New member here with a recent K100RS acquisition.  The bike had been dormant for a few years and I like the challenge of bringing things back to life. Thanks to the excellent posts on this topic from a variety of people, I reached the conclusion the Ignition Control unit is not providing the necessary ground to the starter solenoid coil.  Clutch switch, neutral light and starter switch all working fine.  If I supply a ground to the red with green wire at the starter solenoid, everything works as it should, the bike fires right up and runs well. 
My question is, by installing a permanent ground to the solenoid, will that be bypassing the safeties within the system like no crank while running etc?  Can the Ignition Control unit be repaired?  Thanks and looking forward to the K adventure.

I don't understand, you should not supply earth to red/green wires, they are positive, and there are no wires like that at the starter relay (solenoid)

Do you mean there is no earth at the fuel relay? because that has red/green wires (and five pins) You do not want to supply a permanent earth to this relay, as you say its a safety feature. In case of a crash, if the computer detects that the crankshaft isn't turning, it denies earth to the fuel relay so the fuel pump stops, you don't want fuel pumping out at a crash site.

Yes, the ignition control unit can be repaired, but more likely the electrical plug just needs a good clean with the help of deoxit. You want to check that the ignition control unit is getting all the required inputs before worrying about the unit itself, or have a close look at the yellow/brown earth wire, and its connections.

Well that is good way to confuse people, give the wrong colour code on a wire - my apologies.  It is the brown with red stripe to the starter solenoid (see pic) that if grounded, everything works fine. 

Identified as BRRT from the Ignition control unit to the starter solenoid on this schematic: http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/kwiring/K100/K100RS.RT.1986+.SLP(K100US_RS_85).V1.0.pdf

For whatever reason, the ignition control unit is not supplying ground as it should be.  At least there is a work around for now.

DavidKS wrote:
My question is, by installing a permanent ground to the solenoid, will that be bypassing the safeties within the system like no crank while running etc? 

I think your solution is a pretty good one, you don't have to worry about that, because even though it's only supposed to get earth while the start button is pressed (to pin 85) it also only gets power while the start button is pressed (to 86) I guess you have eliminated the earth wire and it's connections as the cause.

That problem often occurs with bricks though, for other reasons. That's why a lot of forum members, including myself, have installed a battery isolator switch.

Thanks for the input Daveyson, I will move forward with that plan.  Looking forward to riding it, it will be my first time on a brick.  And as the saying goes, "you always remember your first one"