BMW K bikes (Bricks)

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mynameisrodney

mynameisrodney
active member
active member
Hi Guys,

So I have my tank pulled apart at the moment, looks like somebody put E10 in there at some point and all the rubbers have turned to goo. 

The fuel sender has shat itself. Rusty as hell, and a bunch of broken wires. The prices for replacements were above what I was willing to spend, so I thought it try to repair it on the cheap. So I made up a little circuit board with some vero and some 15 ohm resistors. The vero is oriented vertically, so the arm wipes across the tracks as it would across the wires. Obviously this is going to go in steps rather than a continuous gauge, but i can live with that. I have been soaking a spare bit of vero and a spare resistor in petrol for a few days now and they look unchanged so far, so should be able to hold up to the petrol. Testing the sender now gives a range of 0.5 - 120 ohms (I read some people saying the max should be 110 and some saying its 130). 

So far, it looks ok, will be slightly off and in steps, but thats ok with me. It's cost me $3 and some time. But the fact that I've never seen somebody do something similar gives me a funny feeling that I'm being stupid and haven't thought about something. So, am I an idiot?

Cheers,
Chris

Cheap repair for fuel sender - Am I an idiot? Img_2010

Cheap repair for fuel sender - Am I an idiot? Img_2011

EDIT: Sorry, forgot to also ask, is the 0.5ohm minimum value a concern?

    

MartinW

MartinW
Life time member
Life time member
Cheap repair for fuel sender - Am I an idiot? 177912 Submersible EFI rated fuel line can be had from https://motorsportaccessories.com.au/ Around $17.00 Au per metre if you become a member which it free. They also stock the correct size EFI hose clips. Fuel pumps can be had from AliExpress http://www.motobrick.com/index.php/topic,10054.msg86054.html#msg86054
Regards Martin.


__________________________________________________
K75s Hybrid
    

Suzi Q

Suzi Q
Life time member
Life time member
My kind of fix, love it!  Cheap repair for fuel sender - Am I an idiot? 112350

0-100 Ohm is commonplace in fuel senders, so you could also graft a resistance track in from a cheapo scooter sender, but what you've done is genius.


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Sunova Beach!  I threw away a sender a month ago because the wires were shot.  Darn!

That's a brilliant fix!


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

volador

volador
Life time member
Life time member
Brilliant!
What is a vero and where can I buy 1?

Do you have sketch of your circuit?


__________________________________________________
1984 K100RS  1991 K100RS  Reap The Wild Wind... Ever Commute Is An Adventure
    

mynameisrodney

mynameisrodney
active member
active member
Thanks guys, feeling a bit better about putting it in with nobody pointing out flaws in the plan.

Volador, Veroboard is just blank strip circuitboard. I bought it at Jaycar in Aus I think, it was lying around from a previous project. Electronics/hobby shops should have it.

https://en.wikipedia.org/wiki/Veroboard

I didnt draw up a circuit diagram, it is just 8x 15ohm resitors soldered between the adjacent tracks. I then tinned the strips to fill the holes so the contact wouldnt get stuck in them. Here is a pick of the back of the board.

Cheap repair for fuel sender - Am I an idiot? Img_2012

    

zaubertuba

zaubertuba
Silver member
Silver member
As long as the materials can survive the fuel--which you're testing--I'd say it's an ingenious fix!

Can I ask, though, what kind of gauge are you guys sending this signal to, anyway?  This is quite a bit more involved than the dual low-fuel dash lights on my RS, right?


__________________________________________________
1985 K100RS
    

MartinW

MartinW
Life time member
Life time member
Brilliant solution. If you're concerned about fuel effecting the resistors and board you could coat it with a fuel proof coating, just leaving the track exposed.
Regards Martin.


__________________________________________________
K75s Hybrid
    

mynameisrodney

mynameisrodney
active member
active member
@zaubertuba wrote:As long as the materials can survive the fuel--which you're testing--I'd say it's an ingenious fix!

Can I ask, though, what kind of gauge are you guys sending this signal to, anyway?  This is quite a bit more involved than the dual low-fuel dash lights on my RS, right?

Hi mate, believe it or not I don't have one yet haha. I'm going to add one in the future, but while I had the tank apart I thought it made sense to fix the sender now. 

Martin, good point on the sealer. I have a can of tank sealer in the garage for a rusty kawasaki tank i need to fix. Maybe I can paint some of that over the resistors.

    

mynameisrodney

mynameisrodney
active member
active member
zaubertuba, you made me have another think about this. I missed the part about the K100 fuel sender working opposite to aftermarket ones. So I'll swap the lead wire to the other side of the board. looking at a few aftermarket gauges they use 0-100 ohm rather than 110/130ohm. So I'll swap every second resistor out from a 15ohm to a 10ohm. Then I'll have a 0-100 range.

Cheers,
Chris

    

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