Do you have enough "hands" and dexterity to position the two sharp-tipped probes of a meter against two exposed metal crimp contacts of a harness connector...while slowly rotating the flywheel? The 3-wires to the tach assembly form a fairly simple circuit. There should be a fixed voltage between two of the wires...and a voltage that alternates a few Volts depending on a white or black stripe under the sensor. There have been previous OGR threads on this.
IF you want to get down into the nuts-and-volts of the matter I think you can isolate where the problem lies.
