I have a faulty ground connexion on pin 86 of the fuel pump relay Yellow Brown wire.  
If I shunt it to the bike frame the relay quicks in and the fuel pump even runs

My question is would it be ok to just get that wire permanently connected to the ground ?


Or, do I need to investigate further :  ignition control unit ?


Clue :  I just High pressure washed "K-T" then started the engine fine, and let it idle for a few minutes, after releasing our 5 sheep from their pen, I found the engine stalled & would not restart.   


Any advice will be greatly apreciated.

The ground brown wire on pin 1 of the ICU is OK.
Will do more testing tomorrow, continuity test wire ICU pin 7 to pin 86 Fuel Pump Relay.

Pressure washing is well known for getting water into electrical components that then refuse to function. It's not a good idea to put a permanent ground on that pin 86 as it prevents the control of the fuel pump by the ICU.

Thanks Rick, I will look further into it, I wonder why the ICU is controling the fuel pump via the ground connexion, and not the + 12 V lead ?   It seems esoteric, at least, but HEY !  They are the Bavarians, not me.  
Does the ICU control the speed of the fuel pump, or just ON/OFF ?
Is there a diagram of the ICU itself, somewhere ?
I already replaced the fuel pump, the FP relay, and cleaned the contacts everywhere, still looking for the Yellow Brown wire on the ICU side pin 7.
Time to let the sheep out, and then back at it .

Yellow/Brown wire OK, 
PCB of ICU does not look too bad as far as corrosion damage.
It is a bit difficult to pull the PCB out of the box, & I don't want to damage it.
The pin 7 PCB track runs to the upper right corner of picture, to what might be a power transistor (to regulate/switch the fuel pump relay coil ?, or maybe a rectifier (to prevent polarity inversion accidents) ?
I can't see it, and I might just go for trying to run the bike with the ground bypass additional wiring via a switch, for the time being, since I need to ride this Brick ASAP due to my van undergoing an automatic transmission home rebuilding.
To be continued...

Found a nice almost NEW (19.000 miles) replacement ICU.


https://www.ebay.com/itm/BMW-K100-K75-Ignition-Control-Unit-Bosch-OEM/184421252090?hash=item2af05cfbfa:g:Y9oAAOSwqA1e1wmf

Your order's in! 

You should get it by Mar 13.


Why is it always cheaper to order from USSA than Germany ?



Just in case I blow the old ICU by shunting the ground.



$40.00 Shipping + $ 35.00  import tax.    on $ 100.00 item.

You might want to check the operation of the Hall Effect Sensors. I think their output has something to do with the signal for turning on the fuel pump.

It does.

Good point Point Seven Five.

Another point if the hall sensors check out good, I could see that high pressure water could short out one of the positive inputs (pins 6 and 10)

Sposing you don't have power at pin 6 (cranking signal) with the start button pressed, I spose the ICU wouldn't send a signal to pin 7.

Thank you Point Seven Five.


It would make sense since that's the dirtiest mudiest part that I High Pressure washed,those tractors aresure leaving a lot of dirt in this beautiful countryside. 
So I took a few pics and it does not look wet inside the cover.



I just read this :

Quick explanation of the principle:
What's happening when you press the starter switch:
The starter relay and Fuel Injection relay close, Hall sensors get power.
The FI relay energizes (+) the fuel pump, the idle switch, the injectors and the air
flow meter.
The LE-Jetronic receives (pin #4) the signal that the engine is starting and it sets
itself on starting mode (enriching the air/fuel mixture)
Electronic ignition module pin # 9-10-14 send power to the ignition coil
Electronic ignition module pin #8 send engine rpm info to pin #1 of FI computer
Electronic ignition module pin #7 sends ground to the coil of the FI relay as soon
as the starter switch is depressed. The ground will stay on after the starter switch is
released as long as the Electronic ignition module receives a signal from the Hall
sensors.

Thanks alot Crazy Frog for this:  K100 troubleshooting Starting.pdf 

Tomorrow, I will test if the fuel pump runs when depressing the starter button switch !!!
Gee, I 'm such a noob, did not know all that elctronic "voodoo".

The fuel pump needed replacement, the bottom intake filter was squished and ripped open due to the rubber sleeve "shock damper" crumbling,and letting the fuel pump sit on the bottom of the tank.
The bumpy roads must have killed it, there was some brass / copper filings in the gas.
I also replaced Both fuel filters, the rubber sleeve, and monting bracket, so it should be OK for another 120.000 Km.

Will keep you guys updated...

In which case I will have a spare ICU (the first thing to go wrong in case of EMP with the EFI computer).
I will keep it in a Faraday cage, as an insurance that I will never need it.

Was trying to edit while you posted. Wondering if at actually starts for two seconds or so, or just cranks, and if it cranks slower than usual.

Depends a bit which areas copped a spray, say under tank, head stem, handlebar switches etc.

Like with Rick hopefully just cleaning connections and allowing them to dry might fix it.

Thanks Dai & Daveyson.
I did not pressure wash under the tank,but some mist can go anywhere...
I high pressure washed it quite a few times since I bought it in 2013, mostly the rims and fairing, not the handlebar, or key switch.
Most likely, the fuel pump is dead, the engine was cranking OK until the battery got tired (6 months old).
The fuel pump was mechanically blocked , cause when I try to power it from the 4 poles socket it didnt start, even after replacing the fuse.
So, confused with the wires color, yellow/green here in Europe is for EARTH =  -  so I reversed the wires and the pump made a bad noise, but ran, witnh the + on the green/yellow & the - on the brown, (correct wiring), it ran with no suspicious noise.
It probaly spat a piece of copper / brass in reverse mode.
Now it seems OK, but I would not trust it (this bike is to heavy to push).
Good bike !  It failed right here at home sparing me trouble, and towing money.
Same thing with the transmission of my Transporter T4 van, it would not drive another meter forward, I had to drive it in reverse into the barn / garage.
Earlier last winter I had to replace the radiator, no biggie.
Still in love with my Brick !  

