Hi All,

I've spent way more hours than I care to admit on this BUT here it goes...

I have what appears to be a fully functioning instrument cluster minus the speedo and odometer. RPM, gear indication, 7L/4L gas warning, choke, battery, oil, temp, clock, neutral, turn light indications all work.

My father used to own this bike and from what I remember (this is like 25 years ago) the speedo intermittently worked then. He would tap the speedo side with varying success, sometimes the speedo would pop right back up and other times not. So my first thought was some cleaning with deoxit.

What I've tried with the instrument cluster on the bike:

1. I removed the cluster and cleaned with deoxit the following:

• All 23 male and female main connectors into the cluster
  
• The 4 male and female connectors on the speedo amplifier board (long board at the bottom)
  
• The 3 male and female connectors on the "arm" at the top of the unit that connect into the horizonal circuit board
  
• The 2 male and female connectors on the horizontal circuit board
  
• The potentiometer under the horizontal circuit board
  

2. I ensured the female clips (don't know what these are called, the clips you bend inwards to ensure good connection with the male part) on the 4 pin speedo amplifier and 3 pin "arm" at the top of the unit were tight
3. I rotated the 4 male pins on the speedo amplifier board about 5 degrees (Something I read on the forums that may help with connection)
4. I rotated the 3 male pins on the "arm" at the top of the unit that connect into the horizonal circuit board about 5 degrees (Something I read on the forums that may help with connection)
5. I rotated the potentiometer slightly (maybe to help if it had a dead spot)
6. I removed the rubber covering around the 23 main connectors into the cluster, no signs of burning or loose pins 
7. Ensured the speedo face plate is nice and flat, it has not delaminated causing the needle to hang up or stick
8. Ensured the speedo needle moves freely and when manually turned anti-clockwise it returns to its zero position 
9. Removed the impulse speed sensor from the final drive and cleaned it
10. Disconnected the impulse speed sensor near the battery/coolant tank and cleaned both yellow and brown male/female connectors with deoxit
11. Tested the resistance of the impulse speed sensor at multiple locations:

• at the first connection near the coolant tank: ~280 ohms
  
• at pins 22 and 23 on instrument cluster female side: ~280 ohms
  
• at pins 1 and 2 on the 4 pin speedo amplifier board (these connect to pins 22/23 main connector): ~280 ohms
  
• at pins 4 on the 4 pin speedo amplifier board and pin 3 on the 3 pin "arm" at the top of the unit that connect into the horizontal circuit board: ~280 ohms
  

After doing those steps, with the bike on the center stand I run the bike up through gears 1, 2, 3 with back wheel spinning pretty good and I get no speed/odometer. With the wheel spinning I've played the 3 pin and 4 pin connectors at the back, wiggling them back and forth and still nothing.

Though I don't think the impulse speedo sensor was the issue, I decided to eliminate the bike all together and got K_DIAG up and running with the appropriate 3.5mm connector and a 12V 2.5 amp power supply and the following setup:
1. Power +12 V on pin 6
2. Power ground on pin 13
3. Impulse yellow on pin 22
4. Impulse brown on pin 33

I set K_DIAG to 100km/hr and with the 3.5mm unplugged I have verified I'm getting a pulse that changes with speed setting. I plug the 3.5mm in and when I hook up to 12V I get a slight bump on the speedo and RPM arms, and by slight I mean maybe 2mm, just a bump and it returns to neutral. The odo also spins slightly, I can see the gears moving slightly and then all stops. I turn on/off the 12V and I keep getting this slight bump over and over.

I've read pretty much every speedo discussion on this forum I think and tried what I found.

What I haven't tried is testing the stepper motor or any other board components, I saw one post about a guy saying his stepper motor had died. How would I go about testing the stepper motor? Any other suggestions? I'm not quite at the 'throw it across the garage stage' but I'm getting there.

Appreciate the input. Sorry for the book. Thanks.

