BMW K bikes (Bricks)


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glennpm


Silver member
Silver member
Rc varies by the sweep of the float pickup but Rn is a fixed value.
What is the impact across the linear sweep of the pickup?


1/Rt = 1/Rc + 1/Rn
1 = Rt * (1/Rc + 1/Rn)
Rt = 1 / (1/Rc + 1/Rn)


Ω
BMWVWRtvGauge Reading % Error
90.00
107.01
90.00
Empty
0%
87.50
93.64
80.34
 1/8
-7%
75.00
80.26
70.29
 1/4
-5%
62.50
66.89
59.82
 3/8
-3%
50.00
53.51
48.89
 1/2
-1%
37.50
40.14
37.48
 5/8
0%
25.00
26.76
25.55
 3/4
1%
12.50
13.39
13.08
 7/8
1%
0.00
0.01
0.01
Full
0%



Last edited by glennpm on Fri Nov 22, 2024 11:28 am; edited 3 times in total

    

glennpm

glennpm
Silver member
Silver member
Now to spec out the Rn resistor. 560Ω resistors are readily available, just need to figure the wattage.

restoration of the fuel sender ...a work in progress so far  - Page 2 Wattag11


... and on ebay ten 560Ω resistors rated at 1/2 watts for $2.22 https://www.ebay.com/itm/394630335733

    

glennpm

glennpm
Silver member
Silver member
When I took apart a fuel level sender that I had replaced years ago, I found that at least one reason for this sender's inaccuracy was due to bunching of the circuit board wires. In my picture this bunching was predominately where you can see the black coloring below the wires.

I first cleaned the board with Deoxit then straightened the wires with a stiff nylon brush and brass brush as required. I then cleaned the board again. I checked the sweep of the sender's pickup and taped off the board. I applied JB Weld to the top portion to afix the wiring.

"JB Weld’s formulation is designed to provide strong, durable bonds that can withstand various chemicals, including gasoline. Its epoxy resins and hardeners create a bond that, once cured, is resistant to heat, cold, and chemical exposures, making it a contender in the realm of gasoline-proof adhesives." from https://welderology.com/is-jb-weld-gasoline-proof

restoration of the fuel sender ...a work in progress so far  - Page 2 Dscn2722

    

glennpm

glennpm
Silver member
Silver member
An update.
I measure 0 to 81.5 Ωs for the VW sender I bought. It sure looks like it came from the same factory as the BMW.

I tested it by making leads and plugging into the bike's tank plug. I soldered wires to #1 brads, 1.5mm diameter. For whatever reason, the lowest reading was 3/8ths to Full. It would not return to zero.

restoration of the fuel sender ...a work in progress so far  - Page 2 Img_3313

restoration of the fuel sender ...a work in progress so far  - Page 2 Img_3311restoration of the fuel sender ...a work in progress so far  - Page 2 Img_3312

I repaired an old BMW unit and I'll be using it and not the VW one.

Glenn

    

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