That model Yale has a module in the distributor. It should ground the neg side of the coil and then open that circuit to make it spark. If you are measuring 4 volts on the negative side of the coil, with the wire attached, then I would say that's the problem. It should measure 0. Part number for the module is 9013578-08. The rotor just pulls off. No need to take the distributor out.
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