Originally Posted by bostewart
Well, this didnt work the first go-round; I am installing my sportvue heads up display, which requires a tap into the bikes RPM wire. I have different answers from different shops, and none are correct yet. I need to know which wire is the rpm sensor wire from the crankcase to the gauge. If anyone can figure this out- please let me know! My bike is in a state of chaos!
That is a hard one to find. Most go to the inst cluster which shows a thing called a tach. The tach (German for speedometer) is the speedometer.
The tach (English this time) is found all alone on schematic #VI of the schematic booklet for the K1200RS BMW part # 01 99 9 799 685 for the bikes starting in 1997 and probably lacking some changes starting in 2001 or 2002 US having to do with the Integral ABS, etc.
Anyway, I'm telling you how to build a watch and you asked for the time...
The black wire from the Motronic, pin #31 goes to the instrument cluster connector pin # 5 and then to the instrument mounted tachometer (still English). Pin #4 is a brown wire and is ground if you need that for the Heads Up. Another point where you can find the same signal as pin #5 is on the coils connector, pin #3 there, and still a black wire. BMW uses black for the signal (switched) wire to the coils. The other wires at the coil connector are green, #2, which is +12 volts and brown #1 which is probably ground but isolated from chassis ground...maybe for noise issues from the coil current on it since it is a return for the coil primary current. I would think the coil connector is easiest to find for the rpm signal, but the same wire at the instrument cluster connector may be the easiest for the Heads Up sender depending on where you put it.
This black wire has quite a high voltage signal on it when the bike is running. Apparently, the Heads Up has an attenuator in it to lower this pulsing voltage to a small signal somehow. The signal goes directly to the tach and the tach has an attenuator in it for the same reasons. All tachs work this way and deal with the large signal.
The green/black wire if you see them anywhere is fused and is the ignition switch +12 wire after the fuse and is convenient to use as power which turns off with the bike's key. Electrically the green/black is the same circuit as the green wire, but on the other side of a fuse.
I'd be interested in hearing how the thing works.