Goodo that you found the problem quick.

Ignore this post if your van has a manual transmission. If it's an auto (dunno much about VW) it can sometimes happen if you're in a hurry and quickly go from drive to reverse before the van has fully stopped, that both forward clutches can get stuck on, leaving you with only reverse.

If so reverse a small amount, stop, put it in forward, in low, in neutral, in park. Do it again, maybe a few times. If lucky it will unstick and work normally in a couple of minutes of messing with it, and wiggling it.

So, confused with the wires color, yellow/green here in Europe is for EARTH =  -


Maybe the case for house wiring (as in the UK) but standard earth on BMW bikes is brown.

Mike

Easy mistake to make in Australia too, since yellow/green is earth in domestic work.

All righty !
Tested it and when I depress the starter button the fuel pump (old one used to test as I don't want to run the new one dry with the empty tank), runs for a couple of seconds like it was ment to do.
All those "idiot proof" safety features that would prevent a start if not in neutral, or whitout pulling the clutch if in gear, and prevent damaging the teeth of the starter gear and windings above 710 RPM and other features :


Quick explanation of the principle:
What's happening when you press the starter switch:
The starter relay and Fuel Injection relay close, Hall sensors get power.
The FI relay energizes (+) the fuel pump, the idle switch, the injectors and the air
flow meter.
The LE-Jetronic receives (pin #4) the signal that the engine is starting and it sets
itself on starting mode (enriching the air/fuel mixture)
Electronic ignition module pin # 9-10-14 send power to the ignition coil
Electronic ignition module pin #8 send engine rpm info to pin #1 of FI computer
Electronic ignition module pin #7 sends ground to the coil of the FI relay as soon
as the starter switch is depressed. The ground will stay on after the starter switch is
released as long as the Electronic ignition module receives a signal from the Hall
sensors.
Note: Under 1300rpm, the timing is set to 6 . ̊ Then, the Electronic ignition module
will modify the timing according to the engine rpm.
At 8777rpm, the timing is reset back to 6 . ̊
At 8905 rpm, the fuel injection is shutdown by cutting the signal from pin #8
(Electronic ignition module) to pin #1 (LE-Jetronic) and will resume when the rpm
come back down to this value (This is to avoid over running the engine).
When engine runs over 710rpm, the Electronic ignition module shuts off the
ground to the coil of the starter relay (pin #11). (This is a higher rpm than what
the starter is capable. and at this point the Electronic ignition module assumes
that the engine is running).
If the engine stalls (no more signal from the Hall sensor), the Electronic ignition
module will shut off the ground to the FI relay and to the coils. (This is to protect
the output transistors and coils).

All nice things that makes the bike safer and last longer.
Compared to my old HOREX 450 mono gylinder I had in 1973 where if you forgot to turn down the timing before kickstarting it, you'll end up doing a back flip salto, &  get a broken foot.  I know it because the previous owner sold it to me for that reason. 
As for the Kawasaki 500 Mach 3 - 2 stroke (widow maker) I had in 1975, you better knew what you were doing, because only drum brakes @ 240 km/h took 2 Km to decelerate, not much engine breaking allowed because the oil was premixed in the gas, and if you shut off the throtle, you cut off lubrification, and seize the central piston.
But I am still around, now at 68.
Amazed that this brick was so much ahead of it's time & competion way back in 1985.
So now back to put it all back together, after I replace the headlight bulb with an HID type.
Thanks to all who joined in helping.

Do you have the engine running properly now?

It will run tomorrow.
I still need to dress her up, fuel tank, + fairing, left glove box, saddle, etc...
I installed that new HID headlight bulb & ballast first as I needed to access the headlight before rebolting everything.
The fuel pump is getting some juice now, the starter cranks it up, but without gas lines hooked up, no GO.

daveyson wrote:Goodo that you found the problem quick.

Ignore this post if your van has a manual transmission. If it's an auto (dunno much about VW) it can sometimes happen if you're in a hurry and quickly go from drive to reverse before the van has fully stopped, that both forward clutches can get stuck on, leaving you with only reverse.

If so reverse a small amount, stop, put it in forward, in low, in neutral, in park. Do it again, maybe a few times. If lucky it will unstick and work normally in a couple of minutes of messing with it, and wiggling it.

Thanks for the tip (only the tip ?)   
But it is DEAD, moving all our stuff from Belgistan to Hungary in 3 round trips of 3.200 Km pulling a double axel trailer loaded with the Bike, and lots of heavy stuff up and down the Alps in Austria at 150 Km/h was the "coup de grâce" for that automatic tranie.
It was slipping for the last 2.000 Km.
Then it would spin like if it was in neutral.
I' ve got all the new brakes, clutch disks, an parts to rebuild it, still need to take it down, and fix it then put it all back in, a job that will cost me less than  1.000, rather than have it done for € 8.000.
Great van, now turned into a stealth camper with 160.000 Km of use for my previous industrial refrigeration / AC business.
Year 2000 T4 - 2.5 L  TDI 5 cylinders 102 HP  174 Km/h and average 8 L / 100 Km.
Full options (it was a tax write off).

At last  !
Engine running properly (great !).
Still need to fix the lower left fairing, glove box, and ready to go for a spin tomorrow.
I added a switch in the yellow brown ground wire, that cuts off the fuel pump relay as an immobilising measure, since I never lock the steering, too hard to do with the fairing, no roomfor my big hands.
So they can always try to hot wire the key switch...


Again, thanks for the support.