Got a curious case here of the speedometer on my 89 K100 deciding to suddenly stop working out of the blue, the clock and tachometer are unaffected.
Guessing it's either a failure of the three-pronged plug or the speedo motor in the hub but not sure how to tell without removing the unit (and the RS fairing by extension) to tell, is the odometer/trip counter connected to the speedo motor as well or could I use that to gauge if it's getting a solid connection?

First, remove the rear wheel sensor, which is the signal generator for the speedometre, in the final drive, carefully, and clean off the pickup. 

Mind the delicate o-ring. The sensors do get scummy sitting back there in that hypoid oil with nothing else to do.

Changed out the oil (probably decades old at this point) and cleaned the plug, to be honest the pickup looked completely fine, light oil film but no debris or corrosion.
It is once again working but I'm skeptical the sensor was the cause; guess I'll see in the upcoming days.
In the process I've discovered an issue with the rear brakes as well, guess I'll post that in another section.
Thanks for the assist!

Follow the two wires up to a connector on the right hand side frame that disappears under the back right edge of the fuel tank. See if it's disconnected. Beyond that, the speedo head might need to come apart to check connections, and those wonderful little soft rubber gears that go kaput.

Just a thought. ( I know ...dangerous).

Sometimes the speedo needle warps a bit and the needle can stick on the instrument glass. I had this problem a few times. If you give the glass a tap when it is not working and it suddenly works,  it might be worth a look inside. As TWB mentioned continuity at the connector plug below the tank is also a common culprit.

Hope you get to the bottom of it.

Cheers
Mick

Well, it was working fine all last week and then on the last mile stretch of a 200-mile trip today it stopped working again right as I crossed over a bridge into town, wasn't even bumpy or anything; curious.  Guess I'll check it out again tomorrow when I have daylight and the luggage racks off.

This bike is quickly turning into a headache like my other two, I measured the resistance for the speedo sensor and it's at 330ohms, about 100 more than it should be.
Worse than that though the starter clutch decided to give out instantaneously and with no prior warning while running on an engine full of rotella t4 15w-40 diesel oil; ya know, the stuff that's explicitly recommended to clean and prevent that from happening.
I can hear the starter motor spinning; the tachometer revs up to 3000 but there's no engagement, I can get the bike bump started in second gear and away it goes but still no engagement after shutting it off and trying again with the button.
I'm convinced me shutting off the bike while it was idling in second on the center stand to test the resistance of the speedo sensor has something to do with this, it makes no sense otherwise.

EDIT: Bumpstarted it, got it warmed up and rode it around 20 miles, still won't engage, tried to jimmy it back and forth in 5th, still nothing.  It's alternating between two states the 1st and most common is where I can hear the motor spin up, see it hit 3k on the tach and nothing, the other is it gets up to about 1000, I can hear the fuel pump start to prime and then the starter lets off, the fuel pump then dies and the motor restarts and this process repeats.  This tends to only happen maybe 1/10 times when trying to start it.  On the other hand, the speedometer is working again, and I did nothing to cause that to happen, so I still don't know what the issue is there either.

Yote wrote:I'm convinced me shutting off the bike while it was idling in second on the center stand to test the resistance of the speedo sensor has something to do with this, it makes no sense otherwise.

If the bike's idling then the starter sprag isn't engaged so that shouldn't impact it.

Another possible point of failure for the speedo is the three pin connector inside the cluster:

Link: https://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/flakyspeedo/flakyspeedo.htm

Thanks duck, I think checking the connection in the instrument cluster is the next step for me regarding the speedometer's flakiness. I noticed riding in the dark last week that the tach backlight is burnt out, so it needs to come out regardless for a new bulb, would you happen to know what type of bulbs these use for the main backlights?

I am for the moment chalking up my starting issue to a nearly dead battery causing something funky with the l-jetronic.  I took it in to have it tested and it was only measuring 59 amps out of 400 while at full voltage; that was after spending all night on a battery tender so presumably its sulfated.  I'm pulling a smaller 200cca battery from one of my other bikes right now and am going to see if it gives me a different result.  It seems strange to me that the starter motor was definitely turning and not engaging on what I am to believe is a purely mechanical clutch engagement system, but I can't argue with the fact that the battery is certainly part of the problem.

UPDATE: Yeah, it was the battery causing the starting issue, I've never had a starter motor have enough juice to spin freely but not engage the motor before, I guess that's owing to the unique clutch design on this bike.  I'm tempted to buy another $35 210cca battery and build a housing for it since it's clearly enough to start it with no issue and I run a tender.  A part of me however is inclined to spend the extra to see if a fully juiced battery will cause my code 7 issue with the abs to go away now that I know the battery has been barely hanging on these past months.