Look, I'm not confident about this at all, cause the odo also doesn't work, but in the mean time I would give the following a try, so here she goes... Remove the wire from the flasher unit to the speedo, at the instrument cluster end, or cover the pin with a piece of paper, to see if the speedo then works.

daveyson wrote:Look, I'm not confident about this at all, cause the odo also doesn't work, but in the mean time I would give the following a try, so here she goes... Remove the wire from the flasher unit to the speedo, at the instrument cluster end, or cover the pin with a piece of paper, to see if the speedo then works.

Thanks for the suggestion daveyson, appreciate it. I just got around to trying this today. I removed pin 21 and tried again. Still no speedo. What was the thinking behind that thought?

I'm going to hand the unit off to my electrical engineer brother. He designs circuit boards so he'll have a look at it over the Canadian winter.

To future readers, I'll update this post if we figure out what the issue was. His first thoughts are maybe old capacitors. My hope is to update by March 2024.

I said to meself, daveson, I said, what would happen if that wire was earthing out to the frame? What would happen, would that result in no speedo, and the half pulse? And I said to meself, dunno. But then I said, it could, then I said it would, then I said, be honest now, dunno. Then I said, but that wouldn't effect the odo would it, and I said dunno, then I thought, no it wouldn't, then I thought, be honest, dunno. One thing I did know was I better say in my post, I'm not confident about it.

The solution would be good to find anyway, and I also think you're brother should join the forum too, it's an interesting place, I'm confident he will agree, well I'm pretty sure about it.

Chris350 wrote:Hi All,

I've spent way more hours than I care to admit on this BUT here it goes...

I have what appears to be a fully functioning instrument cluster minus the speedo and odometer. RPM, gear indication, 7L/4L gas warning, choke, battery, oil, temp, clock, neutral, turn light indications all work.

My father used to own this bike and from what I remember (this is like 25 years ago) the speedo intermittently worked then. He would tap the speedo side with varying success, sometimes the speedo would pop right back up and other times not. So my first thought was some cleaning with deoxit.

What I've tried with the instrument cluster on the bike:

1. I removed the cluster and cleaned with deoxit the following:

• All 23 male and female main connectors into the cluster
  
• The 4 male and female connectors on the speedo amplifier board (long board at the bottom)
  
• The 3 male and female connectors on the "arm" at the top of the unit that connect into the horizonal circuit board
  
• The 2 male and female connectors on the horizontal circuit board
  
• The potentiometer under the horizontal circuit board
  

2. I ensured the female clips (don't know what these are called, the clips you bend inwards to ensure good connection with the male part) on the 4 pin speedo amplifier and 3 pin "arm" at the top of the unit were tight
3. I rotated the 4 male pins on the speedo amplifier board about 5 degrees (Something I read on the forums that may help with connection)
4. I rotated the 3 male pins on the "arm" at the top of the unit that connect into the horizonal circuit board about 5 degrees (Something I read on the forums that may help with connection)
5. I rotated the potentiometer slightly (maybe to help if it had a dead spot)
6. I removed the rubber covering around the 23 main connectors into the cluster, no signs of burning or loose pins 
7. Ensured the speedo face plate is nice and flat, it has not delaminated causing the needle to hang up or stick
8. Ensured the speedo needle moves freely and when manually turned anti-clockwise it returns to its zero position 
9. Removed the impulse speed sensor from the final drive and cleaned it
10. Disconnected the impulse speed sensor near the battery/coolant tank and cleaned both yellow and brown male/female connectors with deoxit
11. Tested the resistance of the impulse speed sensor at multiple locations:

• at the first connection near the coolant tank: ~280 ohms
  
• at pins 22 and 23 on instrument cluster female side: ~280 ohms
  
• at pins 1 and 2 on the 4 pin speedo amplifier board (these connect to pins 22/23 main connector): ~280 ohms
  
• at pins 4 on the 4 pin speedo amplifier board and pin 3 on the 3 pin "arm" at the top of the unit that connect into the horizontal circuit board: ~280 ohms
  

After doing those steps, with the bike on the center stand I run the bike up through gears 1, 2, 3 with back wheel spinning pretty good and I get no speed/odometer. With the wheel spinning I've played the 3 pin and 4 pin connectors at the back, wiggling them back and forth and still nothing.

