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Fuel tank sending unit - now rebuilt!


BarrettN
Go to solution Solved by Einspritz,

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Just a head's up so if you want to wait to see how it turns out instead of buying a new unit.

 

My sending unit has a broken wire - with where it's broken I could probably fix it , but I started digging in as I saw so many people having problems with this. I think I have tracked down the right wire, I ordered and a 100' spool of it shipped today, so if yours is broken, just hang on a week or so for me to get it and play with it. I suspect I'll have mine redone and will be mailing a bunch of people wire so they can fix theirs also. I need the good karma, so assuming it works (hey, it's 40 gauge bare Nickel Chromium resistance wire @ 70.2 ohms/ft - it's just wire, right?!) I'll just have anyone who needs some send me a SASE and I'll snip them off a chunk.

 

I'll update here on how it works out.

 

 

Barrett

Edited by BarrettN
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Koboldtopf - '67 1600-2

Einhorn - '74 tii

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Barrett

Per the old BMWCCA Tech Tips book (circa 1976) the resistance wire on the gas gauge sender is approximately 6 ohms per inch, and when the resistance is measured across the terminals it should be 75 ohms when empty (float at the bottom of its travel) and near zero at full (float at the top).  So your 70.2 ohms/ft should be pretty close.

 

Keep us apprised, and save me a couple of feet of wire.  I have several broken wire senders I'd love to fix.

 

cheers

mike

Edited by mike

'69 Nevada sunroof-Wolfgang-bought new
'73 Sahara sunroof-Ludwig-since '78
'91 Brillantrot 318is sunroof-Georg Friederich 
Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette) & Kermit, the Bugeye Sprite

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50 minutes ago, mike said:

Barrett

Per the old BMWCCA Tech Tips book (circa 1976) the resistance wire on the gas gauge sender is approximately 6 ohms per inch, and when the resistance is measured across the terminals it should be 75 ohms when empty (float at the bottom of its travel) and near zero at full (float at the top).  So your 70.2 ohms/ft should be pretty close.

 

Keep us apprised, and save me a couple of feet of wire.  I have several broken wire senders I'd love to fix.

 

cheers

mike

Happy to have something that may be of use for you  Mike.

Koboldtopf - '67 1600-2

Einhorn - '74 tii

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Where would you buy a new unit anyway ? AFAIK hey are NLA and used ones are a crapshoot... I had my gauge recalibrated to match my 3rd Ebay used sender ;-(  Turned out expensive if you factor in the bad ones I bought and the gauge redo. Good luck with the broken wire, best way to go if that's your problem.  

--------------------------------------------------------------

73 inka 2002 w/ fuel injection & 5 speed, LSD

 

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OK, if you want a piece of resistance wire, PM me for my address and send me a SASE (self addressed stamped envelope) and I'll send you a chunk.

 

 

I took a piece of 3" PVC and used a dremel to cut notches in it to use as a removal tool - you can easily control and feel the amount of torque you're applying and reduces the risk of something slipping and bashing a terminal or something.

 

20190219_080946.jpg

 

My unit was pretty cruddy, I soaked it in carb cleaner first, then used a toothbrush on the nut on the end. Once clean, I soaked it in PB Blaster - I did not want to twist off that end! Note or mark the orientation of the top to the tube for reassembly. When removing the outer tube, twist it as little as possible. If you must twist it, turn the whole unit upside down. (There's circle molded into the plastic base of the tube that engages the float when it's all the way down - and if you twist the float, you put strain on the wires). Who knows, your unit might have intact wires and not be working right for another reason - no need to break the wires if you can avoid it. I'll edit this later and add a picture of the whole unit without the outside tube so it's clearer what I'm explaining.

 

Here's what the underside of the top of my sender looked like

 

20190218_144000.jpg

 

 

 

I used a bamboo skewer and a scotch-brite pad with engine cleaner, carb cleaner, and finally electrical cleaner on it to clean it. I really should have broken out my dremel and found something small that would fit in there.

 

I used solder wick to remove the old solder from the hook ends of the terminals - it will probably take more heat than you think, the sender makes a good heat sink.

