Hey everyone, first post!
You guys are an amazing wealth of info...been stalking this site for months and picked up so much good info.

Okay, here is the issue. 87'K100 Basket case bike- Bike was put down and never started again about 5 years ago. Original owner pulled ALL the wiring thinking it would be something simple...it wasn't.

I have torn this bike down to its guts and rebuilt from the ground up including most the wiring. Turning it into a more scrambler style bike now.

Here is the issue. 5 years after nothing but sitting- It turns over and "almost" runs...meaning it runs if I hold down the starter button...it roughly revs but not a clean rev and seems to want to run...but, I let go of the starter and it is game over...just shuts down. 

Has to be a short somewhere but I have no idea where! I keep thinking in the left hand controls but where?

To be honest, I have removed the clutch override (victim of first owner) but that is about the only change from original OEM electronics. 

Any thoughts? 

Thanks everyone!

Most likely thing is the temperature sensor on the water riser just behind the radiator on the left side.
If it is short circuit the computer thinks the engine is hot and gives a lean mixture but when the starter button id down the mix is enriched to help starting so if the engine is cold it will start and needs a richer mixture to keep going till it's warm. The choke does not enrich the mixture as one could think but is only a fast idle to help it warm up.
Make sure you get the correct sensor if you buy one as the 16v engines have a similar looking but different type.
Don't measure resistance across the 2 terminals but from each terminal to the body of the sensor. There are 2 sensors in the one unit.
You can also measure from pin 10 on the EC plug to ground.

Also check the connection to the computer under the seat. Had a K75 do same thing recently and the problem was that multi connector. Disconnect and reconnect it and have a look at the pins on the computer.

Okay, checked temp issue....Not it...

Will try the CPU connect and clean tonight.
It is odd that when the starter is pushed it cranks right up and runs as long as the stater is pressed...but the second I let go it dies.

Jim P- 87' K100RT wrote:Okay, checked temp issue....Not it...

Will try the CPU connect and clean tonight.
It is odd that when the starter is pushed it cranks right up and runs as long as the stater is pressed...but the second I let go it dies.

The starter button also causes the fuel pump to pressurise the fuel injection system. But once the engine is running this is no longer the case. After the engine starts and runs the injection system fuel pump gets its signal from a different location, and a different pin into the ECU, from the Hall Sensor [if I am correct]. If this second signal is not present then the engine will cut out as soon as you let go of the starter button.

What you do now know is that your fuel pump is working and the engine fires. A bad connection at the ECU will give the same effect as a bad sensor, it cant tell if its a connection or a sensor.

At the age the Ks have achieved connections are the more common bugbear. You have to be methodical because some of the sensors do go funny but there are lots of posts. Once you have this sorted you should be good for a long time.

Hey Jim,


I believe that when you depress the starter button, there is a change to the richness of the fuel delivery. This maybe worth taking into account in your diagnosis. Good luck with it.

Jez

Do the lights on the instruments go out when the engine stops?

It is highly unlikely that the hall sensor is the problem. They either don't work at all or stop when the engine gets to operating temp.
Don't go and throw money at expensive electrical components the solution is very often a simple fix, you just need to find it. I have seen so many do this and it usually ends up with a no go and empty pockets.
Check the kill switch and start button they are not the toughest thing that BMW put on a K and UV tends to make them brittle.
It is very possible that the start button is supplying the power for the ECU and ICU when it is pressed and not the kill switch.
If you need a workshop manual with good circuits them PM me or there is an interactive wiring diagram in the portal page.

In the troubleshooting guide at the portal I think it is a "how to" test the hall sensors.

When the Hall sensor fails it usually does so after the engine has warmed up, not at starting. I should have said that. Its far more likely that there is a connection problem. There are other things to check out before going near the Hall sensor. Even then the Hall sensor seems to be quite reliable, lots on the forum on how to deal with it but it hasn't affected very many of us. We have quite a few Ks over here in Ireland and they all seem quite reliable. Our climate is not one for having mint Ks simply because we can use them all year rather than park them up and shine them.

The forum technical side has a lot of stuff up there because at this stage it covers almost everything so be careful you don't get the feeling that every K gets all these problems- they don't. When you do meet a problem, the forum usually has the answer.

Keep your wallet in your pocket for now, time and method are your tools along with lots of good advice from here.

Take up the advice on the wiring diagrams, along the way check all the connections and particularly the earths and the earth under the petrol tank.

I don't mean to spoil the party, gents, but the topics have been merged. No real need for duplication.

Jim P- 87' K100RT wrote:Lights are not affected in anyway during this process. They come on when I turn the key and function fine throughout the process.

This strikes me as odd. I'd expect the load shed relay to do its job (kill the lights while cranking), but your statement gives me the impression that it is not functioning. That points to wiring error in my book.

Jim P- 87' K100RT wrote:I have torn this bike down to its guts and rebuilt from the ground up including most the wiring.

Did you redo a lot of the harness wiring during your rebuild?

Uh, OH!!!
Yes I did...I took it down to nothing and started over...

I just checked...the light is on with the key...shuts off when I press the starter. Is that right?

Yes, that's right.

Disconnect the ECU and ICU then press the start button with the key on and see if the power gets to either or both with the start button pressed and not down that will tell you if the problem is actually in the wiring or the computers

Okay....AAAARRRRRRGGGGGGGG!!!!

