No-start/run diagnosis. Solved

[erik325i]

I hope this diagnosis helps someone in the future, so I’ll post up my story.

Backstory: The bike is a ‘85 K100. I’ve had it for a couple years. Since I’ve had it, it would always start on the second attempt. The first attempt to crank, would always only crank for half a second then stop. Since it would fire up the second attempt, I just accepted it.

Fast forward to a month ago. I’m riding in to work one morning, and the engine dies on me. I try to restart it, and the starter only cranks for half a second, then stops for a second, then cranks for half a second, then stops, etc as I hold the starter button down. Can’t get it to start.
So at this point, I have a strange intermittent crank, and a no-run problem.

*remove/inspect and bench test starter. Operates fine. The power supplied to the starter is cutting in/out causing the weird starter activity.
*trace the power up to the starter relay. Gets a good constant power supply from battery. I check the power signal wire to the relay from the starter switch. It gets a good signal when I hold the starter button down. Something is causing the relay to click on/off, so finally I test the ground. The ground is switching on and off. I use a jumper wire on that terminal of the relay and jump it to ground, and now get a solid crank. (Still won’t run though).
*I analyze the wiring diagram and see that the starter relay gets its ground from the Ignition Control Unit (ICU). Now we’re getting somewhere! Finally something that could cause both problems; no crank, and no run.
*I buy a used ICU and toss it in. No change... Something else is telling the ICU to cut out.
*I look at the wiring diagrams again and also the flowchart Johnny suggested I check out. I see one of the only other imputs to the ICU is the Hall effect sensor.
http://www.motobrick.com/index.php?action=dlattach;topic=5067.0;attach=6480
*I buy a new Hall effect sensor, and install it. Cranks strong now!
*Reinstall the tank, and it starts up and runs!

Hopefully that helps someone. I’ve seen a few other threads with a similar on/off starter symptoms, but none got a difinitive solution.

[Laitch]

I’ve seen a few other threads with a similar on/off starter symptoms, but none got a difinitive solution.

You did not find and read through enough of them. This problem has often surfaced here and been successfully diagnosed and treated. The causes vary.

The flowchart you consulted is useful if carefully followed as you have done, and often can prevent the purchase of unnecessary parts. Thanks for posting your detailed description of how you found the remedy.
 :clap:

[The Mighty Gryphon]

Thanks for the detailed follow up.  Those Hall Effect Sensors are a known weak point, but normally don't give problems when the engine is cold. 

Since they're critical to making the engine run and difficult to get, when I travel I carry a complete plate with sensors because you never know when they're going to crap out.