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Can you calibrate the fuel gauge ? (genius at work here)


deschodt

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45 minutes ago, abe2002tii said:

Ok...so what if your sender checks out ok and your gauge reads a little too low when it is full and when it is empty?

Is there a way to adjust the gauge itself instead so that it reads accurately when it is full?  Any screws behind it ...I do not want to start bending the needle itself.

 

Abe

First thing I'd do is short the sender terminals as close to the gauge as possible.  Knowing that zero ohms is a full tank, this would narrow the problem down a bit.  If you now see the gauge reading correctly at full, you have an issue in the wiring leading to the sender.  I'm assuming you have confirmed that the sender itself reads zero ohms on a full tank.

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I took the sender apart and cleaned the hairline wire as well as the connectors that attaches to it and the spade connectors that are on the outside.  I did not check for resistance but it looks like the  problem  is increased resistance across the range thus the gauge never looks completely full and when it reads empty when there is still gas in the tank. I know  I have good ground everywhere....I double grounded everything in the instrument panel and sender.  So maybe it is the wiring or the sender itself.

I was just hoping that the gauge itself could be adjusted avoiding dishing out money for a new sender or new wiring.   My 69 E type uses a similar system but with resistance increasing and decreasing more like toilet handle and float.  The gauge at the instrument panel also has two screws ...one to set the needle where full is ... and the other to set the needle at empty.

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10 minutes ago, abe2002tii said:

I took the sender apart and cleaned the hairline wire as well as the connectors that attaches to it and the spade connectors that are on the outside.  I did not check for resistance but it looks like the  problem  is increased resistance across the range thus the gauge never looks completely full and when it reads empty when there is still gas in the tank. I know  I have good ground everywhere....I double grounded everything in the instrument panel and sender.  So maybe it is the wiring or the sender itself.

I was just hoping that the gauge itself could be adjusted avoiding dishing out money for a new sender or new wiring.   My 69 E type uses a similar system but with resistance increasing and decreasing more like toilet handle and float.  The gauge at the instrument panel also has two screws ...one to set the needle where full is ... and the other to set the needle at empty.

Hi,

 

What is the resistance reading in ohms when the sender is at full and when it's at the other end?  Can you post pics of the disassembled unit that might show the float?  Perhaps the contacts n the float can be adjusted but never having seen the insides of one, pics would help.

 

Thanks

Edited by Healey3000
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FWIW my tank is a late Tii tank and measures from topside of flange where sensor rests to inside bottom of tank at 8-13/16".

Sensor length from bottom of lock flange to bottom of sensor housing at 8-3/4"

Sensor reads 81ohms with float resting on housing bottom and gauge needle is just about at the top edge of the word "Tank" in the dash.

With 3-1/8" fuel level from bottom of tank sensor reads 57ohms and gauge needle point is just touching the top edge of the red reserve marker in the dash.

The bottom of the fuel pickup is about 1/8" off tank bottom but has the dam around it that limits fuel level to about 1" before no more fuel will be sucked in.

So in a pinch, there is quite a bit of fuel if the needle gets close to the reserve mark.

A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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  • 3 months later...

I started this spanish inquisition, thought I should report...

My gauge (all of a sudden) seemed to not go below 3/4 tank and I ran out of gas in my driveway because I trusted it - lucky. I opened and cleaned the sender and measured resistance, the #s were normal per other posts but the float was a little catchy some of the time unless you upended the sender somewhat violently... So I obtained a spare sender, a new sleeve and sealing ring... Cleaned that too just in case, installed, and I thought it looked fixed because as of this morning I was reading slightly below 1/2 a tank (which is lower than the previous sender read since the issue started). 

 

Still, the fuel pump was getting a little louder on my way to work and I figured I could do some math instead and not risk running out of gas on a bridge... So I filled up and to my surprise put 11.7 gallons in the tank !!!!!  In other words (based on 12ish fuel capacity?)  I was damn close to running out again... 

 

%$^$%^%$^

 

Gonna look at the instrument cluster grounds - it's possible I disturbed something there since I had been playing with the odometer, but I don't understand this. To me a ground is either good, or it's not grounded and the gauge pegs, or the wiring is damaged and it's intermittent with a fluctuating gauge...But mine is solid. Is there really an "in between" where grounding affects the reading by a %? 

 

Edited by deschodt

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

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

 

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You could definitely have a bad ground that has a set amount of resistance, although this would be rare and I think in one of the earlier comments it shares a ground with a flasher or turn signal circuit so it would be negatively affected as well. I think I would disconnect wiring at sender and connect various resistors and see if your gauge "sweeps" up and down as intended. You could probably even use a potentiometer and sweep it up and down to see your gauge working without the fuss of resistors, should do same thing depending what final ohm reading the potentiometer is designed to produce, which would also "exercise" gauge as well maybe...without disassembling your dash.

