No power to positive wire in the fuel level sending unit - 1988 BMW K75S

[ikayyal]

Hello,


I live in MA.  Just got a 1988 BMW K75S.  It ran great for about 10 miles then left me stranded on the road all of sudden.  I traced the issue to the fuel level sending unit.  When I press that start button, I have power going to the "green" wire of the tank connector, but none of the wires on the sending unit's side is receiving power (I pulled the sending unit out and hooked it to a test light).  I am expecting the white wire on the sending unit's side, which is hooked to the pump to have electricity, when I press the button.  Any assistance is appreciated.


Thank you.
Issa.

[johnny]

greetings...

what is voltage at battery with key off... do all fuses test good...

with moto on centerstand key off kill switch run position trans in neutral...

turn key on... what happens... you gotts headlight dash lights horn...

when you press the start button what happens... is it turning over...

j o

[ikayyal]

Hello,


The battery is new and always on a tender.  I will test it again once I am back with the bike, but I am sure it will be over 12V.  All fuses work.  Dash, horn, & light are fully functional when I turn the key on.  When I press the start button the bike turns over and I get power to fuse #6 as well as the green wire at the tank connector.  The problem is that the power does not seem to continue to the other side of the connector (white wire going to the pump) that is hooked up to the circuits on the fuel level sending unit.


Thanks,
Issa.     

[TX brick]

Hello,


The battery is new and always on a tender.  I will test it again once I am back with the bike, but I am sure it will be over 12V.  All fuses work.  Dash, horn, & light are fully functional when I turn the key on.  When I press the start button the bike turns over and I get power to fuse #6 as well as the green wire at the tank connector.  The problem is that the power does not seem to continue to the other side of the connector (white wire going to the pump) that is hooked up to the circuits on the fuel level sending unit.


Thanks,
Issa.   

The fuel pump pass through the sending unit is a known failure point. Most people have had to replace the$$$ fuel sending unit. Maybe if you remove the unit, you can find the fault and fix it.

[ikayyal]

The battery is reading 13.04V when the bike is off.  I have removed the level sending unit.  Can I somehow do a "bypass" from the green wire on one end of the unit to the white wire (going to the +ve end of pump) on the other end of the unit ?

[Martin]

Before you go pulling the sender unit. You need to thoroughly clean and check the 4 pin connector, under the RHR of the tank. This connector is a well known for suffering from connectivity problems. Cleaning with Deoxit is recommended. If worst comes to worst replace with a waterproof 4 pin connector which can be sourced from Jaycar. A smear of dielectric or silicone grease will keep the air and moisture out which causes the corrosion.
Regards Martin.

[rbm]

The battery is reading 13.04V when the bike is off.  I have removed the level sending unit.  Can I somehow do a "bypass" from the green wire on one end of the unit to the white wire (going to the +ve end of pump) on the other end of the unit ?

No, I don't think a bypass is possible.  That pass-through connector is gas tight and any work you do on it could compromise the integrity of the connector and gas will start to leak out.  I've read several threads this year (especially) like this one where owners are seeing intermittent failures with the fuel sending unit in the tank.  It's odd that so many of these occurrences happen this year compared to years previously.


A new unit is several hundred dollars.  There might be a McGiver repair you can attempt that will save spending that money, if you are clever with a soldering iron.  If you never plan to use the fuel level sending functionality of the unit, it might be possible to unsolder the white fuel pump wire from its terminal as well as the green wire going to the fuel level resistor.  Then solder the white fuel pump wire onto the now vacant terminal.  Then rewire the connector at the harness to put +12V onto the White wire instead of the Green wire.  This probably sounds confusing, and if you are not electrically savvy, then it's best not attempted.


It is also theoretically possible to build from scratch a replacement level sending unit plate with just the liquid tight connector and the wiring for the pump. You'd have no low fuel sensor so the dash would not work, but it could be cheaper than a new level sending unit.

[ikayyal]

Thanks ... the connector pins look clean, but I will clean them anyways and see if this fixes the issue.   Thank you.

[ikayyal]

Thanks RBM ... I was actually thinking of doing something similar:


1. delete the green wire for the gauge (since my bike does not have one) at the unit
2. Tap the green wire into the white wire outside of the unit between the connector and the plate
3. Move the white wire to the location of the green wire at the unit, since the green wire does get electricity if I hook up +ve current to it.


I was not sure if it is safe :) .... but it sounds like it may work.   btw .. local dealer is selling the unit for $190.

[The Mighty Gryphon]

Whatever you do, try to resolve the issue asap.  At the rate BMW is raising parts prices and discontinuing parts for K bikes, if you wait until December you will have to pay at least $250 for the sending unit if you can still get it.

[Laitch]

The problem is that the power does not seem to continue to the other side of the connector (white wire going to the pump) that is hooked up to the circuits on the fuel level sending unit.     

You checked for continuity on the wires of the under-tank connector and the white wire had no power on the sender side of the connector? If that is the situation, wouldn't replacing the under-tank plug be the solution? I need clarity here, although I guess that wouldn't be important to anybody else. :giggles

[johnny]

greetings...

at this point... really in the 1st post... photos would end speculation...

where is agent toxicity when you need her...

j o

[ikayyal]

Thank you everyone for the assistance.  I will order a new sending unit (to err on the safe side) and will try and bypass the wires on the old one and it keep as a backup.   Once I switch the wires on the old one, I will post a photo.


Regards,
Issa.

[CNRED]

I bought my Sender at Euro Motoelectrics, http://www.euromotoelectrics.com, I still paid $200.00. At least they had it.  My 90 K75rt had an issue much like yours. Solved my intermittent stalling problem.

[rbm]

Rethinking my advice, I'd recommend you avoid getting a soldering iron near the gas tight passthrough connector just to be safe.  First, I'd cut the white wire at going to the pump as close to the gas tight connector as possible.  Second, I'd clip off the green wire going to the float resistor as close to the resistor as possible.  Third, I'd splice and solder the white wire to the green wire, then coat the connection with some covering impervious to gas, like Liquid Tape, Rhino-liner or similar.  Remember to reposition the Yellow wire's connector on the plug to mate to the Green/White power connector from the harness.

[ikayyal]

Thanks RBM .... I will make the modifications and keep the unit as a backup.  I will also order a new unit, since I want the peace of mind going on the long trips .... cannot wait to start putting miles on it.

[ikayyal]

@CNRED .... I will be ordering mine from http://www.dunbareurosports.com/

[White Dog]

With the 4 pin plug disconnected, do you read any resistance to the fuel pump wiring?  Also, take a look at the female side of the 4 pin plug.  Mine were wallowed out to the extent the male pin(s) did not make good contact.  So I flattened the female ports a tad.  Works like a charm.

[ikayyal]

Thanks White Dog .... the power is available all the way until it goes into the "gel" part of the sending unit.