 

I'd recommend putting an overhand knot in both ends of the wire - it's so fine it slide through most pliers. I wrapped the wire around the tip of my multimeter probe to make some loops in it - it seems like the nichrome wire doesn't take to solder well, so I wanted a couple of loops around the terminal. Again, it'll probably take more heat to solder it than you expect. The wire holds its form, when you tighten the loops up by pulling the free ends it stays that way while you solder it. I found a pair of tweezers useful to maneuver the loops over the hook end of the terminal.

 

20190218_140033.jpg

 

20190218_140348.jpg

 

With one end secure, route the resistance wire.

The pictures below show the routing of the wire through the float contacts and around the crosstree at the bottom. Note that the wire only touches the plastic of the crosstree, it runs under the plastic ledge which keeps it from contacting the nut of the central shaft.

 

 

20190218_143842.jpg20190218_143931.jpg

 

 

For the second part of this you'll want a vise or some way to secure the sending unit - or six hands!

 

The tricky part is soldering the wire to the second connector while keeping some tension on it. I did it by routing the wire and noting the approximate point where it would attach to the second terminal. I made the same loops as for the first connector, but a little further down the wire. I then double checked the routing of the wire, maneuvered the loops over the second terminal, and then locked a pair of visegrip pliers on the wire further down. I pulled the wire tight and let the visegrips hang to use their weight to keep tension on the wire so that I had both my hands free to hold the solder and soldering iron.

 

 

 

OK - the important part - the results:

20190218_152103.jpg20190218_152044.jpg

 

Not quite the 0 ohms and 75 people report on a working unit. I'm going to disassemble the sender again and clean the contacts of the float to see if that's where the added resistance is coming from. I'm assuming that if you get 0 ohms with the float at the top normally that means the resistance wire is out of the circuit - the fact that mine shows 6.5 ohms with the float at the top implies that there's some additional resistance somewhere else and it's not due to the wire itself being off.

Edited by BarrettN
mispelling
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Koboldtopf - '67 1600-2

Einhorn - '74 tii

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  • BarrettN changed the title to Fuel tank sending unit - now rebuilt!
  • 1 month later...

Thanks for doing this.  I've always cursed the fuel gauge in my car for its inaccuracy.  

 

Yesterday it became pinned at FULL.  I know the problem is in the sender as I disconnected the wires at the sender and gauge needle fell to the ignition off position.

 

I will dismantle and see if there is obvious problems.  AND I will re-read your DIY post again to help resurrect my sender.

 

Cheers to you for giving me some incite into this.

73 Inka Tii #2762958

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On 2/19/2019 at 6:48 AM, BarrettN said:

Not quite the 0 ohms and 75 people report on a working unit.

 

I just tested mine and can report that the range is 4  to 71 ohms.  Still trying to wrap my head around the significance of those numbers.

 

4   - full tank (s/b 0)

71 -  empty (s/b 75)

 

I'm also wondering what is the ultimate way to clean the contacts on the float.  It is a very delicate unit. I'd forgotten how fine the wire is.  In the right light I can see some "shiny" inconsistencies in the surface of the wire, presumably from 40+ years of wear.

Edited by PaulTWinterton

73 Inka Tii #2762958

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I'm going to try it first on the sending unit from my parts car, but I'm planning on using a thin strip of 1000 grit emery paper slid up and down between the two contacts, once on each side. Just putting this out there in case someone wants to try it out - it'll be a while before I'm ready to try it. 

Koboldtopf - '67 1600-2

Einhorn - '74 tii

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38 minutes ago, BarrettN said:

I'm planning on using a thin strip of 1000 grit emery paper slid up and down between the two contacts

 

That would work on a unit without wire, but I'd hate to try fitting an abrasive in between the contacts and the wire.

 

I was thinking that soaking the float (at the bottom position) in Evapo-Rust and then rinsing with brake cleaner.  That would clean up the metal surface if there is any rust, but Evapo-Rust tends to dull rusty metal surfaces (that were previously shiny) and I'm not sure that is a good thing at the contact points.  Does dull metal give less conductivity than shiny metal?  I'm not sure.