I am having a hard time understanding how the starter not engaging is making the tach read 3000rpm when you try to start.  Never seen that before.

Yote wrote:I've never had a starter motor have enough juice to spin freely but not engage the motor before, I guess that's owing to the unique clutch design on this bike.  

That IS odd. I've owed lots of Ks and run down more batteries than I care to admit and never had that happen.

I'm gunna chime in and hope not to muddy the waters too much. Yote, a known gremlin with K bricks is as the bike ages, the starter motor brushes wear (easily replaced) and the dust accumulates, causing all manner of strange electrical gremlins, since the system is grounded through the starter. It's been posted and commented on in this and other K forums ad nauseum. I'm not suggesting this is why your speedometre isn't functioning as you'd like, but it's a good thing to look at. On my classic bricks, over the years, I would call it a maintenance item; something that should be looked at at least every 20,000 to 30,000 miles or so.

duck wrote:

Yote wrote:I've never had a starter motor have enough juice to spin freely but not engage the motor before, I guess that's owing to the unique clutch design on this bike.  

That IS odd. I've owed lots of Ks and run down more batteries than I care to admit and never had that happen.

I know how it sounds, that's why my first inclination was the starter clutch was bad, but I can confirm the starter motor was spinning and the tach was rising as the starter spun up to speed with zero engagement for the main motor.
Finally got the new battery delivered, charged, and installed.  Started right up, I'll try to clear the ABS code again tomorrow and see if the new battery keeps it from reoccurring.
This bike has almost 56k miles so cleaning the starter motor is probably good advice although it will have to wait a few weeks (along with seeing if the final drive splines are on the way out, or if they've even ever been greased).

56K is low miles for a starter to be dirty.

Just my personal (lazy) preference but I'd swap in a lower mile starter before doing I'd bother with taking apart a starter and cleaning it. You can find starter motors for "nothing" on Ebay. Example: Here's a 28K starter for only $16.56 shipped.

https://www.ebay.com/itm/364874366804

(I have no affiliation with the seller.)

I want to know how a starter that is not engaged with the engine can make the tach read 3000rpm.

Point-Seven-five wrote:I want to know how a starter that is not engaged with the engine can make the tach read 3000rpm.

Indeed. That's very odd since the tach signal comes from the ICU wire to the 1/4 coil.

Guess: It could be caused by the weird grounding through the starter.

My view is that the problem is not electrical, because the starter motor spins.
Look for a mechanical cause, such as a faulty sprag clutch.
These things are a known sore point on many motorbikes.

Spose the battery has over 12 Volts but low on Amps, that can sometimes make the tacho chuck a wobbly. 

An oiled sprag clutch with smooth steel surfaces want's to snap into engagement pretty quickly in order to engage, even if it's cranking at normal speed (when not just spinning) 

Modern oils typically have low friction additives and often high shear strength, that's almost always good, but makes it harder for the clutch to engage.

Low Amps with a lazy clutch engagement, combined with modern oils, then it might just spin instead of engage.

I don't think it's been mentioned yet if the odometer and trip meter also stop working when the Speedo stops.

I posted earlier it was the battery, I had a smaller one on hand that had ~200cca's and it started it just fine.  I got the new 400cca battery in yesterday, started up right away, also allowed me the clear the code 7 fault from the abs and it to stay clear, so now I have a working ABS as well.

I'm guessing the starter clutch needs a quick initial burst of inertia to catch and a weak battery won't do it even if it spools it up regardless.

I don't see why bother replacing the starter motor when you can just disassemble and blow it out with compressed air, this bikes FI so it's not like anyone's been cranking the starter for 30 seconds at a time to get float bowels filled up constantly.  Either way battery was the issue here regardless

A lazy way that can clean the starter, roll it backwards in gear with the clutch in, then release the clutch.

daveyson wrote:
An oiled sprag clutch with smooth steel surfaces want's to snap into engagement pretty quickly in order to engage, even if it's cranking at normal speed (when not just spinning) 

I suppose that might be true, but how come I can just get a sprag clutch to grab at near-zero revs, in my hands?

Dunno if the sprag in your hands is oily. Your hands don't need much torque, but to crank a brick engine over, you need a lot of power to do that, so that needs a stronger grip.

The tacho (and Speedo) gets earth from the battery through the frame, so I don't think it should make a difference as to the condition of the starter.