I have a 1983 K100 (very early model) and it has an instrument cluster with the horizontal pin stripes on it, so from a later bike.
The neutral light stays on all the time and the gear indicator does not indicate anything, it is blank.  It has been like that since I got the bike 5 years ago.
The gear indicator switch was replaced by the previous owner and when I check continuity between the pins at the connector behind the battery, it does change when I change gears so I'm hoping the switch is OK.
Are the clusters from earlier and later K100's fully compatible?
Thanks
Jan

They are compatible. There could be a problem with the low fuel light/s. With the earlier instruments there were two warning lights..5 litres and 7 litres left in the tank. The newer instruments have one warning light at 7 litres. Because the tanks are different and the fuel light sending units are different you may have the low fuel light on in the later instruments mounted on the earlier tank. The only way around it is to remove the warning bulb in the instruments.
Gear position indicator may not be working because of an issue with the sender unit or wiring along the way.

If you want to have fuel lights change the PCb (electronic board) in newer instrument with one from early instruments. Newer instruments have one red light and it will signal at 7 ltr in tank. Its simple operation. I did it when changed instrument cluster for newer one.
Best regards

Maybe something in this thread could be of use

http://www.motobrick.com/index.php/topic,6941.0.html

The later single low fuel light comes on at 5 liters, not 7.  From the rider manual:



Here's how to diagnose if it's the GPI switch or circuit board is bad:
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/gpi/gpi.htm

Aside from the pin shape the only wiring differences are:

83-85: Pins 7 & 8 are for the low fuel lamps

86-on: Pin 7 is low fuel. Pin 8 is ABS.(only on ABS-equipped bikes)

Thanks for all this info, especially Duck.  That link is exactly what I was looking for.

Jan