First post so apologies if it's not in the right place.

I have a 1990 k100rs cafe racer project on the go and I've recently put everything back together after removing the abs. I have kept all of the original electrical components (relays, sensors etc) but I have shortened some parts of the loom where there was excess lengths of wire. Bike started up first time every time before I started the project. 

I've now got everything back together but can't seem to get it to start. It's turning the starter motor, pressurising the fuel rail, I can hear the fuel pump prime, relays click, neutral light, sidestand switch, all good. But doesn't fire. I've checked the injectors on the bench by manually applying 12v and they all spray perfectly. So I've then checked the injector plugs. When testing with a multimeter, I get 12v on one pin and earth on the other when they are ALL unplugged and the starter button is pressed. But when I plug any one of them in and check the other 3, I get 12v at both pins, no earth. Now I don't know if this is normal as I know they should pulse, or whether it points to an issue somewhere, maybe the ECU? I've pulled the fuel rail out with all the injectors and plugs still connected pressed the starter to see if the injectors spray, which they don't. So there's obviously an issue, but I'm not sure what it is. Any input from you brain boxes and k100 geniuses is much appreciated. Hopefully I've provided enough information, if not, let me know what you want me to check/look for/explain further. 

Ride safe

The injectors are fired by GROUNDING them.  One pin will have 12V all the time the ECU is powered up.  The other pin isn't connected to anything until the ECU grounds it through a switching transistor.

The reason you are seeing 12v on both pins is that the voltage is feeding through the injector you have plugged in and going to the ground wire.  Because the wire isn't being grounded to fire the injectors, the 12v is not being pulled down to ground.  Hence you are reading 12v.

It looks like your wiring is okay.  If the injectors aren't firing the problem may be in the ECU or the Hall Sensors that provide the signal for the injectors as well as the ignition.

Hope this makes sense.

Thanks for the response point-seven-five, that makes total sense. The question now of course is what would have caused either the ECU or hall sensor to go bad whilst the bike has been sitting (the ECU has been in storage). And how would I go about testing these for a fault? If I'm correct the hall sensor is on the front of the engine block below the radiator, behind an access panel? This has wires that come out and plug in to the main loom yes? I have opened the ECU casing to see if there is an obvious burnt out or damaged area inside, bit it is absolutely immaculate! I was speechless when I opened it and saw how clean it was, it looked new, not 30 years old! This isn't to say there's not an issue with it of course. I have also opened that access panel to the hall sensor to look for any obvious broken connections etc, but again, immaculate. I didn't touch anything on either, I just did a visual inspection for a glaringly obvious fault and then closed them back up. 

Ride safe

Check the output of the ECU to the injectors to make sure it is pulsing them. 

You will need a 12v LED.  An LED is the only way to test the ECU output because it's the only thing fast enough to respond the the very short pulses going to the injectors.  You can find them at most autoparts stores.  Connect it to the injector plug on the harness making sure to get the + of the LED to the correct terminal.  Crank the engine and watch for the LED to pulse.  Pulsing injectors indicate everything from the harness to the ECU to the Hall Sensors are okay.

If the injectors are getting pulses, and the engine won't start, I would seriously consider a new set of spark plugs.  Last winter, I had a bike that ran okay, but a little rich and put a bit of carbon on the plugs.  After sitting 4 months I couldn't get it to start even though I could see a spark when the plugs were out of the engine.  Turned out, there was enough carbon residue on the spark plug's insulator to bypass the spark under the compression of the cylinder.  New plugs solved the problem.

The FI ECU works as a slave controlled by the ignition ECU. Just one wire does this - yellow with red stripe that connects both ECUs together. Check this is present if you have altered the loom. It signals the FI ECU to fire the injectors.

The ignition ECU also has some control over the FI relay, which provides power to the whole FI system when energised. The ignition ECU controls the relay by switching its earth supply off if the engine stops running or over-revs. The wire that does this runs from the igntion ECU to the FI relay coil. The wire is yellow with a brown stripe. Check this is present and functioning - i.e. the FI relay clicks when the starter is pressed.

Edit: If you are getting 12v on the Green/Red wire to the injectors, with the starter pressed, then the FI relay is working. Green/Red is the wiring colour for the part of the circuit that is supplied with 12v from the FI relay, it feeds the FI ECU, the AFM, the TPS, and the injectors. The 'problem' circuit seems to be the Yellow/Grey circuit, which,as stated, is controlled by the FI ECU and should drop to ground to fire the injectors. Maybe check continuity on this circuit from the injector plugs back to the FI ECU plug?

Edit#2: Not sure if I've missed this - but you can easily check the Hall sensor by checking for a spark: Spark okay = Hall sensor okay.

Just one thing, the post mentions side stand switch and it's a 1990 RS so 16v engine?

No MAF on 16v engine.

In that case......ignore the entire post. It only applies to 8v Jetronic engines.

