Hello, I'm getting all of my family's bikes ready for riding this summer, but my dad's 85 k100rs is being difficult as I can't get the fuel pump going. 

Started out by trying to crank it but the battery was dead, turns out the trickle charger doesn't work anymore. Went to jump the battery witha spare car battery we have and stupidly put the jumper cables on the spare battery backwards. Alternator wire started smoking but I quickly noticed and fixed the issue. Once the cables were on right I went to start the bike and it started right up with no issues, so I don't think this is my problem, but I am including just because it is a possibility. 

Went for an hour and a half ride to see if the battery would hold a charge, it didn't, so I bought a new battery and swapped it out. Other than the battery not holding a charge the bike ran really well. After sitting overnight, the bike had trouble starting cold and only started about half the time, and had trouble idling cold (wouldn't idle at low idle but would poorly at high idle), but the fuel pump was running and the bike would start. This was a problem last year as well so I think that there's a new issue. Now, the next day, the bike doesn't turn on at all and there is no power coming to the fuel pump. 

The fuse seems to be in working order as I swapped it with a known good one and there is no power at either end of the fuel pump fuse when cranking. I put a wire from the positive end of the battery to the fuel pump fuse and the fuel pump does work. I think that I tested the fuel pump relay correctly and I was getting +12v to pin #86, -12v to pin #85, and +12v to pin #30 with the key on as per this post: https://www.k100-forum.com/t1686-fuel-pump-not-powering-up The only discontinuity I have with the information in that post and my relay is that pin #30 was only getting power with the key on and wasn't powered with the key off. 

I am a complete novice when it comes to diagnosing, and I have no idea where to go from here.
Thanks for your help in advance

The pump will only run when the engine is running and when the start button is pressed and for 1.5 seconds after the button is released.
First thing is to check the ignition switch to see if power is getting to it and leaving.

The bike cranks over well so I don't think it's the ignition switch. I was testing the fuel pump fuse with the engine cranking

The four pin connector under the RHR of the tank is a known source of problems. If it starts to play up it is best to replace it with a quality waterproof connector.
Regards Martin.

Hi Martin, 
I think that I have ruled out it being the tank connector as the pump ran when I jumped it from the battery to one end of the fuse. If the connector was faulty it's my understanding that the pump wouldn't be able to run

I think I found the connector you're talking about. It was on the frame on the right side of the bike above the coolant overflow tank. The pins look fine and I played around with them with a pick but nothing changed. Is there any other way to check this connector?

How have you determined the fuel pump isn't operating when you press the starter button? Did you check for fuel pumping out of the fuel rail connection or turbulence within the tank? If not, remove the four screws holding the fuel tank cap and take a look. Check for loose interior fuel hoses and loose pump connections, too, while you're in there.

How old is the fuel in that relic? Was it ingesting E10 before hibernation? Did you add clean fuel when you rode it? Has the fuel filter been changed sometime in the last few decades? A partially clogged one can cause intermittent stubbornness. All these are the least complicated points of interest to explore.

If the pump is truly stalled, there might be a fault in the wiring from, or to, the fuel sender from the four-pin plug.

I "bench tested" the pump by jumping a wire from the positive battery terminal to the terminal on the fuel pump fuse. The fuel pump did run and sounded good. When I bump the starter I cannot hear the fuel pump run as it should for a second or two after cranking (as it was doing last night).
I put gas in it two days ago before my hour and a half ride but the gas that was in it was about a year old. Before last year the bike had basically been sitting for 3 or so years but I had drained all of the gas out of it and then it got a clean bill of health from a BMW dealership. Since going to the dealership it has been driven and had a tank or two of gas run through it.

Check for 12V on Green/ White wires at fuel injectors with start button, that will confirm fuel pump relay is working if you are getting the 12V. Then check wiring from fuel pump relay to pump. Also the ground for pump should be coming from stud/lug under tank on frame (I think).

If your description is accurate that you cannot hear the pump power down when releasing the starter button but there is power at the four-pin when the starter is turning—like caveman indicates, the fuel injection relay would be faulty if there wasn't—then checking continuity at the fuel pump connections might be appropriate. The wiring from the four-pin to the fuel level sender and from there to the fuel pump can break down in neglected or seldom ridden motos with ancient fuel in their tanks.

