K75 Starter randomly engages

[NC]

1990 K75 Starter randomly engages.
Issue:
The starter will engage at any given time (when trying to start it or driving down the road).
Sometimes I can start it a 100+ times (quick on/off) with no problems. Sitting for long time (weeks) with no problem, other times immediately.
Driving down the road & sometimes it will just engage the starter.

Replaced:
- New known good battery (same thing with old battery that tested good).
- 2 new starter relays.
- modified a 3rd relay to have more snap on the spring when it disengages.
- checked starter.
- disconnected & jumped both the clutch switch and the side stand switch.
- Battery voltage off = 12.6
- Battery voltage key on engine off = 12
- Battery voltage at idle = 12
- Battery voltage at 1500 RPM = 13.1
- Battery voltage at 2000 RPM = 13.7

Starting to think it maybe a faulty alternator.

Thanks for the help.

[pdg]

Have you checked the start button on the handlebars? To be honest, that's where I would've started looking.

Whatever the fault with the alternator I fail to see how it can engage the starter... Even if your system voltage goes down to 3V or up to 30V it's not going to do anything (well, not with the starter as you describe, it'll probably bugger up other things).

The 'thing' with low battery voltage actually welds the solenoid contacts shut so the starter cannot disengage - I'm not aware of it randomly engaging the motor.

[NC]

I agree that the alt should not be doing it, but I am starting to think that if it is not charging fully, ..... but that would not cause it to do it driving down the road.

I'll check the button & se if I can clean it.
Thanks

[K75RT Keith]

It might be possible that the sprag gear is starting to stick on instead of off. 

Did you clean the starter and replace the brushes?

[Zampano]

If the relay is good I second the starter button. Especially considering how the starter motor is sporadically running. Does it also stop randomly or do you need to shut it down? Could you have a worn wire; possibly grounding itself as you ride and triggering the relay? That's a real longshot, and until I have some coffee I have even less of a clue than normal.

These bikes start up so easily I would gladly trade the starter button, load shed relay, starter relay, starter motor, feet of connections to the headlight, battery, "control units," etc etc for a piece of steel and cog. And my foot.

Now I'm curious to add up the cost to fill a shoebox with everything related to the electric start. Dealer prices would be good for a laugh.

good luck. My money is still on the button.


p/s
I forgot to mention the motor itself but second what RT Keith posted. I replaced my brushes when I had issues but think my problem was the relay. I am a somewhat ignorant of the inner workings of the starter. I'm not sure if it can fire itself up without the juice from the relay.

[Mongrel]

I've always wondered just how heavy of a modification you'd have to make to install a kick-start, or if it's even possible given the engine & tranny layout.

[TimTyler]

> disconnected & jumped both the clutch switch and the side stand switch.

K75's don't have side-stand switches.

I think that, if you really wanted to, you could remove the starter from the electronic start procedure and wire it directly to the battery with a heavy-duty on/off switch. You'd still need to press the green start button to get the bike going, but at least your starter would not run without you wanting it to.

Or you could just insert a breaker into the thick 12v starter cable and flip it OFF once the bike was running.  :dunno

[pdg]

I am a somewhat ignorant of the inner workings of the starter. I'm not sure if it can fire itself up without the juice from the relay.

No, it can't.

There has to be a voltage present on the wire to the starter for it to operate. To get a voltage on that wire it has to pass through the solenoid/relay (nomenclature may vary with region). For this voltage to get through, the solenoid contacts must be closed.

There are two ways to close these contacts - present a sufficient voltage to the solenoid winding for it to operate - or - a fault that allows the contacts to 'wobble'.

With the 75, pressing the starter button with the engine running shouldn't operate the starter, so I am going to revise my previous comment. ECU or wiring...

I will however stick by my assertion that it's not the alternator or the battery voltage causing it.

[pallum]

Does your headlight turn off when the starter runs every time? My starter button broke quite some time ago and will randomly spin at times as well-- only when the RPM goes under 770 (like abruptly pulling in the clutch after accelerating) as per the starter safety lockout. If the starter is spinning like you say it is but the headlight doesn't go out from the load shed relay that might point to some other problem. Otherwise, it's annoying to know it's "broken," but not going to really hurt anything.

[rbm]

The start button is not "operational" unless the clutch lever is pulled in or the bike is in neutral.  Furthermore, if the Hall Sensor tells the ICU that the engine is turning over faster than 700 RPM, the ICU will not enable the start circuit. In fact, ground is provided thru the ignition control unit and on thru the Hall sensor during startup.  That circuit design exhibits weirdness such as an intermittent HES problem causing on-off-on-off starter operation with the starter button held down, for example.

So, even if you press the start button while pulling in the clutch or while the bike is in neutral with the engine running, the starter motor will do nothing because the start circuit is disabled by the ICU.

In the OP's case, something is failing that causes voltage to appear at the start relay while the engine is running.   Constant power is wired to Pin 30 of the starter relay and if any fault in the trigger circuit caused the starter relay to engage, the starter motor would try and turn.  I would hazard a guess that the ICU or possibly the Hall Sensor is acting up.