Though I don't think the impulse speedo sensor was the issue, I decided to eliminate the bike all together and got K_DIAG up and running with the appropriate 3.5mm connector and a 12V 2.5 amp power supply and the following setup:
1. Power +12 V on pin 6
2. Power ground on pin 13
3. Impulse yellow on pin 22
4. Impulse brown on pin 33

I set K_DIAG to 100km/hr and with the 3.5mm unplugged I have verified I'm getting a pulse that changes with speed setting. I plug the 3.5mm in and when I hook up to 12V I get a slight bump on the speedo and RPM arms, and by slight I mean maybe 2mm, just a bump and it returns to neutral. The odo also spins slightly, I can see the gears moving slightly and then all stops. I turn on/off the 12V and I keep getting this slight bump over and over.

I've read pretty much every speedo discussion on this forum I think and tried what I found.

What I haven't tried is testing the stepper motor or any other board components, I saw one post about a guy saying his stepper motor had died. How would I go about testing the stepper motor? Any other suggestions? I'm not quite at the 'throw it across the garage stage' but I'm getting there.

Appreciate the input. Sorry for the book. Thanks.

Sounds like the zenner on the output of the amplifier board has gone short,  this will cause the motor not to operate   and the signal going to the flasher unit to be effecivly a short circuit.... i imagine that the zenner (9.6 v) is there incase somone or something goes wrong down stream   . You vould disconnect the motor drive 3 pin connector and measure the out put of the amp board and earth to see if this is the case   ....low ohms  ...should be a short circuit..... with lunchbox - speedo disconected from loom 

Just thought
Have seen this issue twice

*****Solution*****

The issue was with the speedo amplifier board, thin long board at the bottom of the backside of the speedo.

The backside of this board had some broken electrical traces and one capacitor that was suspect. Some built up corrosion was present around the capacitor and general area of the broke traces.

The capacitor was changed and the traces were repaired with external wire (forget the technical term for this stuff). Speedo fired right up upon hooking up to a frequency generator.

I could have saved many hours of guessing and testing had I removed this board and inspected immediately even before trying deoxit everywhere so I suggest you do that before doing anything. Rookie mistake. The front of the board looks perfect with zero indication of anything wrong on the backside. The key was measuring the traces point to point to find the breaks. @robmack created some OEM schematic diagrams that were useful here, see first link in the references below.

Even if I had found this issue myself it was probably beyond my current soldering and electrical DIY skills. Someone who had all the right gear helped me repair and test.

I'm sure my deoxit cleaning won't go to complete waste.

Thanks to @daveyson for giving me some hints to troubleshoot
Thanks to @charlie99 for pointing me to check the amplifier components which lead to seeing bad traces and capacitor
Thanks to @robmack for creating some schematics that we used to help track down the problem

Some reference material that I used:
https://www.k100-forum.com/t13127-reference-bmw-instrument-cluster-schematics
https://ibmwr.org/index.php/1995/03/08/fixing-a-flakey-speedometer/
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/speedocal/speedocal.htm
https://www.k100-forum.com/t18481-k75-speedometer-not-working
https://ibmwr.org/original/ktech/speedo-cal3.shtml
http://velobanjogent.blogspot.com/2011/08/k100bmw-motometer-instruments-german.html?m=1
https://servimg.com/view/19788995/12
https://forums.bmwmoa.org/showthread.php?88549-89-K100RS-Speedo-quit

Onto the next maintenance item. Thanks all.

A good result ......in the end

I have finally got around to building an instrument test box so it plugs in and can check all the inputs and outputs.

The remaining thing to do is the rev counter RPM input from the coil. I'm guessing this triggers on the back EMF from the coil and is not as simple as a 0-12v pulse.

Before I dos so, has anyone out there built atest/simulator circuit to make the rev counter go on the bench?

TIA, I'll post all the details of what I've made when I'm done.