 

I wish I had a sacrificial unit to play with.

 

Another option might be to just use spray cleaner for electronics.

 

On 2/19/2019 at 6:48 AM, BarrettN said:

I used a bamboo skewer and a scotch-brite pad with engine cleaner, carb cleaner, and finally electrical cleaner on it to clean it.

 

Just a thought: Evapo-rust would clean up your whole unit nicely.  It's amazing and very friendly.  Rust be gone!

 

For some reason my unit has very little rust. I should use a magnifying glass or a high rez photo to look at the float contacts.  That might answer some questions.

73 Inka Tii #2762958

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On 2/14/2019 at 12:55 PM, deschodt said:

Where would you buy a new unit anyway ? AFAIK hey are NLA and used ones are a crapshoot... I had my gauge recalibrated to match my 3rd Ebay used sender ;-(  Turned out expensive if you factor in the bad ones I bought and the gauge redo. Good luck with the broken wire, best way to go if that's your problem.  

 

I always start for a look on Maximillian BMW and it is hidden in an odd place. It is under Instruments/Measuring Systems->Instrument cluster in the 62/8 view.

 

Prices are $218 or $264 depending if you need the short or long unit and just the can itself is $106. Pricey, but at least the o-ring is under $4 -ha ha.

 

Go there for part numbers and then Google up a search for lower price or go to Blunt to see what he can get it for.

 

Randy 

1975 - 2366762 Born 7/75

See the whole restoration at:

http://www.rwwbmw2002.shutterfly.com

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On 2/19/2019 at 6:48 AM, BarrettN said:

I took a piece of 3" PVC and used a dremel to cut notches in it to use as a removal tool - you can easily control and feel the amount of torque you're applying and reduces the risk of something slipping and bashing a terminal or something.

 

Fantastic DIY tool.  Crossed screwdrivers are recommended in the Service Manual, but especially installing a new o-ring can make that procedure a little difficult.  Pressing down while manipulating and turning screwdrivers can be tedious.  Applying pressure and rotating the DIY tool...presto!  

 

Thanks for that stroke of genius.  3 bucks and 10 minutes of grinding.

SenderWrench1.jpg.44c3c8097e63028ac41b20b4de317f2f.jpgSenderWrench2.jpg.d4dea05ebf7ef97b8dfacb69d17f979f.jpg

 

 

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73 Inka Tii #2762958

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  • 1 month later...

As some of you know, I am rebuilding a 02turbo that's been sitting festering for some years here in Malaysia.

The sender unit has suffered by sitting in half a tank of decomposing fuel...it's interesting that the worst corrosion is at the surface of the fuel level, not at the bottom of the tank...i guess it's the oxidation of the fuel - alkane~alcohol~aldehyde~carboxylic acid and it's this acid which corrodes everything.

IMG_20190607_182723-1190x1587.thumb.jpg.73ef59e3d2d9b4bf5cb0a88b434d6e0d.jpg

 

As you can see, the central guide rod inside the sender has rusted thru...and the float is rusted in place at the fuel level...

The rest of it is rather crusty...but it might be saveable..

IMG_20190607_182732-1190x1587.thumb.jpg.31f348dc74b3d31c343f3f4805532315.jpg

 

IMG_20190607_182743-1190x1587.thumb.jpg.e7f339aff1a616aef1e3203b13016791.jpg

 

So, I'll need to fashion a new rod and string some new resistance wire..

 

The turbo tank is much deeper, the sender longer... Do you think they used a different resistance wire?? So that the same dash assembly as standard could be used?? Or maybe it doesn't make much difference....???

'59 Morris Minor, '67 Triumph TR4A, '68 Silver Shadow, '72 2002tii, '73 Jaguar E-Type,

'73 2002tii w/Alpina mods , '74 2002turbo, '85 Alfa Spider, '03 Lotus Elise

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I'd suggest getting a potentiometer or an adjustable resistor and see what resistance ranges are required to get the gauge needle to move full scale.

Sent from my SM-G960U1 using Tapatalk

Koboldtopf - '67 1600-2

Einhorn - '74 tii

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