Sorry for the run on sentence..brain gets going.

Sent from my SM-G950U using Tapatalk

Dave R.

1975 Inka 2002a

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I can do you one better... I now have a spare so I can connect it outside the tank and rotate it up and down and observe the gauge... Great idea... Should have thought of that.. But I'm 99% sure it's the sender(s), I got 2 used ones broken in a a similar way.  I've heard from friends with similar contraptions that the shaft / float always gets a worn spot and "catches", which is hard to reproduce on an empty sender but I got it to catch a few times on full when gently rotating up upside down and right side up.... 

 

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

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

 

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EDIT: FIXED !  I hooked up my spare (original) sender to the sender wires in the trunk. Had my kid check the fuel gauge level when I tilted the sender to move the float.

My gauge only went from FULL to a smidge under 1/2. No lower (unless power is off, so it's not binding).  I then measured the sender Ohm and I have Full 5 Empty 71, which is close to spec.

So despite all I said before, looks like the issue is *at* the gauge if the sender delivers close to the correct results...

 

I pulled the instruments cluster which took all of 3 minutes (2 of the 3 minutes taken bending my aging body under there, 2 nuts and a speedo cable, boom - love 2002s).

Saw nothing loose in terms of grounds but for good measure, I pulled the main connectors, cleaned up the pins, and reinstalled...My gauge immediately showed 3/4 instead of full... Ah ha ?  Did the test again with the spare sender, FULL amplitude, empty to full !!  Yes - fixed ! So partial grounding / partial connection is a thing !  

Edited by deschodt
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--------------------------------------------------------------

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

 

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  • 1 year later...

Gently use a jeweler's screwdriver and pop the metal edge off the plastic portion. (pry gently between the metal edge and the light colored plastic surface) Once that's off, you'll see a (6mm?) nut. If you loosen that nut, you'll be able to see the innards of the unit. 

 

IMG_6317.jpg

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Paul Wegweiser

Wegweiser Classic BMW Services

Nationwide vehicle transport available

NEW WEBSITE! www.zenwrench.com

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Gently use a jeweler's screwdriver and pop the metal edge off the plastic portion. (pry gently between the metal edge and the light colored plastic surface) Once that's off, you'll see a (6mm?) nut. If you loosen that nut, you'll be able to see the innards of the unit. 

 

 

Thank you,wegweiser.
I was able to reach the nut with your advice.

20200115FAQ.jpg

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So despite all I said before, looks like the issue is *at* the gauge if the sender delivers close to the correct results...

 

I'm following this thread because I've been down this road recently.  I was sure that my sender was at fault for my poor readings.  Other than cleaning the resistance wires with a fine polish, I could find no problems with my sender.  The tests showed decent resistance at full, half and empty levels.  

 

 

Please tell me the procedure or article.

 

 

Maybe the following picture will help with your diagnosis.  Eventually I replaced my fuel gauge in the pod and have much better readings.  I believe it's the resistor in the gauge that suffers from old age.

Collage2.thumb.jpg.377dacdd233f606f9cf88b7ca017e0a5.jpg

 

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73 Inka Tii #2762958

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When removing the tube be careful not to rotate it, you can break the resistance wires that way. At the very least, hold the sender upside down as you disassemble it. The float settles down over the crossbar on the bottom of the float rod, twisting the tube with the float at the bottom turns the float. I did a write up on rebuilding sending units, there are some close up pictures in the write up that display this well. 

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Koboldtopf - '67 1600-2

Einhorn - '74 tii

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I'm following this thread because I've been down this road recently.  I was sure that my sender was at fault for my poor readings.  Other than cleaning the resistance wires with a fine polish, I could find no problems with my sender.  The tests showed decent resistance at full, half and empty levels.  

 

 

Maybe the following picture will help with your diagnosis.  Eventually I replaced my fuel gauge in the pod and have much better readings.  I believe it's the resistor in the gauge that suffers from old age.

Collage2.thumb.jpg.377dacdd233f606f9cf88b7ca017e0a5.jpg

 

The resistance measurement I measured is as follows.

  ■ FULL: 5.6Ω
  ■ 1/2 FULL: 34.6Ω
  ■ TANK EMPTY: 66.5Ω

Thanks to Paul's resistance measurement as a guide for judgment.

 

At a later date, connect to the vehicle and check the meter level.

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When removing the tube be careful not to rotate it, you can break the resistance wires that way. At the very least, hold the sender upside down as you disassemble it. The float settles down over the crossbar on the bottom of the float rod, twisting the tube with the float at the bottom turns the float. I did a write up on rebuilding sending units, there are some close up pictures in the write up that display this well. 

■ With your advice, I was able to remove the aluminum tube without straining the resistance wire.
■ Thank you, BarrettN

■ I removed the resin plate at the bottom of the sender unit to remove accumulated dirt.

 

FAQ-001.jpg

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