fuel pump wiring continuity and 4 pin connector

[Eville Rich]

I'm working through a no-start situation. Cranks-but-no-start, similar to the multitude of other posts here.  I went through the procedures outlined in the no-start wackness list.  Other than the side stand switch.  I don't think my 1987 K75s has one.  But did the deoxit, rebuilt the ignition switch, knocked it backward in 3rd gear, etc.  I will still probably pull the starter and clean it just to close the book, regardless.

Anyway, I've pulled the tank and then the fuel pump.  Applied 12v to the pump and it ran fine.  Squirted the remaining fuel right out.  I did a continuity test at the fuel pump power leads.  I get continuity between the black negative lead and the brown wire pin on the 4 pin connector.  I'm not getting any continuity at the fuel pump's positive lead (brown, why is this brown?) and any of the remaining three pins from the four pin connector.  But I'm not sure whether I should be or not.  Sort of feels like I should.  Any pointers on this would be great. 

To some extent I hope it's the four-pin connector.  I'm taking advantage of the situation to replace the battery and fuel filter.  If the pump is working, I'm happy to not replace that.

Thanks much,
Eville Rich
'87 K75s
'15 WR250R
'16 Super Tenere

[bluebossa]

Have you checked fuel pump fuse?

[Eville Rich]

Yes.  The fuses are good.  I'll be doing a few other electrical tests over the week.  Right now I'm wondering about whether I should expect continuity between any of the pins in the under-tank plug and the wire that connects to the positive terminal of the fuel pump (ground had continuity).

I don't see why I wouldn't expect continuity, but there are enough nuances to the circuitry on this bike that I want to make sure I'm understanding it correctly.  And if I should be getting continuity at the positive wire for the fuel pump, but am not, then I've also isolated on at least one issue with it not starting.

Thanks much,
Eville Rich
'87 K75s

[Laitch]

get continuity between the black negative lead and the brown wire pin on the 4 pin connector.  I'm not getting any continuity at the fuel pump's positive lead (brown, why is this brown?)

That wire might be the result of a previous repair.

Did you test the for continuity between the sender and the 4-pin connector or between the sender and the pump?

[Eville Rich]

At this point I've only tested continuity between the pins on the plug and the leads that attach to the fuel pump.  I have not tested the in-between at the sender itself.  It's still in the tank and bit awkward to reach with my multi-meter leads.

I'm not hearing that I shouldn't expect continuity from a plug pin to the end of wire that attaches to the fuel pump. If that's the case, then there is an issue with either the plug, the sending unit, or the wires in-between.  I'll pull the sending unit and make further tests unless someone tells me continuity should NOT be expected between one of the pins and the end of the fuel pump's positive-side wire. For example, I could imagine BMW being daft enough to require the fuel level sensor to close the circuit for the fuel pump (I hope that's not the case, but there are a lot of interactions in the circuitry across the bike, and it would explain my current test result).

And I do think there is some chance of a PO having made a repair. I have reason to suspect that the tank is not the original tank (by inference from a couple of observations) and there have been several other non-stock solutions I've identified throughout the bike. Luckily, I'm treating this as a restoration project, so have been willing to keep pulling the thread on the sweater until I get to an end-point.

Eville Rich
'87 K75S

[Eville Rich]

Update:

I pulled the sending unit out of the tank.  There is continuity between the fuel pump end of the wire and where it's soldered to the sending unit base.  There is no continuity between the green wire on the plug and the sending unit base. In contrast, I get continuity between the yellow wire and sending unit base for one of the actual level sensor wires.  Ground wires show continuity through the system.  My conclusion is that there is a problem at the plug end, with the fuel pump not receiving power. 

Now I need to decide on spending $$ on a new sending unit or just fixing the plug.  The base of the sending unit was showing some corrosion (on the side inside the tank).  Or do I just keep it and fix the plug issue.  Decisions, decisions. 

If more experienced minds have a perspective on my diagnostics or pathway to fixes, I'd appreciate the thoughts.

Eville Rich
'87 K75s

[Eville Rich]

Woo-hoo!  Got the K75s running again.  Replaced the sending unit.  As noted above, there was a lack of continuity to the fuel pump power.  It was great to hear it fire up.  During the time she was down, I also took care of a few other things:
1) New spark plugs
2) New battery - Odyssey PC680
3) Air accumulator - no connection to the engine block, but absent - just some lines from the overflow/venting openings. 
4) Tank filler neck rubber ring
5) New fuel filter
6) Rebuilt the ignition switch
7) Rear brake switch (unrelated but needed doing)
8) "Z" hose

I found that the new four pin connector that came with the sending unit definitely had a more positive engagement with the female end of the plug.  I could have probably got away with a trailer light harness, but since I had the entire tank apart, I felt like starting with a fresh sending unit and putting it back together as "new" as possible. 

The rear brake switch was very satisfying.  The previous owner of the previous owner had put a piece of electrical tape over the BMU warning light on the dash.  The previous owner thought it was the BMU that was faulty. Even though the rear brake did not actuate the brake light.  I thought otherwise and found that the brake switch was bad.  New one fixed it.  Now my rear brake works and the warning light goes out. OCD is somewhat satisfied.

And she runs again. Time to motorbrick a bit.  Then onto other restoration.  I was going to tackle somewhat loose fairings this fall, but that may need to wait until the spring.  Or will I do the high seat conversion (from low to high)?  Or the brakes?  Hmmm.  One thing at a time. I'm planning to ride for now.

Thanks everyone for the various pointers, tech articles, past posts, etc.  The issue was a bit more than just deoxit would fix.  But in terms of "get to know your motorbike," this was a pretty good exercise.   

Eville Rich