Electronic tachometer
connections, wire colors, retrofitting to earlier models with mechanical tachometers
How to check your tachometer calibration.
©
Copyright, 2012, R. Fleischer
tach.htm-37A
(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. As
was reported to me, You might also note that any tach spec'd to
work on a Harley Davidson "dual fire", such as Drag specialties
or similar, will also work fine on any wasted spark airhead
ignition.
(2) Tachometer calibration:
You don't need to buy, or borrow, any fancy tachometer testers.
You do not 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 that rotates. 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 alternating current
frequency. The frequency is held VERY precisely to 60 Hz
in the USA (also called '60 cycles'); old-fashioned motor-driven clocks were
powered by that, and kept perfect time for YEARS, so you need not
worry about accuracy for checking your tachometer!!
Disconnect all battery negative wires, and then remove the front cover of the engine. Reconnect the battery (disconnect again when done before replacing cover). Clean the end of the alternator rotor, and mark that end with SIX (6) white paint lines. Try to keep them thin and straight. These lines should be radial, relatively evenly spaced, from the center outward. You will need the area to be illuminated with a common fluorescent light. 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. NOTE that some types of spiral or other compact fluorescent lights run via their own internal frequency generators, and will not work for this application....but, you will have no way of knowing what is inside that CFL, so just try it, unless you have long tube shop lights, which are almost always OK for our purposes here. In my own use, I make up a small piece of black paper with white lines, and use fast drying rubber cement to hold the paper to the end of the rotor.
Note that you do not have to use 6 lines, in fact you can use one, two, three, or four. You just need to know how this all works, so as to interpret the results. I will give another example here, and how this works. First, you must understand that old-fashioned fluorescent lamps are gas discharge lamps, the electrical discharge causes phosphor coated on the inside of the glass to light up. This happens very fast. The flashing occurs so fast that our human eye and brain do not distinguish the flashing as such, but as continuous light. The picture on your TV screen flashes a picture too, at 30 times a second. The fluorescent lamp turns on and off with the power line, which is an alternating current, which goes through a complete cycle of off; on-positive; off, on-negative, and back to the starting off position, twice per cycle. So, that is 120 flashes PER SECOND. The incandescent (heated filament) lights do the same, but the INERTIA of the heated filament heating/cooling happens so fast, that these types of lamps are not good for strobe effects.
Let us suppose you affix a SINGLE white line to the rotor, from the center to one side. Let us suppose the engine is rotating that crankshaft at precisely 1800 rpm. You will see FOUR lines under the fluorescent lamp. Why? Because at 1800 rpm that is the same as 1800/60, or 30 revolutions PER SECOND. The lamp is flashing at 120 times a second. Thus, 120/30 is 4 lines appearing.
So, what about 2400 rpm? Same mathematics applies, and you get THREE lines.
So, you do not have to use 6 lines.
Simply note the tach error, if any, and I suggest you check the tach at several places. Just what places you check, depends on your number of lines. Near idle, ~1/3 red line would be adequate, although you certainly could 'calibrate' the tachometer at ~yellow line too.
rev:
03/16/2006: Add calibration information + minor editing for clarity.
01/14/2009: was 38A
06/14/2009: minor type fixed; plus add information on use
of CFL's.
02/15/2012: check article again, particularly to be sure the strobe lamp
usage is fully explained.
© Copyright, 2012, R. Fleischer
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