$70 dollars may seem like a lot but I would not go thru the work or money that it would take to diagnose this problem. The easiest quickest way to diagnose this problem how most shops troubleshoot the problem is by hooking up a shop tach to the engine end check results.
Most shops are probably like myself and will put this money towards the actual repair. In other words if the diagnosis only takes 30 mins they will put the other 1/2 worth 0f labor charge towards the repair (either tachometer or rectifier).
You were asking about the "sender" on your engine this is the rectifier/ regulator. This part is responsible for changing your voltage on your engine ac/dc (charging your battery) and sending the tachometer pulses to your tach. Sometimes the motor will charge just (sometimes for years) and only the tachometer circuit be fried. Sometimes the signal will come and go eratically (tach may work for while and then quit again)
If you have any more questions pm me or call me at (706)891-9921 I'll be glad to help.
If you do need a tach they run between $70-$100, if you need a rectifier they run around $200-$250 best I can remember.