91 K75S fuel pump problems, again.

91 k75S, started and ran fine last fall. As people here have suggested, I let it sit through the winter. I guess that was a mistake. I just went to start it, and of course, no fuel pump hum when I turn the key. Also, I see it is blowing the pump fuse when I turn it over. So it looks like by sitting unused for a few months, the fuel pump has locked up. New pump a year ago. Tank all cleaned up inside. New gasket on sender unit, etc. Any suggestions on how to unfreeze the pump without taking the fairing off, draining the tank, pulling out the pump and then shifting the little balls at the bottom of the pump for 30 seconds to unfreeze it?

I have to say, removing these tanks on K75 bikes to massage the fuel system is getting a bit old.

Just sitting didn't do that. My K75S's sometimes sit for months to a over a year. Zero pump issues. Usually start first push of the button.....

You can't reach everything from the filler?

dieselyoda said:

You can't reach everything from the filler?

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The pump on a K75 does not hum (energize) when the key is turned on. It only does so when the starter button is depressed. It may not be stuck at all. It should start and run a few seconds if the starter button is just stabbed.

Yes you can reach everything from the filler. No need to touch the fairing and no need to remove the tank on any K75. Remove the 4 little screws and the entire cap assembly lifts off. Reach in and depress the two tabs on the nylon ring and the pump lifts out far enough to remove the hoses.

It is often possible to unstick a pump by reversing the polarity of the power to the pump. I place the pump in a container og gasoline and attach jumper wires at least 3 feet long. I then make and break the connections at the battery end to avoid sparking right above the gasoline. By alternating the polarity of the power the pump tries to run backwards and often unsticks itself.

Good suggestions. If I can get to the pump without taking the tank off, which from what both of you say is the way to go, that will be really nice. Save me a lot of time and effort. I'll try reversing the pump polarity, and if need be, fiddle with the rollers at the bottom of the pump. It's a new pump, hardly any use, so I have no idea what could be causing this. I'll post again after the work is done. Thanks.

jconway607 said:

Good suggestions. If I can get to the pump without taking the tank off, which from what both of you say is the way to go, that will be really nice. Save me a lot of time and effort. I'll try reversing the pump polarity, and if need be, fiddle with the rollers at the bottom of the pump. It's a new pump, hardly any use, so I have no idea what could be causing this. I'll post again after the work is done. Thanks.

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Back up and retest: The pump will NOT come on with the key. It comes on when the starter button is pressed. So, open the filler cap. Just stab the starter button to make contact. Listen at the open fill cap for a hum for a few seconds. Report back.

I was able to get to the pump fairly easily through the filler opening. I pulled it up enough to attach some leads and test it, and it ran okay. So I don't think that is the problem. Checking, I find that when I turn on the key, the fuse stays whole. But I believe as soon as I hit the start button, it melts. The red "fuel" light is on when the key is on, bike not running, but I assume that is normal. I think it may well be the dreaded fuel sender unit. The bike had sat for quite a while with bad gas. I removed the unit and cleaned it up, and it had been working.These units seem to be rather fragile. I know the wires are thin as hair, (for what reason I can't fathom). I am going to try by-passing the unit, by disconnecting the clip to the tank, and jumping from the power wires directly to the pump terminals and see if the fuse stays intact. I assume that will tell me it's the sender unit that's causing the problem. Any other suggestions?

Anyone had any luck rebuilding these units? Does anyone do that work? You can't find them used anywhere, or they are worse than mine. I see car units, and sender units for scooters, etc, for around $50. BMW wants $310 for theirs, $280 from EuroElectrics. Yep.....

https://forums.bmwmoa.org/showthread.php?72833-Fuel-Tank-Pump-Level-Sensor-Wiring-All-Early-Kbikes


Spend 10 minutes and test it with an ohmmeter.

Let us know what you find. They can usually be repaired fairly easily.

replace w/ $10 ebay one

I replaced mine a # of yrs ago w/ cheap one and keep spare in bike storage. It has always gotten me down the road.
Every once in a while after bike sits the fuse will blow, but just replace it and works fine.
if you do go the smaller ebay one you'll need to had fashion a conversion bracket &
have to make sure you get the right filter with it becuase the bigger filters won't fit.

keep em runnin...........
and there fun to ride

rwik81 said:

I replaced mine a # of yrs ago w/ cheap one and keep spare in bike storage. It has always gotten me down the road.
Every once in a while after bike sits the fuse will blow, but just replace it and works fine.
if you do go the smaller ebay one you'll need to had fashion a conversion bracket &
have to make sure you get the right filter with it becuase the bigger filters won't fit.

Click to expand...

I was confused with this, but I'm thinking you are referring to a replacement for the pump itself. Is that right? My pump seems to be working okay, and it's almost new. I'm trying to figure out if it's a problem with the fuel gauge unit. It is so specialized, I didn't think there were any $10 replacements out there.

Lee,
There are four wires going into the tank, so could you explain what you meant by checking with an ohm meter? To blow the fuse, I assume there is a dead short somewhere, but there has to be continuity through the wires for current to flow. Should I be only checking the two wires that feed power to the pump? I'll need to figure out which ones they are. There's a YouTube video on a guy working on one of these units and I believe he lists them.

jconway607 said:

Lee,
I'll need to figure out which ones they are.

Click to expand...

Did you look at the link that I posted two days ago?

98lee said:

Did you look at the link that I posted two days ago?

Click to expand...

Sorry Lee, I missed the link the fist time, but I did read through it. Well, here is the situation:

1) I unplugged the connector and hooked up a VOM to the green and brown leads coming from the bike. Key on, switch on, push the start button and no voltage shows on the meter. Seems odd.

2) Placed VOM leads between green and brown holes in sensor (tank) half of connector and it shows full continuity.

3) With fuel sensor disconnected, key on, switch on, push starter and fuse stays intact.

4) Reconnect fuel sensor, key on, switch on, hit starter button=fuse blows.

The first picture shows the setup for the VOM to bike harness test, the second one is where I hooked a ground up to go to both the VOM and the connector, to be sure there was a good ground.

I can't figure out how the pump runs if there shows no current to the power wires from the bike.
But in any case, am I mistaken that the short causing the fuse to blow is most likely in the sensor
inside the tank? Next step, take it out? Or...?

In picture #1 your meter is connected to the tank, not to the bike harness. That's why you aren't seeing any voltage.

And if you used the same hook up to test the Ohms between the green and the brown and got zero Ohms, you have a dead short either in the pump or the sender plate. Remove wires from pump (easier to remove pump from tank first) and tape them off so they don't touch each other or anything else, then redo you Ohm test on the green and brown wire the way you have pictured. If you now get no continuity, the wires and sender plate are probably ok.

Test across the terminals on the pump. Should be a couple ohms at least. If zero, the pump is shorted out.

Which ever you get zero ohms on is the problem, pump or sender plate. If it is the sender plate, you need to remove the plate (remove tank andremove plate from bottom of tank. Then test plate on the bench to determine where the problem is and how to fix it.