

If you do run your car from the magneto supply or wish to run the car on either at different times, then the circuit shown in figure 2 would be connected between the magneto and the master vibrator circuit board. Note: The original circuit combined both on one circuit board and may be done that way as well with this revised circuit.

Figure 2: The fuse, half-wave rectifier, and filter to change the magneto output to DC.

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