Have you checked the pickup coil, the one that surrounds the shaft? Its wires move with every motion of the vacuum advance, and they're subject to breakage from millions of cycles of movement. Disconnect the 2-wire connector from the pickup coil to the module, and check it with an ohmmeter while moving the wire harness around. It should not go open-circuit at any time. Then, connect a low-scale AC voltmeter to the connectors, and crank the engine. At cranking speed, you should see at least 1/2 volt AC. The faster the distributor spins, the higher the voltage will be. If it fails either test, replace it. You'll need to remove the distributor shaft to do that- - - -there's a roll pin through the gear at the bottom of the shaft. Be sure to memorize the assembly of the gear, the shaft, and the thrust washers- - - -it's possible to reassemble things wrong. Also, you'll probably need to use some carburetor cleaner on the shaft- - - -sludge builds up and makes it hard to pull through the bushings.
Jerry
