Thanks guys. What does the TFI do?
The Thick Film Ignition module, type IV serves two functions.
1. It accepts a very weak signal from the distributor (a 'trigger" signal, by a Hall Effect method) and uses that trigger signal to open and close a solid state switch to the ignition coil.
2. It measures the current loading to the ignition coil, and adjusts the current sent to the coil to the optimum so it reaches saturation, but is not overheated with excess current.
Basically then, it replaces the function of the mechanical breaker points in controlling the ignition coil, which had two parameters. One, the timing as to when the points opened, controlled by a cam., And two, the dwell (time points are closed, usually stated in degrees of rotation of the distributor) to charge the coil. Both the timing and the dwell are critical to engine performance, and points require regular maintenance. Even when performing well, there are still shortcomings, especially at high engine speeds.
While originally electronics came to automobile ignitions for improved performance and reliability; the advent of emissions regulations made them mandatory in order to minimize maintenance needs, and even just to meet requirements.
An associated note--- the TFI-IV has a history of problems when it is MOUNTED DIRECTLY on the distributor. If overheated, it will simply not function, though usually recovers when cooled sufficiently. Many hobbyists who use the module in their vintage cars use a wiring adapter and move the module itself to a cooler location under the hood, and keep the advantage of the electronics but avoid the overheating troubles. Ford Motor Co. settled a class action suit over the issue years ago.