Tacho problem on 12v71TI's

I have a pair of 12-71TI´s´ in an old 1979 Broward. Each motor has 1 mechanical tacho in the engine room and 2 electronic, one at each helm. Both port side electronic tachos have ceased working. I suspect as both are out that it wont be an individual tacho problem? but either a sender unit or broken wire?
Any body familiar with the set up for the electronic tacho´s for these engines and how the system works so I´m sure I looking in the right places?
Any assistance and test procedures much appreciated. Tom

Found problem

appears the electronic tachs are driven by the Glendinning sync on which the tach cable from the engine isn't driving. This is a glendinning part. Never had problems with 12v71 parts but surpose there may be a first.
Thanks Tom