I reverse engineered the LCD Gear Indicator board as part of my project to build a drop-in replacement for the LCD.  I then proceeded to reverse engineer some other boards and draw their schematics, as well as collected schematics from other sources and re-interpreted those drawings.  The flasher unit schematic is not my work; it is available online.  I'll add more as I convert the BMW OEM circuits into schematic drawings.

GPI  This is the board that is located in the OEM cluster underneath of the Tachometer on the right side, containing the LCD that shows the currently selected gear:




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Basic Circuit Description
Power is supplied through pin 15 (+12V) and 31 (Ground).  A diode, V7, ensures that reversing the polarity of the power will not destroy the components on the board.  VIN is fed to the Start Enable circuitry and to the regulator V9.  Regulated VDD supplies the chips with power as well as other parts of the circuit.

The TGPI switch connects to A, B and C inputs.  Diodes, V3, V4 and V5, form a neutral detect feature that operates the Start Enable circuitry.  The three transistors, V8, V9 and V10, creates a +12V signal on the output, LEERL, that drives neutral and the starter relay.  Diodes, V1 and V2, detect when the selected gear is higher than 5th and will send a signal to IC1 to blank the LCD display.  IC1 decodes the inputs and displays a digit on the LCD via PAD-A through PAD-G.

IC2 is an oscillator that creates a high power signal that powers the LCD.  This signal is output on PAD-COM and consists of a 67Hz 100V 50% duty-cycle squarewave.

Speedometer and Odometer
This board is located in the OEM gauge cluster and converts the output from the speed sensor in the final drive into a signal that drives the speedometer servo motor.





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Tachometer
This board is located in the OEM gauge cluster and converts the output from coil #1 into an RPM reading on the tachometer gauge.




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Early Fuel Level Sensor (4L/7L lights)
This board is located in the OEM gauge cluster for the early K-bikes prior to 1985.  The fuel sensor was a pair of thermistors.  These devices illuminated a lamp in the OEM gauge cluster that showed when the fuel remaining in the tank was 7 litres and 4 litres.  This is an interpretation of a schematic posted by Bert Vogel on K100-forum.




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Flasher Unit
This schematic was posted online at the K100-forum and I am including it in this collection.




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Clock
This schematic is of the clock in the OEM dash.  To test that all segments of the display are working, press both switches together at the same time.
All segments will light up.  When the switches are released, the clock will reset to 1:00.   One complaint of the clock is that it only shows 24 hour time. To get the clock to display 12 hour time, use an Xacto knife to cut the trace at pin 22 (labelled "S5" in the schematic) and then take a 30GA solid wire and join S5 to VIN.  This will change the clock to display hours between 1-12.
 



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Bulb Monitoring Unit
This board is located in the relay box.  It's purpose is to monitor incandescent bulbs in the rear tail of the motorcycle, the brake light and running lights as well as the brake switches themselves.  If any bulb or any switch is defective, the BMU will illuminate an indicator on the dash. Since there are no parts identification silk screened on the board and my schematic makes reference to part identifiers, I arbitrarily assigned reference numbers to parts on a photo of the board. This photo helps to show the location of the various components and references them by their part identifier on the schematic.



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You learn something new every day. I had no idea that you needed an oscillator to drive the LCD. I'd assumed that they were a bit like LEDs; add power and away you go.

Thanks for sharing. This is great work.

Have you ever done one for the speedo amplifier ?

Ringfad wrote:Thanks for sharing. This is great work.

Have you ever done one for the speedo amplifier ?

This is on my TODO list.

fantastic work yet again rob

and thanks for sharing

I can download with the link provided.

One more post needed, I think.

Hi

I have a question about my speedometer. 1984 k100.
Tachometer is working.
Speedometer and odometer not working.
Speedometer became erratic just before it stopped working.
All internal components seem to be ok..
Speedometer face is fine, speedometer needle moves freely on the shaft.
Not certain how to approach this problem.
Incredibly frustrating.

Thanks

Gabriel.

Have you tried this?
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/flakyspeedo/flakyspeedo.htm

Thanks for sharing, very useful indeed.
I had a problem last year and it took 2 batteries, one alternator rebuilt and some other stuff before unplugging the ABS that was the root of the problem. One pump was defective and it damaged the instrument cluster, battery and alternator. Installed a used one I bought on Ebay but the odometer doesn't work (maybe because it is on miles and my bike is on kms). Anyway, once the ABS was unplugged with the new speedo everything started working. I switched back to the old speedo and it doesn't charge the battery.

Anyway, the 2 questions are: 1. Can the odometer be fixed? 2. Any idea which circuit is responsible for draining the battery? Thanks,

Thanks,

Vin

Check all the connections and the gears. Facebook Ricardo palao for new gear sets.  The alternator needs the charge bulb in the circuit to work. If the bulb is blown it of course will not light nor will it charge.

I will try this also as my speedo is intermittent, thanks much for this help. Lon

WarrenD wrote:Check all the connections and the gears. Facebook Ricardo palao for new gear sets.  The alternator needs the charge bulb in the circuit to work. If the bulb is blown it of course will not light nor will it charge.

This is true with Airheads but not with K bikes, in my experience. There are no idjut lights on Big Block, in fact, no instrument cluster at all, and she cranks out all the voltage juice she needs to keep the battery charged without a bulb (or LED) in the circuit.

I wonder if and which differences exist for bikes with the ABS brakes - K75, K100, K1, K1100?

With weather like we're having here, I'd rather be riding, smelling new-mown hay and feeling G-forces rather than contemplating circuitry but, as it turns out, I must return to stacking firewood. 

Anyway, here's a diagram of a K75RT ABS system and a 1993 K1100LT/RS ABS system for comparison. The accuracy of these diagrams is subject to your verification. 

K75RT ABS



1993 K1100 LT/RS

Laitch wrote:With weather like we're having here, I'd rather be riding, smelling new-mown hay and feeling G-forces rather than contemplating circuitry but, as it turns out, I must return to stacking firewood. 

Anyway, here's a diagram of a K75RT ABS system and a 1993 K1100LT/RS ABS system for comparison. The accuracy of these diagrams is subject to your verification. 

K75RT ABS



1993 K1100 LT/RS

Thanks - but I was more interested to find out if there is any difference in instruments  circuitry, save for two ABS bulbs?

No differences in the instrument cluster internals except for the bulbs, as you point out.