I'm puzzling this out, but posting in hopes one of the smarter K gurus knows this issue:

91 K100RS, with very little (mile)age-related problems because it got put away with 39k miles in 2004

The fuel pump is more or less intermittent: once running I havent had it cut out (yet), but starting is an issue. Pump runs briefly as normal when bike is powered and all is working BUT more often now, no pump and of course no start. 4-pin pump/sender connection has been cleaned, ignition lubricated, fuses and holder cleaned. While fidgeting with the pump connector, and getting no result, I bumped the blue 2-pin in the same spot and found that moving THAT connector seems to have a major but inconsistent role. While bike is running, touching ONLY that connector can make the pump cut out. Likewise, if pump isnt working changing the position of that connector will get the pump connected and running again. Connector has subsequently been cleaned thoroughly, but moving/pulling/otherwise abusing the connection has no consistent effect on pump power. It just seems to stand, right now, that futzing with THAT connector is the thing that consistently affects pump operation.     
To confuse me further: this morning bike started and ran great, but now fuel level float isnt showing on gas gauge which now sits at empty on a nearly full tank. SO! I suspect a link, but I dont understand.
The black 4-pin is fuel pump power and fuel sender, right? 
What does the blue 2-pin connect?
The only other possible mitigating factor (and I have NO idea if its related) is that while the bike runs gorgeously, the fake choke/throttle advance NEEDS to be in the start position for the bike to start (as opposed to my 1100 which almost never needs that choke lever moved), and while cold is does not like sudden throttle-off. A quick drop in RPM will stall it. This too goes away once up to temp.

Thank you if you made it this far.

What color are the wires of the connector that gets bumped and creates magic? Are you starting your K100 4V on the side stand, on the center stand, or standing over with side stand up? Did you know your K100 Brick—like your K1100 Brick—has a side stand switch that effects starting?

Are you warming up the Brick by idling the engine before riding, or starting the engine and riding right off? Bricks can differ in their need for the use of the increased starting speed lever for warmup yet all be equally good runners.

Green & yellow. This is the suspect:


The side stand switch was already bypassed by the PO and seems healthy enough.
I usually only idle after I start for a minute at most, and then usually 10 min of more or less gentle around town speed.

Electric issues are always a great source of entertainment - i have seen similar issues. This is what did it for me ... which is by no means a guarantee that it will do it for you (sorry)

Generally - when fiddling with electric connectivity to get things to work AND things work sometimes and other times not - then you need to get the gremlins out of the system. And this can take time. But it is not impossible. Mostely it is one of the following things that is the reason (sorry if i point out the obvious):
a. ground issues
b. connections/wire connectors that are not connecting (including circuite breakers/switches)
c. fused gone (do not think this is what you are working with)
d. faulty wiring/wires that have had damage (or wrongfully wired)
e. components starting to die/wrong components being used (most seen is the cheap ebay/amazon sensor for the cooler)

List of things i did:
1. ground location (by left foot peg) - clean-up, tighten and give "connectivty spray" (translated from Swedish). Then check that ground is present all locations around this part of the electric system
2. hot-wire around the part of electric wireing that can alter behaviour - start with jsut moveing the ground, if this does not do it (and fix permanently afterwards - including soldering wires before they get into new/renovated stick - as oxidation is then minimised/removed from wires)
3. Air sensor - get it cleaned. And check connectivity
4. The sensor from the cooler - check wireing. Check that this is an original Bosch sensor - if not change to Boach (and use right version), if it is - try a new one
5. Motronic unit - look at the wires in the multistick/connector - i had one instance where connectivity went - after cleaning the green gui-stuff off and using connectivity spray - then it worked
6. Check battery voltage - this might also come from low voltage on the battery. You should have no less than 12.3 volt on the battery before starting up (optimal  14.2V)
7. Try out an other fulltank unit - this one is last. for two reasons, it is the least seen issue ... and the most expensive approach (i assume that changin filter and clean fuel have been tried before you wrote this - right. Likewise the thing with trying to run the fule pump reverse to get stuff out of it .. if this is an issue) 
8. When i get to this point in time - if things are not working. i let the bike get cold (not hard around here) , and then put a thermal camera on the wiring. You will be surprised how often you can se where there is shortcircut, or connectivity on/off - you can detect heat. Or alternatively use a RMS Clam meter ( i use a "FLUKE 323 True RMS clamp meter") to se where there is amps going throung the wires. 