Direct observation into the tank during the starting festivities might be useful if you have found yourself asking people to "Speak up!" lately. 

caveman wrote:Check for 12V on Green/ White wires at fuel injectors with start button, that will confirm fuel pump relay is working if you are getting the 12V. Then check wiring from fuel pump relay to pump. Also the ground for pump should be coming from stud/lug under tank on frame (I think).

Thank you so much. Theres no power to the injectors. I will get a new relay and hopefully that works

laxer wrote:

caveman wrote:Check for 12V on Green/ White wires at fuel injectors with start button, that will confirm fuel pump relay is working if you are getting the 12V. Then check wiring from fuel pump relay to pump. Also the ground for pump should be coming from stud/lug under tank on frame (I think).

Thank you so much. Theres no power to the injectors. I will get a new relay and hopefully that works

Take the relay into the shop with you. If you don't, you're likely to be sold a DIN-2 standard relay which has a different pinout to the BMW OEM ones.

Hold the phone! If the injectors are not getting power (12V) then the fuel pump (injection) relay may not be getting energized. The Hot (12V) comes from Fuse 1 and the ground comes from the Ignition control module (that I believe gets it's signal from the hall effect senders/pickups.

So I checked the relay with the key on and got the values that I stated on my first post. I feel like I need to test it while the engine is cranking (or while the pump should be on) to see if the relay is getting energized but I don't know how

I'm not too good with electrical stuff so I find coloured electrical diagrams a big help. 

The fuel pump relay doesn't work until you press the start button.

Hopefully it's just a bad fuel pump relay. I'd remove the relay and connect a battery to pins 85 and 86, it should then make a click sound, then you should have continuity between 87 and 30, which should provide enough power for say a headlight. 

If it doesn't click, good, you need another one (depending)

daveyson wrote:I'm not too good with electrical stuff so I find coloured electrical diagrams a big help. 

The fuel pump relay doesn't work until you press the start button.

Hopefully it's just a bad fuel pump relay. I'd remove the relay and connect a battery to pins 85 and 86, it should then make a click sound, then you should have continuity between 87 and 30, which should provide enough power for say a headlight. 

If it doesn't click, good, you need another one (depending)

Tested the relay with a battery and I did have continuity and it clicked. Not sure where I should go from here

If you have checked that the green/yellow is hot (12v from fuse 1) at the coil hot side (pin 86 or 85, it really doesn't matter) the other side will be looking for a ground from the ignition controll unit on the yellow/ brown.....

"then clip the lead of test light to BTT+ and probe - (yellow/brown) side of coil with led or low watt (6 or less) light (I have no idea how strong the driver is in the ignition module) and see if  the relay is getting what it needs to energize."


The ground to energize the relay.

Sorry it's been a few days since I've been able to do any work in the garage. I just checked the relay base again and realized that pin 85 is not grounded. When I connect my multimeter to check if it's being grounded I got a value between 0.5-1 volt and then I checked again and I'm getting nothing. I've looked at a few wiring diagrams but I cant tell where pin 85 is supposed to come from anyone have any advice on where to go from here or have a diagram that shows where 85 comes from?

What color is the wire on pin 85? If it is the ground side it should be yellow/brown and only pulled to ground when start button is pressed and the ignition module and hall effect sensors or working properly.

caveman wrote:What color is the wire on pin 85? If it is the ground side it should be yellow/brown and only pulled to ground when start button is pressed and the ignition module and hall effect sensors or working properly.

85 is yellow/brown. Pin 86 is hot (positive) when the key is on and then goes off when I start cranking. 85 does not go to ground with key on nor when I'm cranking. Looking at a wiring diagram, it looks like 85 goes directly to/from the ignition control unit.

Are you loosing power at fuse 1 when cranking?

Fuse 1 goes from 12 volts to about 11 volts

Use a test light as well to make sure your maintaining continuity. Hell clip a test light between pin 85 and 86  at coil socket and see if it lights. Voltage alone means very little, voltage drop under load will point you in the right direction.

On fuse 1, the test light flickers really rapidly. The light never actually goes off, just dims a little and then full brightness. Hooked the test light up between 85 and 86 and got nothing while cranking

Ultimately, power to green/yellow wires comes from the kill switch, so that's another place to look.