Or buy a 8v bike that won't start and fix that instead 

Just sayin'

Is the side stand switch working properly?  Is the stand down?  About every other time I try to start my 4v K100RS I forget that damn stand.

Ok, so I've done the led test this morning. The light flashes on and off once when I press the starter button in, and once when I release it. It does not flash repeatedly whilst the starter button is held in. The flash is in time with the fuel pump starting, when it primes the fuel rail, and when the fuel pump stops, once the fuel rail is pressurised. Is this good news or bad news? 

I have checked the sidestand switch with the multimeter and it appears to be fine. I bypassed it just incase there is a fault inside the switch that I'm not picking up, but that's made no difference. 

So if the led test shows theres no problem on the injector plugs where do I look next? If it shows there is a problem on the injector plugs, erm, where do I look next? 

Also, thanks for pointing out the misunderstanding in chris846 post, saved me looking through the loom for something that's on a different bike 

Appreciating the input so far, don't know why I didn't join this forum sooner! 

Ride safe

Oh, I'd just ignore Chris846 full stop...

Are the spark plugs wet or dry after you've tried starting the engine?

The LED should flash at least once per revolution of the crankshaft as the starter spins the engine to start it. 

That it only flashes once when you press the start button and when you release it sounds like you have a problem somewhere in the path from the Hall Sensors to the injectors.  Possibly in the ignition amplifier or the ECU.

I think your bike has a Motronic ECU which has some rudimentary diagnostic codes.  Have you tried reading them?

The spark plugs are dry, which isn't too much of a surprise after doing the led test. And how would I go about checking the diagnostics on the motronic ecu?
 Is there anything else I should be checking in the meantime? I've heard there are a lot of start conditions that rely on one another with these bikes, if one isn't met, you get no start. Ideally I would like to avoid buying parts just to see if they solve the issue. What's stumping me is how something could go wrong with the ECU/hall sensor whilst in storage. 
Thanks for the advice and help, keep it coming, even if it doesn't solve the issue it's useful to know and tick off the list 

Ride safe

Go to the "Portal" page.  Right above the graphic is a link to the "K100 Forum Tech Page".

Click on it and agree to the terms.  A large page of links will open up.  Go down to the lower left corner and you will see a catagory "Motronic".  Click on the link to "Motronic fault codes".

You might want to peruse the topics on the tech page.  This is the mother lode of K bike information.

Will do! I'll start reading! In the meantime, if anyone thinks of anything I might have been dumb enough to miss, let me know. It might be something stupidly obvious, but I'm sure we've all scratched our head looking at a job only to find it's a missing screw or a missed plug... No? Just me? 

Ride safe

"Oh, I'd just ignore Chris846 full stop... "


Been tryin' for years 

Did it run well after your surgery on the loom? I've lost the plot.

The surgery on the loom was minor. All I've done really is remove the abs which is a separate mini loom, and shorten wires that were too long after shortening the frame. I've not changed anything really. Just repositioned stuff to look cleaner for that cafe racer style. Although I have replaced the old "brotbox" instrument cluster with the bep 3.0 replacement that allows for digital instruments. But that's just a simple plug and play replacement

Tattooist Keith wrote:All I've done really is . . .

I guess that means before.

Sorry, didn't really answer that did I. No, it hasn't run since I've done the work on it. It's been an ongoing project around my day job, I've only just got it put back together to a point where I can test things. My plan was to put it back together after the major jobs were all done, fire it up, test, then dismantle and send for paint. Probably a bit naïve to think it would be that simple...

Bit of an update in response to point-seven-five's earlier advice. I've used the troubleshooting section for the motronic and found some things. Firstly, the flashing led fault codes didn't seem to work, I just kept getting a solid light, no flashes. I'm not sure if this is pointing towards a significant problem, or just due to the fact I've removed the abs and have the bep 3.0 clocks replacement. Either way, I followed the next set of trouble shooting tips, testing resistance in the pins in the big plug to the motronic. For the most part everything was well within perameters, but one issue came up with the co2 potentiometer. The advice was pretty simple, chuck it and get a new one as its knackered. So I guess I'll try that. I've also removed the loom again, checked all my connections and put it back on. Could probably do this with my eyes closed now. I know having made changes during the cafe racer conversion makes finding faults a bit trickier, it's possibly even considered sacrilege to some of you k bike purists, but I appreciate the help so far.

Ok, an update from my electrical nightmare. I've changed the c02 potentiometer, no change. So I've replaced the injector part of the loom with an untouched original injector loom to remove any issues that I might have caused with my adjustments. No luck. 

So my options that remain are: a) the ECU is kaput, b) there is a problem with the front half of the loom that I've made adjustments to, or c) there Is a problem with the hall sensor. 

So my gut is telling me the hall sensor is fine as I've not removed it or touched the wires that come from it at all. I've also been told numerous times that the motronic ecu is bulletproof, so that sort of makes me question that as a possible part of the problem. I could source a new untouched main loom to see if it's a problem caused by me and my primitive monkey brain, but I wanted to throw it out to the higher forms of intelligence that lurk in this forum. 