I am aware that not all have extra parts to try to swap everything out. But bimmer drivers are usually extreemly nice to loan/help out around here. 

Let's us hear how it goes with getting the gremlins out of you electric system

Drive. Be safe. Have fun
Jensen

Has that bike got an alarm system on it? Green is a power feed and yellow is the headlight.

j1s1 wrote:Electric issues are always a great source of entertainment - i have seen similar issues. This is what did it for me ... which is by no means a guarantee that it will do it for you (sorry)

Generally - when fiddling with electric connectivity to get things to work AND things work sometimes and other times not - then you need to get the gremlins out of the system. And this can take time. But it is not impossible. Mostely it is one of the following things that is the reason (sorry if i point out the obvious):
a. ground issues
b. connections/wire connectors that are not connecting (including circuite breakers/switches)
c. fused gone (do not think this is what you are working with)
d. faulty wiring/wires that have had damage (or wrongfully wired)
e. components starting to die/wrong components being used (most seen is the cheap ebay/amazon sensor for the cooler)

List of things i did:
1. ground location (by left foot peg) - clean-up, tighten and give "connectivty spray" (translated from Swedish). Then check that ground is present all locations around this part of the electric system
2. hot-wire around the part of electric wireing that can alter behaviour - start with jsut moveing the ground, if this does not do it (and fix permanently afterwards - including soldering wires before they get into new/renovated stick - as oxidation is then minimised/removed from wires)
3. Air sensor - get it cleaned. And check connectivity
4. The sensor from the cooler - check wireing. Check that this is an original Bosch sensor - if not change to Boach (and use right version), if it is - try a new one
5. Motronic unit - look at the wires in the multistick/connector - i had one instance where connectivity went - after cleaning the green gui-stuff off and using connectivity spray - then it worked
6. Check battery voltage - this might also come from low voltage on the battery. You should have no less than 12.3 volt on the battery before starting up (optimal  14.2V)
7. Try out an other fulltank unit - this one is last. for two reasons, it is the least seen issue ... and the most expensive approach (i assume that changin filter and clean fuel have been tried before you wrote this - right. Likewise the thing with trying to run the fule pump reverse to get stuff out of it .. if this is an issue) 
8. When i get to this point in time - if things are not working. i let the bike get cold (not hard around here) , and then put a thermal camera on the wiring. You will be surprised how often you can se where there is shortcircut, or connectivity on/off - you can detect heat. Or alternatively use a RMS Clam meter ( i use a "FLUKE 323 True RMS clamp meter") to se where there is amps going throung the wires. 

I am aware that not all have extra parts to try to swap everything out. But bimmer drivers are usually extreemly nice to loan/help out around here. 

Let's us hear how it goes with getting the gremlins out of you electric system

Drive. Be safe. Have fun
Jensen

Tack min vän! I finally isolated it to the 4 pin pump/sender plug and installed a new one, I've put on about 300 miles since it went on without issue. 
Now the big mystery is my front brake switch - rear switch works great, brake light is not stuck ON. But the front switch, which was functional before I did a bunch of "stuff", is now not working. Time to chase that down 

On my 4v k100rs that front brake switch was a microswitch that was in a spot where every drop of brake fluid that leaked ran onto it.  The switch can be had on eBay for about $10 and takes a couple hours to remove and replace. 

If I recall, the switch is configured normally closed and the brake lever holds it open, allowing the contacts to close by internal spring force when you pull the brake lever.  When the brake fluid(or dirt) gets into the switch it gums up the works, preventing the switch contacts from closing and turning on the brake light.

Point-Seven-five wrote:On my 4v k100rs that front brake switch was a microswitch that was in a spot where every drop of brake fluid that leaked ran onto it.  The switch can be had on eBay for about $10 and takes a couple hours to remove and replace. 