Checked pins, all is good. 
Took apart the starter button and cleaned points and scrubbed.
Checked all grounds...cleaned and good.

ECU disconnected and checked power...all good.
ICU (which is were I am headed!!) was good an showed no issues.

Put it all back after checking...same deal...hold the starter in an it revs and runs like a champ...let go of starter...just like hitting the kill switch...I mean ...INSTANT off.

Do you have the chance to substitute parts like the ECU and ICU?  It might be one of these acting up; if so, I'd bet on the ICU.

Check that when you turn the kill switch on you get power to the coils at terminal 15 should be a green/yellow wire to both coils.
If you don't get power try it by pressing the start button and see if you get power.

This thread has been dead for awhile but I'm hoping there was a solution as I'm experiencing  the exact same problem.  Bike fires right up and revs smooth as long as I hold starter button down.  When I let off starter button, the motor promptly quits.  I'm in the process of moving about 45 miles away and my K is the last of my belongings not moved yet.  I don't want to trailer it and am contemplating taping the starter button down so I can get her to my new garage.  Would that be unwise?

You could hold the button down by taping it, I would be sure to have ready access to the battery terminals just in case other things becomes evident by the smoke  escaping.
There was no resolution to this thread, a common problem but re-reading the thread makes me believe the ICU was the problem or the connections to it.

Update-problem was solved and it wasn't electrical at all.  It was rather simple and left me feeling a bit simple in the head.  I had just replaced fuel pump, vibe damper, plastic collar thingy, fuel filter and all new lines in tank due to the bike sitting for a couple of years with fuel in tank.  Damper had turned to goo and gummed up the pump.  I bought an aftermarket bosch fuel pump (https://www.amazon.com/gp/product/B000BZGGXO/ref=oh_aui_detailpage_o08_s01?ie=UTF8&psc=1) 
of smaller diameter than the stock unit and a foam adapter sleeve from Euromoto Electric (http://www.euromotoelectrics.com/mobile/Product.aspx?ProductCode=FP-576FOAM ) as well as the filter,damper and other related goodies.  Well, included in the order were two flexible in tank fuel lines to use on either end of the filter and they  seemed a good bargain considering the cost of traditional submersible fuel line.
http://www.euromotoelectrics.com/mobile/Product.aspx?ProductCode=FP-HoseKit
They are hard plastic w/o much elasticity.  The problem was that no matter how hard I tightened the house clamp, the rigid line was not sealing completely around the metal line in the tank that leads to the tank outlet.  When the starter button was depressed the pump was supplying enough pressure so the bike would fire right up but when the button was released the fuel pressure was escaping through the poor connection.  The turbulence of gasoline around this joint was visible when I finally took the time to look.  I was too busy assuming the worst and guessing my problem was electrical in nature.  Replacing the hard plastic line with flexible submersible hose solved the issue.  The hard plastic line is working fine for pump to filter connection.  I think the metal line in the tank is just a wee bit smaller than the other connections in tank.  The K is home now and in a garage at last.  I'm looking forward to getting my hands dirty and making some repairs/improvements.
I just wanted to pass this along in case it helps some other K owners who, like me, may temporarily abandon common sense and miss the forest for the trees.

That was a very informative post ad very well worded. Never twigged the fuel pressure was higher due to the absence of the effect of the fuel pressure regulator when the starter button is pressed.

Another one for us to watch out for.

"When the starter button was depressed the pump was supplying enough pressure so the bike would fire right up but when the button was released the fuel pressure was escaping through the poor connection."
I don't understand your reasoning behind this as the pump runs all the time the engine is running.
I would have said it likely that the enrichment function when the start button is pressed was making up and supplying enough fuel for the engine to start till the button was released and then the fuel leans out due to low pressure and the engine stalls.

Rick, 
Eric, over at Tuckurimba (of MZ fame ) bought a K100 and couldn't get it to run properly. Clouds of black smoke as if the thing was running just far too rich. Baffled for weeks until he took the starter button assy off and let it hang. Bingo - bike ran fine. Little thingy inside was jamming on and richening mixture (OD ing on too much fuel).
Told him that getting on  forum was the easy way to get the answer by way of collective wisdom but no.......

That's one way of getting a K to sound like an MZ!! Wheee putttt puttttt.

Thanks Rick,
Indeed, my reasoning was flawed and your explanation hit it on the head.  Thanks for setting the record straight.  I've learned something again.  My only technical experience comes from air cooled VW's and an old '65 Vespa.  I've got a lot to learn with this bike.

Werewasi wrote:

Clouds of black smoke as if the thing was running just far too rich. Baffled for weeks until he took the starter button assy off and let it hang. Bingo - bike ran fine. Little thingy inside was jamming on and richening mixture (OD ing on too much fuel).

Well I'll be blowed if this forum doesn't just help me out daily. I commuted yesterday in heavy heavy rain 
(the sort that gets the inside of your helmet visor wet).
I went to leave work in the arvo (K parked under cover) and it ran ran like a drunk with a club foot! Black smoke toboot. I had developed the same issue as werewasi's mate through a wet starter switch. A quick blow with a heat gun got her going and sounding like a sewing machine again.

Thanks K forum.

Jez