Electronic tachometer connections, colors, retrofitting to earlier models with mechanical tachometers; How to check your tachometer calibration...ETC.
©
tach.htm-38A

(1)  Tach install  information is primarily from a BMW Service Information bulletin, dated May 1978, and whilst the bulletin seems very hard to read, it appears to be #1063R.   That bulletin specifically dealt with installing an electronic tach of the later type into the earlier pods.  This information applies to all the models at that time, which were R60/6-R100RS (1977)....but some of this information is applicable to later models for wiring, etc. The bulletin has a sketch, but that sketch is not really needed.  In some instances you have to drill two holes for the wire, BMW says to use 2.6 mm.  Nothing critical about that.

If the black lead from terminal 1 of the ignition coil (POINTS connection) does NOT exist in the pod, then you need to drill the holes.   IF the black connection is present, it will be known by an actual connection at pin position #10 of the 12 pin plug.

The tachometer wiring is:  
red = +12, and is pin #12
brown, as usual, is ground, and negative, and this is pin #7
green is the pulse wire, it is pin #10

1977 bikes:  if no pin #10, nor wire to coil, neatly find a way to lengthen the green wire and connect to the points connection on that particular coil.

BMW has a SEAL to block off the mechanical tach takeoff area.  The worm, etc., can be left in place. The seal is 63-23-1-351-257   I am sure that is now obsolete, and you will have to fashion something.

For 1978+; the electronic tach is 62-13-1-243-434, a pricey part, and a junkyard item may be more to your tastes.

(2)  Tachometer calibration:   
You don't need to buy, or borrow, any fancy tachometer testers; nor have to depend on THEIR accuracy.  No matter what type of tachometer you have, mechanical, or electronic, here is a VERY simple way of checking the calibration of your tachometer.  This trick works with anything, frankly.  In fact, it can be used with rotating wheels to determine speed, and many other things.  What you will do is to take advantage of the extreme accuracy of your electric power line AC frequency.  The frequency is held to 60.00+Hz. (also called '60 cycles').   For your Airhead, unfasten the battery negative wires, and remove the front cover of the engine.  Reconnect the battery (disconnect when done before replacing cover).   Clean the end of the alternator rotor, and mark that end with SIX (6) white chalk lines, or use white paint dabs.  These lines should be radial, relatively evenly spaced, from the center outward.  You will need the area to be illuminated with a common fluourescent light.  You could also use a Neon lamp, or any other light source that is plugged into the power line.  When the engine is started, watch the markings.  There will be SIX, fully stopped, at 1200 rpm. Same at TWICE and THREE times that rpm, and so on.  

Simply note the tach error, if any.

rev:
03/16/2006:  Add calibration information + minor editing for clarity.