If I recall, the switch is configured normally closed and the brake lever holds it open, allowing the contacts to close by internal spring force when you pull the brake lever.  When the brake fluid(or dirt) gets into the switch it gums up the works, preventing the switch contacts from closing and turning on the brake light.

Thank you! I figured that could be it - the brakes sat for 15 years, plenty of fluid seeped out and crystalized in there. I know it just takes patience, but do you have any useful tips for chasing that one down the frame and under the tank?

These could help you, possibly. From BMW's K100 4V shop manual, and Clymer. The manual—k1/k1004v—is available at BSK Speedworks in the workshop manual section of its Download page. The connector wires are grey/red and green/black according to Clymer.

Laitch wrote:These could help you, possibly. From BMW's K100 4V shop manual, and Clymer. The manual—k1/k1004v—is available at BSK Speedworks in the workshop manual section of its Download page. The connector wires are grey/red and green/black according to Clymer.

I'd be dead in the water without the Clymer for sure. The first one you posted is form a factory tech guide though! That's REALLY helpful! Lousy scan but hey! Thanks!

KEVVYMETAL wrote:

j1s1 wrote:Electric issues are always a great source of entertainment - i have seen similar issues. This is what did it for me ... which is by no means a guarantee that it will do it for you (sorry)

Generally - when fiddling with electric connectivity to get things to work AND things work sometimes and other times not - then you need to get the gremlins out of the system. And this can take time. But it is not impossible. Mostely it is one of the following things that is the reason (sorry if i point out the obvious):
a. ground issues
b. connections/wire connectors that are not connecting (including circuite breakers/switches)
c. fused gone (do not think this is what you are working with)
d. faulty wiring/wires that have had damage (or wrongfully wired)
e. components starting to die/wrong components being used (most seen is the cheap ebay/amazon sensor for the cooler)

List of things i did:
1. ground location (by left foot peg) - clean-up, tighten and give "connectivty spray" (translated from Swedish). Then check that ground is present all locations around this part of the electric system
2. hot-wire around the part of electric wireing that can alter behaviour - start with jsut moveing the ground, if this does not do it (and fix permanently afterwards - including soldering wires before they get into new/renovated stick - as oxidation is then minimised/removed from wires)
3. Air sensor - get it cleaned. And check connectivity
4. The sensor from the cooler - check wireing. Check that this is an original Bosch sensor - if not change to Boach (and use right version), if it is - try a new one
5. Motronic unit - look at the wires in the multistick/connector - i had one instance where connectivity went - after cleaning the green gui-stuff off and using connectivity spray - then it worked
6. Check battery voltage - this might also come from low voltage on the battery. You should have no less than 12.3 volt on the battery before starting up (optimal  14.2V)
7. Try out an other fulltank unit - this one is last. for two reasons, it is the least seen issue ... and the most expensive approach (i assume that changin filter and clean fuel have been tried before you wrote this - right. Likewise the thing with trying to run the fule pump reverse to get stuff out of it .. if this is an issue) 
8. When i get to this point in time - if things are not working. i let the bike get cold (not hard around here) , and then put a thermal camera on the wiring. You will be surprised how often you can se where there is shortcircut, or connectivity on/off - you can detect heat. Or alternatively use a RMS Clam meter ( i use a "FLUKE 323 True RMS clamp meter") to se where there is amps going throung the wires. 

I am aware that not all have extra parts to try to swap everything out. But bimmer drivers are usually extreemly nice to loan/help out around here. 

Let's us hear how it goes with getting the gremlins out of you electric system

Drive. Be safe. Have fun
Jensen

Tack min vän! I finally isolated it to the 4 pin pump/sender plug and installed a new one, I've put on about 300 miles since it went on without issue. 
Now the big mystery is my front brake switch - rear switch works great, brake light is not stuck ON. But the front switch, which was functional before I did a bunch of "stuff", is now not working. Time to chase that down 

Tjena kompis, i have been through my handle with ismilar issues. With me it was down to wireing and connectors - not a swich issue. Not last time around.
In general you should be able to short the two wires and se if that did it. 
In general,if you are to get new swiches ... keep in mind these bikes rune for many years. Hence get the good once - if you are to put in new spares. More often than not. We see electric issues when cheap/knock off/alibaba 10 cent electric components get introduced.