Hi all, still working on the '90 K75S instrument pod issues, i.e. the speedo doesn't work and the Gear Position Indicator(GPI) is malfunctioning. And would like to ask for suggestions on debugging where the fault lies. I recently took the fairing side panels off so I could get to the instrument pod. Not the easiest thing. And have tried swapping the instrument pod for another and swapped the speedo sensor for another. Neither swap affected the results. One suspicion, It could be the second instrument pod is faulty.
1. Speedo, is there a way of ringing out  i.e. testing that the cables are not the issue? for instance testing that the lines leading into the instrument pod are actually working. Which pins are the two lines coming from the speedo sensor? If they are not the fault then I'm left with trying to repair the inst. pod or at least following the signal path through it.
2. The GPI, works with one inst. pod but shows messed up numerals i.e. 4 shows as an "H", 2 has an extra segment showing, and 5 shows as a "6".  One thought is that it is the same segment in the display that is showing up extra. The other inst pod shows nothing and I assume that is the inst. pod itself failing.

Looking at the wiring Diagram perhaps I should be asking how do the pin numbers get assigned to the instrument pod sockets?

The GPI sensor might want a look see next time the swing arm comes off.

A couple of things which might help to pinpoint the speedo problem. Holding a soldering iron next to the speedo sensor could get the speedo to move. Does the odometer fail at the same time as the speedo? Does the indicator auto cancel feature work as it should?

daveyson wrote:The GPI sensor might want a look see next time the swing arm comes off.

A couple of things which might help to pinpoint the speedo problem. Holding a soldering iron next to the speedo sensor could get the speedo to move. Does the odometer fail at the same time as the speedo? Does the indicator auto cancel feature work as it should?

I took the GPI off once and repotted it. It now works sort of. I've been gifted another one but would like to check the current one out without having to pull the rear swing arm again.

The soldering iron trick. Is that a hot soldering iron and am I threatening the speedo sensor? Should the speedo sensor be pulled out of its normal hole?

Anyone know of any description of the instrument pod sockets and the numbering? I have one of the Prospero's Garage reproduction of the electrical diagram in color and enlarged and if I knew the numbering system should be able to answer the more simplistic questions.

Can you post that diagram? Most of the workshop manuals have electrical diagrams. 

Give all the electrical connectors a good clean, especially the one near the seat between the sensor and the instrument cluster.

Just hold the electrical soldering iron near the sensor.

daveyson wrote:Can you post that diagram? Most of the workshop manuals have electrical diagrams. 

Give all the electrical connectors a good clean, especially the one near the seat between the sensor and the instrument cluster.

Just hold the electrical soldering iron near the sensor.

Not sure about posting the diagram. There might be legal issues I don't want to deal with just now. But acknowledged that I've always had issues reading the diagrams in Haynes or Clymer manuals. It is a very decent colorized version of the diagram blown up to 10.5" x 17". If you are interested they give the following on the diagram you can contact them at prospero@colorwiringdiagrams.com or see https://colorwiringdiagrams.com/ or https://colorwiringdiagrams.com/find-your-car/ols/categories/bmw-motorcycles?sortOption=descend_by_popularity   (Not free but fairly inexpensive.

The reason for wanting to figure out the numbering system for the instrument pod socket is that it would be very straight forward to check the resistance and continuity if I know which leads are which.

WOW. Bother enough people and you will get your answer. Bugged my local K75 expert, Lee F., and he sent me with a bright flashlight to look at the back of the instrument cluster. YEAH! And in very FINE but readable script the wiring numbers are printed in the plastic. Something I had never observed and wouldn't have seen without being told to look for them. Thanks LEE!

And for you guys as well.

The yellow wire from the speedo sensor goes to pin 22. The brown wire from the speedo sensor goes to pin 23

Here's a link to a recent thread with some tests on those pins, and some other tips to boot.

https://www.motobrick.com/index.php?topic=15651.msg138918#msg138918

jjefferies wrote:
Anyone know of any description of the instrument pod sockets and the numbering? I have one of the Prospero's Garage reproduction of the electrical diagram in color and enlarged and if I knew the numbering system should be able to answer the more simplistic questions.

I posted schematics for most of the modules in the K-bike: https://www.k100-forum.com/t13127-reference-bmw-instrument-cluster-schematics. You might find what you are after in that thread.

To ring out the speed sensor, you'll need an oscilloscope.  You're working with alternating signals of very low amplitude, about 2 volts peak to peak at frequencies between 0.5Hz and 40Hz depending on the rotational speed of the rear wheel. That kind of signal is very difficult to measure using a normal digital multimeter. Because the signal is so small, any amount of corrosion on connectors will impede the signal getting to the speedo board inside the cluster. You have connectors under the right battery cover,  a big OEM cluster connector attaching the harness to the gauge cluster and pin sockets inside the housing that could get corroded.  Check and clen those contacts.

The speed sensor is nothing more than a coil of wire wound around a metal core.  It is called a Variable Reluctance (VR) sensor.  It gets a current induced in it by a toothed wheel rotating in the rear housing.  It is kind of like a modern hall effect sensor (HES) but the signal is only 1.5V - 2V peak-to-peak rather than +12V like a HES.