Hi guys

I am struggeling getting my K100 Rs from 1983/84 to start after a good old fashion shine up. 

The weird thing is that I drove the bike from the garage to my home (a 5 minute drive, nothing big) and it ran smooth as a baby butt. 
When I get home I changed the oil and oilfilter and try to start it again - and then it starts farting really weirdly. 

When I say farting, I mean that it may start and run for less than a second and then turns off, the starter motor and everything is trying to start it. 

My brother and I went through the SUPER helpful pdf and get to the point that there is nothing mechanically wrong, because as soon as we pull off the Water heat sensor OR the air flow meter it fires up right away and is able to idle (at a little higher rpm than usual - but stable) 

I changed the water heat sensor as it indicated in the pdf that the resistance was a little off (it was saying that it was colder than it was) and same result as before. 

(Side note. the oil pressure and heat indicator in the instrument cluster does not turn on before I start the bike, don't know if that is indicating anything)

I drove it home from the shop for crying out loud and then nope no more riding for you sir . . . . Help me please!

Best regards
David

Welcome David

Did you all happen to clean the big bundle of grounds that attach to the frame under the fuel tank?  Sometimes corrosion builds up there and causes electrical malfunction such as yours.

Make sure the fuel tank connector is firmly plugged in.

They are known to get flaky so try wiggling it while you start the bike to see if that makes a difference. (Same thing with the ignition switch.)

I have changed the fuel pump thus making sure the cable there is firmly attached.
The grounds under the fuel tank are nicely bundled and clean as a mirror.

The funky thing is that it runs like a charm if you disconnect either of those two sensors. 

Thanks for the input guys!

Maybe have to find a known-to-be-good jetronic unit and swap it out to test if the problem is inside.

Another approach is to open the cover and inspect for burnt components or cracked solder joints on the circuit traces for those sensors.

kennybob wrote:Maybe have to find a known-to-be-good jetronic unit and swap it out to test if the problem is inside.

The thing is that I have talked to a mechanic and he says that the odds of it being the jetronic that is gone bad i slim to non existing, but it is where I was getting at myself as well. :s

Maybe slim odds, but it must be tested to verify it is good (or bad), otherwise how will you know what to rule out when troubleshooting?

i have repaired hundreds of failed Bosch motronic units--the biggest culprit is cracked solder joints due to vibration.  Motorsickles don't vibrate?

The other thing that comes to mind might be a bad temperature control unit in the electrical box under the tank--it seems to have common elements to the dash lights, the temp sensor, and possibly share a common ground with the AFM.

Do the headlights come on? horn and blinkers work?  Checked all the fuses?

Yours will have the bolted down fuel tank, with the single bolt at the rear.

The connector on the outside left front of the tank is a well known problem, it can be loose and you should tighten up the female connectors on the plug a little. I had exactly the same problem you have after a service and this solved it. The earth connectors under the tank are also an essential.

The Jetronic units can go wrong but if you do a search on eBay with the Bosch part no you will find brand new ones as the same unit for the 8 valve engines is also used in a Volvo 340!!

Kenny the lights do come on, horns and blinkers work as they should, but I am not questioning that the jetronic can be the culprit, I just want to rule as much out before getting one of those. I have found a source for it, but really nice to know that it is the same as the Volvo!! Thank you Olaf! Yeah the bolt is missing, and has always been missing since the previouse owner thought it wouldn't hurt it. 

I will check the fuel tank connector tomorrow in day light!

All fuses has been checked! 

I have ensured that it is grounded but I have heard that I may have to check the connectivity between the ground and the sensors, any thoughts and tricks on that?

The temp sensor grounds when you screw it in. That's why it only has one wire.

The air sensor has a ground wire from the L-Jet.

Those should not be an issue.

If you are at the temperature sensor as far as I know that's on the left side of the radiator near the fuel pump wiring connector plug to the tank.....

If that bolt is missing the tank may bounce a bit and that may disturb the connections. The fuel tank connector may look a little brown, indicating heating/shorting issues.

Duck what you are saying is that i would either be the relay or the jetronic then?

The bolt is missing but the tank is being held in place by the rubber bushings in the front and the seat so not much moving around.
and if it is the connector to the fuel pump you are thinking about then it shouldn't work if I unplug the water sensor should it?

Guys I really appreciate your input! THANK YOU!!

Whether the tank is bolted down or not has no impact on anything electrical and will not impact how the bike runs.

So duck you would suggest me to find and try another jetronic?

i'm going to put my money on the L-Jetronic unit as the culprit because both the temperature sensor and the AFM are major input signals for the Ljet. 

It idles with either or both disconnected, but won't idle with both devices attached.

It was riding one day, then quit the next.

My guess is the signal conditioning circuitry (op amp) of the temperature signals for the water and the air.

Yes, swap in another known good L-Jet - but I wouldn't pay for one until you know that's it.

CPH is a pretty big town. Hook up with the local Beemer club or go to local online MC forums to see if you can find another K100 owner who is willing to let you try theirs.

Kenny and Duck you are awesome! I will let you guys know what happens when I find a unit to test it with!

