I dont know how to fix but by using an electronic ignition unit in your car will provide you with another tacho cable which might work for your problem...

outgoing wires from the elec ignition is
-coil negative
-points positive
-ground
-12v input
-tachometer

they take approx 4-6 hours to make 1st time. with practice you can make them much faster. my last was 2 hrs and ive only made 3 of em.

search www.jaycar.com.au for "Universal High Energy Ignition Kit". its $52.95 AU.