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I'm hoping this info will help others in my situation. This will not repair sending units with broken resistor wire, or those damaged badly, but I was able to save one yesterday through gentle cleaning and a lot of patience. Since these are about $200+ each, it was worth the effort!

 

 

I pulled the sending unit out of a fuel tank that had been sitting for 37 years. the tank itself was filled with a 2" layer of crystalized dried up fuel varnish that resembled top soil. I fully expected the sending unit to crumble upon removal.... so I had nothing to lose. 

 

At the bottom of the ending unit are two tiny 5.5mm nuts. (3mm thread) They are arranged so that one locks in the other. If you gently remove these nuts, the outer sleeve / silver cylinder housing will slide off the sending unit. Once it's off, you'll see two hair like wires upon which the contacts of the float travel. If those are broken, you'll have to find another fix.... but on mine they were intact. The problem with mine, was a 2mm layer of rust and varnish on the central shaft/wire upon which the float travels. I wish i had taken photos...it was GNARLY. I *gently* coaxed the float upwards and started lightly sanding the center wire. This took about 20 minutes and multiple applications of brake cleaner / solvent. I finished up with a red scotch-brite pad and made sure there was no gum in the center hole of the float. Now the float traveled freely up and down the central wire. I did not touch the resistor wires - as they're super fragile. 

 

Now....the information I was unable to find here on the FAQ is as follows: If you want to test the sending unit's function, you need to know the resistance values for the terminals as the float travels up and down the shaft. 

 

Full = 4.4 Ohms (or less)

Empty = about 78 Ohms. 

Middle = 38 Ohms +/-

 

Somehow the wires and connections, and resistance wires all checked out. If the float doesn't absorb / fill with fuel, I have saved over $200. 

 

For those of you with Euro versions with the low fuel light terminal, I'm not sure exactly how to test that, but it should be super simple. 

 

So there you have it... the proper resistance values for a 2002 fuel level sending unit. All models / All years (67-76)

 

Photo showing BRAKE FLUID reservoir being tested. Multimeters are an essential tool. Even a cheap one will save you hours of head scratching. 

multimeter.JPG

Edited by wegweiser
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According to an old Tech Tips entry in the BMW CCA Roundel, the resistance wire used in the sender unit is approximately 6 ohms per inch.  The writer found a wire-wound resistor and unwound it to obtain the proper wire, then soldered it in place.  I suspect the wire's thickness--so long as it'll pass through the contact loops on the float--isn't as important as it's resistance per inch to obtain the correct readings for full and empty tanks.  Anyone know of a good source for resistance wire?

 

mike

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I took it apart again today, to illustrate the original post. 

 

The wires are 180mm long (don't let the camera angle fool you.) Measured from glue/solder point to glue/solder point. IMG_6320.thumb.JPG.4d88a8cf05d82202c24f57232d4901f2.JPG

 

The central shaft of the unit looks rough in the photos, I have made sure the float glides effortlessly up and down. The remaining funk in the top gives you a little idea how bad this thing was, when I took it apart the first time.

 

Resistance values listed in the original post are measured between the two connectors on the top of the sending unit. 

IMG_6317.JPG

IMG_6319.JPG

Edited by wegweiser
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I viewed your post Paul with interest. When I removed the sending unit from my Touring, it fell apart, and for the life of me I couldn't see what was supposed to hold it all together..

Whilst my s/unit differs slightly from yours (3 wires and a guide post), until I saw your post I did not realise there was a 2nd nut at the bottom. In case anyone has a s/unit similar to mine, the filter screen can be removed revealing the bottom nut, or, would have done if mine had a nut there (hence the unit falling apart). No clue how he nut came off, and disappeared off the face of the earth, as the screen was still on the bottom of the cylindrical tube.

Re-assembling it with the addition of the guide post is going to be a bugger I'm sure.

DSCN2813sm.JPG.239e994edb514cd460f935bf50953328.JPGunit as it came out of tank

 

DSCN2815sm.JPG.0bdf6e331b503cd5f9fac4a99165d443.JPG inside face

 

DSCN2817sm.JPG.9080d30dd5e5a5df2ed70ac8781940d5.JPGoutside face (screen attaches to this face)

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  • 4 years later...

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