[TR] TR4 mechanical tachometer stopped.

[Randall]

> What's the most likely cause of sudden total failure?  Broken cable?

IMO Yes, brought on by binding within the head.  Sometimes they even bind
only when hot and work more-or-less OK cold.

> I assume I can disconnect the cable at the distributor end, 
> start the car and determine if the distributor-driven 
> connection is rotating.  Any tricks to this?

Nope; tho if the cable is broken, you may find the broken end inside that
connection.

> Can I put an electric drill motor on the cable to test the 
> cable/tachometer combination?  

Sure.  But I'd suggest turning it with fingers first, to see how stiff the
head is.  Should be almost no resistance at all; if it's stiff then the
bushing in the head is binding.

> Which direction to I run the drill motor?  

The cable turns the same way the needle moves.  Run your drill backwards.

> Will I ruin something if I accidently go the wrong way (on a 
> speedometer I would not ruin things, it has to deal with the 
> car reversing)?

Nope, won't hurt a thing.  The tach movement is identical to the speedo,
except for not having the odometer and trip meter mechanisms.

In fact, you can substitute one for the other.  60 mph indicated is about
2300 rpm (assuming your speedo calibration is 1150).

Randall