Hoping some of the electrical experts here can help a newbie out.

I'm trying to understand the wiring diagram for my '93 K75. At the moment I'm looking at the Ignition Control Unit (ICU). I've mostly got it figured out, but there are three wires (black, orange and red) that lead to a "solenoid barrier" (on the K100 diagram, it's "ignition trigger assemblies"). What is the solenoid barrier, and what do these wires do? There's also a brown wire, but I assume that's ground, correct?





For context, the bike is in pieces, and the wiring harness ripped out, so unfortunately I can't just trace the wires visually, and all my reference photos are unclear on this part of the diagram.

It's ignition trigger assemblies the two hall sensors are at 120° for #1 and #3 then the ignition puts the pulse for #2 right in the middle so you get 3 cylinders firing.

The ignition sensors are Hall Effect Sensor (HES) devices, semiconductors that can sense the proximity of metal.  As the engine spins,  a shuttered cup reveals a gap which the HES detects and produces a voltage spike that is registered in the ICU.  The timing of the pulse communicates the position of the crankshaft in its rotation. The ICU needs a crank position indication for each cylinder.  There are two sensors on the K75 positioned at 120° to each other.  The ICU gets direct timing information from the sensors for two of the three 120° positions it needs to know.  It calculates the third position.

Anyway, the wires are:
- Red = HES power source
- Black = power return
- Brown = HES output #1
- Orange = HES output #2

This is the only time on the K-bike where Brown does not mean Ground; it's an actual signal.  There is also a braided shield that surrounds the cable assembly.  This is grounded at the ICU end of the cable and is very important to protect the HES' sensitive electrical signals from electrical noise that could mess up timing.

robmack wrote:The ignition sensors are Hall Effect Sensor (HES) devices, semiconductors that can sense the proximity of metal.  As the engine spins,  a shuttered cup reveals a gap which the HES detects and produces a voltage spike that is registered in the ICU.  The timing of the pulse communicates the position of the crankshaft in its rotation. The ICU needs a crank position indication for each cylinder.  There are two sensors on the K75 positioned at 120° to each other.  The ICU gets direct timing information from the sensors for two of the three 120° positions it needs to know.  It calculates the third position.

Anyway, the wires are:
- Red = HES power source
- Black = power return
- Brown = HES output #1
- Orange = HES output #2

This is the only time on the K-bike where Brown does not mean Ground; it's an actual signal.  There is also a braided shield that surrounds the cable assembly.  This is grounded at the ICU end of the cable and is very important to protect the HES' sensitive electrical signals from electrical noise that could mess up timing.

Thanks robmack, this is super useful information!

One more question: The Brown/Red wire from the ICU looks like some sort of switched ground, but ultimately just connects to the black/yellow wire through the starter relay. Is it safe to just run both ends of this wire (on the ICU and the relay) straight to ground? Or have I completely misunderstood this wire? 

If this isn't a ground of some kind, what about connecting it straight to the black/yellow wire?

Brown/Red is a switched ground wire.   It is connected to one side of the starter relay coil.  Inside of the ICU, it runs through a transistor to ground.  When the start switch is pressed, the ICU knows it through the Black/Yellow wire and internally connects this Brown/Red wire to ground through this transistor.  As the starter turns the engine over relatively slowly, the Brown/Red remains connected to ground through this transistor (the ICU keeps the connection while the RPMs are below 700).  When the engine fires up, the engine RPMs rise to about 900. The ICU knows how fast the crank is rotating via the HES so it disconnects the Brown/Red wire from ground by turning off  the transistor .  Now, if the driver tries to press the start button, the starter relay will not engage and the starter motor will not turn.

It is a safety feature of K-bikes and really shouldn't be bypassed or messed with.

Makes sense. Thanks!