Please send help, I'm losing the will to live!! I've got a baby on the way, if this isn't done before he gets here, it will probably be sat for the next 10 years!!! 

Check on the wiring connections, I suspect that somewhere there is one that is not connected.

I've been through the loom at least 25 times. I've had it completely off the bike and back on again probably 15 times. I've cleaned every plug connector and checked every soldered point. I've been through the wiring diagram every time to make sure everything is plugged in and earthed where it should be. I can't find anything that might be causing an issue through loose or bad connection. I may have missed something, but if I've not found it in those last 25 times, I can't see it being an obvious thing. I will obviously go through it all again and check it against the wiring diagram again. I definitely won't give up.

Tattooist Keith wrote: I may have missed something, but if I've not found it in those last 25 times, I can't see it being an obvious thing.

It might be time to override your gut feelings and check the Hall sensors for function as has been recommended.

Yep, I think that's the way to go. I'm guessing there's a walkthrough of how to do the test somewhere in the forum so I'll find it and get started

Ok, so I've used the guide from the portal section using the led and the 12v power supply. When I connect up the live, I get a faint flicker from the led, then it goes out. It says the led should remain on until a metal instrument such a s a feeler gauge is introduced. Obviously mine doesn't stay on, so there is a fault with something. Is it something I should be looking to fix, or replace? Thanks laitch for giving me the push to do this test. Hopefully I'm on the right track to a fix!

Examine the wires for the Hall sensors for breaks or worn insulation.  Make sure there are no bits of metal in the gaps.  Is there a loose joint somewhere? 

They can be replaced individually, but can be a pain in the azz, since they need to be riveted to the plate.  Complete plates with sensors and wiring can be had from the breakers or on eBay.  The good news is that all the four cylinder K bikes(K100 2&4 valve and K1100) have the same Hall sensor assembly.

Right then, that's a job for tomorrow! I'll have a good look to see if there's any obvious issues. I'll take some photos and post them on here too, because I would describe my eye as "untrained", so I'm sure you friendly folks can spot a fault I might have missed. Hopefully this will be the source of the issue and I'll be able to put it right. And if anyone has a link to a hall sensor for sale just in case, that wouldn't go without appreciation cheers fellas, wish me luck

Tattooist Keith wrote: I'll have a good look to see if there's any obvious issues. I'll take some photos and post them on here too, because I would describe my eye as "untrained",

I'd want to know if the ignition control unit and the Motronic control unit are delivering sufficient current for the Hall sensors to operate correctly in the first place.

How do I test this? Is there a section in the portal I can follow?

Tattooist Keith wrote:How do I test this? Is there a section in the portal I can follow?

If you go to the  electrical diagrams, you can get an image of the various electrical systems. Bert has kindly highlighted each system individually.

Stumpy wrote:

Tattooist Keith wrote:How do I test this? Is there a section in the portal I can follow?

If you go to the  electrical diagrams, you can get an image of the various electrical systems. Bert has kindly highlighted each system individually.

Keith's moto is a 4V so it has a different connection to the Hall sensors.

I'm trying to troubleshoot this using the wiring schematic of a K100RS 4 valve, comparing it with the 2V schematic and the 2V Hall sensor tests then extrapolating from there. The 2V Hall sensor tests in the Portal indicate 12 volts to the sensor plugs with the ignition switch On so I'd be looking for that voltage to be coming from the Motronic to the sensors. If I found it, I would look for malfunction of the sensors themselves. If I didn't find it, I'd backtrack to the ignition module. Alteration of the loom multiplies possibilities and so does connection with the BEP. I'd certainly review the BEP connections.

You should have a wiring schematic for your moto but I can't tell if you do. Maybe somebody with actual Motronic diagnostic experience will chime in here.  

I've tried to reduce the risk of loom alterations causing trouble by changing the injector loom back to the original. I don't have an original for the main harness though. I have removed the hall sensor to see if there's any obvious problems. Aside from some very slight corrosion on the bottom of the plate, I can't see any loose or broken wires. I'll attach photos incase I've missed something obvious. Laitch, when you talk about testing for the power coming from the motronic, how do I do this? I'm guessing this has already been covered somewhere. I will check the bep3 connections. I was assured it's just a plug and play item, you don't even need a clock apparently, it just does the computing job of the old brotbox clocks. Always worth checking though

Laitch, I switched on the ignition and used my multimeter to check the voltage at the plug pins to the hall sensor from the motronic. I got nothing on any of those pins. Is this a sign of something or was it a pointless check? The connections to the bep all seem good and solid. I will try and source an original main harness just incase (unless it's a stupid asking price)

Tattooist Keith wrote:Laitch, I switched on the ignition and used my multimeter to check the voltage at the plug pins to the hall sensor from the motronic. I got nothing on any of those pins. Is this a sign of something or was it a pointless check?

From where are the Hall sensors going to get power to run the engine?

So is this pointing towards a failure in the motronic?