Well, I ended up having to get new senders as well as the four SW tachs.* THAT was expensive, but unfortunately necessary as the 36-year old originals were really getting inaccurate.*
These new tachs require three-wire prallel conections vice the two-wire looped originals.* As the originals were wired in a loop, that is the signal wire went from the senders to the flying bridge tachs before going down to the lower station tachs and thence to ground,*I was faced with the requirement to convert them to papallel.
Since one of the new wires is power, I eliminated the need to run a third parallel wire by taking power to the senders from the engine oil pressure senders of each engine, and power to the tachs themselves was taken from the other instruments of the respective engine at each panel.* The two original wires were easily converted from loops into two parrallel wires by reconnecting from the local TBs to the panels and a couple of short jumpers to get the connections I needed.*
Things are running fine (verified by handheld digital tach), and interestingly, the dip switch settings of the tachs were set differently out of the box but worked fine as they were received without alteration - go figure.