Quist wrote:Kenny the lights do come on, horns and blinkers work as they should, but I am not questioning that the jetronic can be the culprit, I just want to rule as much out before getting one of those. I have found a source for it, but really nice to know that it is the same as the Volvo!!

Be careful buying a Jetronic. Many vehicles of that era used the Jetronic but they are not all the same. They are mapped differently for each vehicle. You might be lucky to find a unit from another vehicle the same as we use in our K100s but I doubt it. I would exhaust easier to test and more likely possibilities first.

Post your VIN and we can find the correct Jetronic part number for your model.

I have ensured that it is grounded but I have heard that I may have to check the connectivity between the ground and the sensors, any thoughts and tricks on that?

You can test  earth connections by testing continuity on the FICU plug. Take a look at the Trouble Shooting link off the portal page. The test table is about 2/3 way down the page. For example between pin #13(earth) and pin #10(temp Sensor) if you read 'open circuit' (infinite resistance) then its likely the temp sensor is corroded and it's not earthing properly.

Hollister, I have found a jetronic from a vendor that says it's from a 1988 K100 so I think I should be good on that part, im not really interested in trying to navigating too vividly in intervehicle parts, specially not the electrical ones. 

I did the entire array of checks that is 2/3s down the pdf page, and there were no signs of any problems.

So you went thru the flowchart on page 5 and worked your way down to page 20 of the 30-page troubleshooting guide (pdf file)?

And you tested the resistance of both the old and new water temperature sensors and compared with the temperature chart?  Were they okay or could both have been bad?

Kenny I went through the flow chart and came to meassuring the resistance on the water heat sensors. The old one was indicating that it was like 10 degrees Celcius colder, so I changed it, but didn't change on the original problem.

Quist wrote:My brother and I went through the SUPER helpful pdf and get to the point that there is nothing mechanically wrong, because as soon as we pull off the Water heat sensor OR the air flow meter it fires up right away and is able to idle (at a little higher rpm than usual - but stable)

When you do that disconnection, David, can you rev the engine?

Both Kennybob and Duck have alluded to a faulty Jetronic, and the more diagnostic steps you take the more it seems to indicate that area.

I'd be tempted to pull the Jetronic plug and thoroughly clean the plug and the pins with Deoxit or another electronic cleaner to be sure there isn't some fine material that inadvertently started bridging the pins—unlikely as that would seem—then reattach it firmly

That's my magical-thinking approach toward a less-costly outcome.

Quist wrote:My brother and I went through the SUPER helpful pdf and get to the point that there is nothing mechanically wrong, because as soon as we pull off the Water heat sensor OR the air flow meter it fires up right away and is able to idle (at a little higher rpm than usual - but stable)

When you "pull off the water heat sensor" what are you actually pulling off?

I may be very wrong, but I seem to recall that at some temperature the engine control unit shuts down the engine.  It could be that a problem in the sensor is making the engine control think that the engine is overheating.  Or possibly, the temperature control relay is malfunctioning. 

Maybe someone more knowledgeable than myself can take this further.

Point-Seven-five wrote:I may be very wrong, but I seem to recall that at some temperature the engine control unit shuts down the engine.  It could be that a problem in the sensor is making the engine control think that the engine is overheating.  Or possibly, the temperature control relay is malfunctioning. 

Maybe someone more knowledgeable than myself can take this further.

If the temp sensor wire is chafed along the way somewhere and shorting to ground then the L-Jet will think that the water temp is a Brazilian degrees.

The temp sensor resistance to the L-Jet can be measured between Pin 10 on the L-Jet connector and ground. At room temperature (68F or 20C) it should read about 2.5ish Kohms.

Quist has said that he's done those tests on the FICU plug from the Troubleshooting page, but without any values posted it's hard to make a qualitative assessment.

The ECU wont shut the engine down if the temperature gets too high but if over 113°C it will prevent the engine starting by preventing the injectors from firing.

Long due conclusion on this matter, after the first succesful ride where it actually works!

It seems that even new stuff can break. 

I changed fuel pump and filter before my last ride, and as it turns out, the ride home from the garage was enough to break the fuel filter, causing my headache...

Thank you for all your replies, and now I have a fully functional bike again!

Thanks for letting us know of the successful outcome.

We you say the filter broke, can you tell us exactly what "broke"?

blaKey wrote:We you say the filter broke, can you tell us exactly what "broke"?

I would guess it did split in the seam, it was a big batch of OEM filters with
a weak seam which entered the market around 2000.

I knew about the bad batch of filters...but that was years ago! I would have thought they weren't around anymore.

Its a good ending but......

Never assume that the dodgy parts in the system have all been used out. We had a similar thing with motorised valves in central heating systems and having to go into whole developments and pull them all. A bad batch coming in. But the annoying thing was the darn suppliers kept selling off the old stock even after they knew they were going faulty.

IngeK as always a fountain of knowledge!

blaKey wrote:We you say the filter broke, can you tell us exactly what "broke"?

I decided to cheap out on the filter, as I bought my fuel pump from them as well - Shouldn't have done that!

Lesson learned.

So, how did the filter break?  Was it at the seam?  Was there a manufacture date on it?