With your test light clip to battery negative and probe to the kill switch output (pin 2 of the right handlebar multi plug on the right hand side under the tank, green/yellow) does the test light remain on when you press the start button? 

With test light clip to battery negative and probe to kill switch input (pin 9, green) does the light remain on when you press the start button?

With test light clip to frame and probe to battery positive the light should glow, confirming the clip contact is good. With test light clip still on frame and probe to pin 9, does the light remain on when you press the start button?

laxer wrote:On fuse 1, the test light flickers really rapidly. The light never actually goes off, just dims a little and then full brightness. Hooked the test light up between 85 and 86 and got nothing while cranking

I would think if there is enough power to light the test light then there should be enough to energize the relay ( like daveyson states from battery + through ignition switch, through kill switch, to fuse 1) but the flickering and dimness suggests there may be some poor connections along the way.

Now move your test light to the Green/Yellow wire at fuel injection relay to make sure power is getting there.

If you have power to the relay unplug Ignition Control Unit and ground pin 7 on the plug, with ignition and kill switch on relay should energize and pump run.

Grounded pin 7 of ecu connector and tried cranking and got nothing. Looking at the flow chart for no start for k100s, I can't tell if this means the module is bad or if I need to keep going

laxer wrote:Grounded pin 7 of ecu connector and tried cranking and got nothing. 

Does that mean a) the connector was reading 12V with the ignition On but 0V when cranking, b) the connector was reading 0V no matter what and not cranking, or c) something else?

caveman wrote:

laxer wrote:On fuse 1, the test light flickers really rapidly. The light never actually goes off, just dims a little and then full brightness. Hooked the test light up between 85 and 86 and got nothing while cranking

I would think if there is enough power to light the test light then there should be enough to energize the relay ( like daveyson states from battery + through ignition switch, through kill switch, to fuse 1) but the flickering and dimness suggests there may be some poor connections along the way.

Now move your test light to the Green/Yellow wire at fuel injection relay to make sure power is getting there.

If you have power to the relay unplug Ignition Control Unit and ground pin 7 on the plug, with ignition and kill switch on relay should energize and pump run.

laxer,
The key words in this post is Ignition control Unit not ECU. The ignition control unit is located up close to steering head tube of frame, no need at this point to try and start to get a signal for it because you will be bypassing the hall effect and ECU that provide the ground to energize the fuel pump relay....
With that said if you did ground pin 7 of ICU and did not energize the fuel pump relay then you have an open between the ICU and fuel pump relay on yellow/brown wire.

laxer wrote:Hello, I'm getting all of my family's bikes ready for riding this summer, but my dad's 85 k100rs is being difficult as I can't get the fuel pump going. 

Started out by trying to crank it but the battery was dead, turns out the trickle charger doesn't work anymore. Went to jump the battery witha spare car battery we have and stupidly put the jumper cables on the spare battery backwards. Alternator wire started smoking but I quickly noticed and fixed the issue. Once the cables were on right I went to start the bike and it started right up with no issues, so I don't think this is my problem, but I am including just because it is a possibility. 

Went for an hour and a half ride to see if the battery would hold a charge, it didn't, so I bought a new battery and swapped it out. Other than the battery not holding a charge the bike ran really well. After sitting overnight, the bike had trouble starting cold and only started about half the time, and had trouble idling cold (wouldn't idle at low idle but would poorly at high idle), but the fuel pump was running and the bike would start. This was a problem last year as well so I think that there's a new issue. Now, the next day, the bike doesn't turn on at all and there is no power coming to the fuel pump. 

The fuse seems to be in working order as I swapped it with a known good one and there is no power at either end of the fuel pump fuse when cranking. I put a wire from the positive end of the battery to the fuel pump fuse and the fuel pump does work. I think that I tested the fuel pump relay correctly and I was getting +12v to pin #86, -12v to pin #85, and +12v to pin #30 with the key on as per this post: https://www.k100-forum.com/t1686-fuel-pump-not-powering-up The only discontinuity I have with the information in that post and my relay is that pin #30 was only getting power with the key on and wasn't powered with the key off. 

I am a complete novice when it comes to diagnosing, and I have no idea where to go from here.
Thanks for your help in advance

Did you check for 12 volts on pin 87 of the relay when the engine was cranking.  The relay should connect pin 30 to 87 when activated.