1
K75 Fuel Pump internals or the tale of 3 tow trucks Tue Apr 12, 2022 1:50 am
jjefferies
![jjefferies](https://2img.net/u/4211/29/48/93/avatars/2834-53.jpg)
Life time member
![Life time member Life time member](https://i.servimg.com/u/f80/11/71/56/74/bmw-5s10.gif)
It didn't start out to be a bad day. A quick, 40 minute, lane splitting ride to SF's VA hospital for a echocardiagram and then down south to visit Greg's "R Bike" shop. Then a quick shot across the San Mateo bridge and up the much maligned I-880 to home. I was doing ... well 70-80 mph in the express (motorcycle) lane when it all went.. dead. Well the motor just stopped and I coasted across 4 or 6 lanes of high speed traffic to fetch up against the sound wall. I couldn't believe how noisy that road is without my earplugs in. Thirty minutes of checking fuses, computer connectors and every thing else I could think of. Bike turned over real well but wouldn't start. Panicky calls to my brother, nephew and Greg, got a lot of advice but no one was in a position to come and help. Did I mention that 8 to 10 bikes had shot past and no one even glanced in my direction. I was weighing my options of walking out, the nearest exit was 1/2 mile, when lo and behold a AAA tow truck swings in front of me and asks if I needed help. Oh, yeah. Expensive no doubt but could he do it? Well, no. But he did offer to put me in touch with a friend who might be able to. And after a bit of phoning around he said they would come and get me but the price was USD $225. for the 10 mile trip. Yeah! Sure will do! He exchanged information with the friend and left. Shortly thereafter another motorcycle capable tow truck pulled up in front of me. I greet the driver but he's not the guy who was summoned but another. How Much? USD $100. flat rate, his response. Oh YEAH! man lets toss it up there. We're rolling it up when tow truck three shows. Driver is a gentleman and doesn't make a fuss about who was there first. Not sure of the legal ramifications but I've not signed anything. So Driver #2 gets me home, I pay him $100 and tip him $20 just because. Seriously doubt that this business went on the books. Anyway now to trouble shoot. Electrics appear to be working but when preparing to get the tank off I check the return fuel line. Nothing, Nada. Oh a culprit. Some serious time spent checking reveals that I've got an open line across the power lines to the fuel pump. I open the tank and pull it. A double check shows it's not working. Checking through my parts bin I find two, a 52 mm one and a 43 mm one. See the photos below. Replace the bad one with the 52mm pump and after the usual time spent replacing, checking etc. I try it out and the bike starts right up.
But why did the fuel pump fail? I've never really looked into one and so having a bad one and ordered a brand new replacement. (Decided to not trust 2nd hand been sitting around for years backup) I open up the bad one looking into it. The first photo is a comparison of the 52 mm vs 43 mm pumps. You can use either but your filter and the vibration damping rubber are different. The plastic mount however is the same. Things I learned. But a K100 plastic mount differs from the K75. The second photo shows the damping rubber and plastic mount (forgive the wires coming from the left). The third photo is of the internal parts. It's basically an electric motor. The fourth photo shows the two brushes but if you look close there is an electric wire in the upper right which has broken from its solder joint. This is the culprit. My best guess is that the soldering may not have been the best and that vibration caused the wire to work harden and then break. Otherwise the pump is in good shape.
![K75 Fuel Pump internals or the tale of 3 tow trucks 52mm_v10](https://i.servimg.com/u/f46/20/23/16/94/52mm_v10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Mounti10](https://i.servimg.com/u/f46/20/23/16/94/mounti10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Intern10](https://i.servimg.com/u/f46/20/23/16/94/intern10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Electr10](https://i.servimg.com/u/f46/20/23/16/94/electr10.jpg)
But why did the fuel pump fail? I've never really looked into one and so having a bad one and ordered a brand new replacement. (Decided to not trust 2nd hand been sitting around for years backup) I open up the bad one looking into it. The first photo is a comparison of the 52 mm vs 43 mm pumps. You can use either but your filter and the vibration damping rubber are different. The plastic mount however is the same. Things I learned. But a K100 plastic mount differs from the K75. The second photo shows the damping rubber and plastic mount (forgive the wires coming from the left). The third photo is of the internal parts. It's basically an electric motor. The fourth photo shows the two brushes but if you look close there is an electric wire in the upper right which has broken from its solder joint. This is the culprit. My best guess is that the soldering may not have been the best and that vibration caused the wire to work harden and then break. Otherwise the pump is in good shape.
![K75 Fuel Pump internals or the tale of 3 tow trucks 52mm_v10](https://i.servimg.com/u/f46/20/23/16/94/52mm_v10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Mounti10](https://i.servimg.com/u/f46/20/23/16/94/mounti10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Intern10](https://i.servimg.com/u/f46/20/23/16/94/intern10.jpg)
![K75 Fuel Pump internals or the tale of 3 tow trucks Electr10](https://i.servimg.com/u/f46/20/23/16/94/electr10.jpg)