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deschodt

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

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(edited)

Good cars die in your driveway. My  2002 is not a good car, it is a *great* car. It's only bested lifetime by an old 911SC I used to own who carried me for 15yrs and  200K miles without a single side of the road breakdown... One corroded fuel pump fuse, one time, in the garage... 

 

I digress, I'm a genius (not). The 2002 is a fuel injection conversion and started to drive weird, missing, etc... I got home up a hill no less, next day it took me 6 or 7 attempts to back out of the sloped driveway, 2 ft at a time - run great, stall, etc... Clearly fuel delivery. 

 

The MSD pump was LOUD so I immediately ordered a new one plus a new fuel filter. Only to find out MSD pumps cannot be sold in california, because... I really don't know, certainly some BS law that should not apply to a generic part used in  a 1973 car which is pre-smog and that I am free to modify. Ordered one on ebay instead, while deploying a finger of my choosing to the state. Today I decided to take a closer look and pulled the inline Mann fuel filter, which immediately shot fuel 6ft in the air (pressurized).  OK, so not the filter...  Trying the pump: I had my kid turn the ignition and observed a dribble of inconsistent fuel out of the pump... Hmm, possibly the pump then. 

 

I then decided to be thorough and obverse pickup so I removed the column thingie that picks up fuel, and laughed... The tank is absolutely empty... Forget that the gauge indicated 2/3 full. EMPTY. There you go, problem solved ! Cavitating pump = loud pump. I'm not cancelling the order, just because California pissed me off... good to have a spare before they start scanning packages too. 

 

So the question remains, how to I reconcile this EMPTY state of affairs with 2/3 full indicated on the gauge ? Defective gauge ? Wiring suggestions ? new sender ?  I should pojnt out grounds aren't the issue, I regrounded my cluster recently and the needles do not move around, temps or gas... Can you calibrate or just buy a new sender ? 

Edited by deschodt

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you said that you attempts to back out of the sloped driveway

 

are you sure all of the gas wasn't at one end of the tank so it couldn't be picked up...just grasping here

 

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Hi,

 

I'd start by seeing what the resistance swing is as you make the sender go through full travel.  Perhaps it's sticking part way and not dropping as the fuel level falls away beneath it.  I'm sure someone on here knows what the min and max resistance is supposed to be.

 

If that passes muster, time to check the gauge by connecting a known resistor close to the gauge to see if it reads full scale.  The calibration ought not to change, but you never know.

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I once calibrated the one in an old VW. I bent the arm down until it just hovered above the bottom of the tank (like by 1/2 a mm).  These are a tube apparatus with a float in the middle.  If all other electrical is ok, I would suspect that it may be hung up.  I suspect it looks something like this, which is out of a 740. image.thumb.png.6907ff3c16efef57b0a6dd4fb4000366.png

 

 

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Consulting my good ole BMWnospaceCCA Tech Tips book from 1975, the writeup on the 2002 fuel sender shows that with the float at the top of its travel (full tank) an ohmeter placed across the two contacts should read essentially zero ohms, and with the float at the bottom of its travel (empty tank) the ohmeter should read 75 ohms.  It's linear, so at midpoint the reading should be 37.5 ohms, etc.  

 

Those hair-thin resistance wires that the float contacts get coated with crud and varnish--especially on a car that sat for some time with little fuel in the tank.  They can be cleaned (VERY carefully) with carburetor cleaner and a Q tip.  

 

I did a column on fuel gauge sender unit repair; if you can't find a reference in the FAQ archive, drop me a PM and I'll send you the column.

 

mike

 

PS--don't break the wires; it's hard to find replacements with the same resistance per inch.

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Try rotating your sender several times to get the float to slide from empty to full. This will "polish" the wire and remove any debris that could cause the float to stick.

When the tank is full, the resistance to the fuel gauge is about 4 Ohms. When empty, the resistance is about 80 Ohms.

As Mike said, take a known resistor (say 25 Ohms) and connect it between the Brown/Yellow wire to the sender and ground. Turn on the ignition, and you should see the gas gauge read about 3/4 full.

A 40 Ohm resistor will indicate about 1/2, and a 60 Ohm about 1/4, etc. 

Make sure the brown ground wire on the sender is clean and making a good ground (in addition to checking the ground on the instrument pod).

I added a second ground wire to the sender, and this cured the gas gauge from bouncing whenever I used the right turn signal. (Right tail light assembly shares a common ground with the fuel sender).

Hope this helps.

John

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Conveniently (or not) the sender works the wrong way around compared to all aftermarket gauges and senders. 

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I disassembled mine and it was relatively clean.  No crud. It looked great. I re-assembled and it still gives a lopsided reading. Very small readings when the tank is full. I get 100 miles in the first 1/4 tank.   At half a tank the gauge plummets.

 

I did not clean the wires.  I'm going in again.

 

 

 

 

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(edited)

FWIW - the increments on a 2002 fuel gauge are not linear. The 3/4 mark is near the full mark and the 1/4 mark is near the empty mark... so the first 1/4 seems like it lasts forever and the second half of the tank seems like it disappears quickly. I agree that you should check resistance values on the terminals on the sending unit with the float in both full, half, and empty positions. If it's a brand new sending unit (or one installed previously by someone else, recently), someone may have neglected to remove the set pin / thin wire that holds the float stationary during shipping. I've seen it happen. It's an easy mistake to make. It's a thin wire that clips on the sending unit and has a portion that inserts in to a tiny hole in the side of the silver cylinder. If you choose to disassemble it (remove the cylinder) be careful and treat it like micro-surgery. The inner wire(s) are delicate. You can clean them gently with solvent / lubricant.

Edited by wegweiser
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I vaguely recall many years ago I accidentally reversed the electrical connections to the fuel sender (standard, no fuel pump) and the fuel gauge remained stuck in the 3/4 position.  Don't know if this is your problem, but I thought I'd share the thought.

 

Jeff 

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6 hours ago, wegweiser said:

The 3/4 mark is near the full mark and the 1/4 mark is near the empty mark... so the first 1/4 seems like it lasts forever and the second half of the tank seems like it disappears quickly.

 

Are you saying that all new 02s out of the showroom were like this?  Can those that bought their cars new qualify this statement?  If it's true I'm not going to mess with my sender anymore.

 

You'd think that the electrical engineers at BMW would not accept this vague performance.  I'm having a hard time believing they'd let that pass.

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6 minutes ago, PaulTWinterton said:

Are you saying that all new 02s out of the showroom were like this?  Can those that bought their cars new qualify this statement?  If it's true I'm not going to mess with my sender anymore.

 

You'd think that the electrical engineers at BMW would not accept this vague performance.  I'm having a hard time believing they'd let that pass.

Paul, that's been my experience.  The gauges on both my '69 and '73 take forever (200+ miles on the highway) to get down to perhaps 3/8 of a tank (between half and quarter) then drop precipitously down to the red mark--often in 30-40 miles.   I've observed the same thing on other 2002s over the years, so that's at least a small sample reflecting that behavior.  

 

 I think part of the problem is that flat tank, and the fact that the sender unit is inside an aluminum tube, which keeps the gauge from jumping all over the place as gas sloshes around in the tank.  Ever notice how long that tube takes to drain when you remove it from the tank?

 

Of course you could easily recalibrate the gauge to reflect actual fuel levels by repainting the marks on the gauge face.  I've just learned to live with it--and I reset the trip odometer whenever I fill the tank.  That way I know for sure.

 

mike

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Yep. Look at the white lines on the gauge below full and above empty. I wouldn't lie to you. 

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It should be relatively easy to determine this non-linear behavior.  Grab a handful of 10 ohm resistors and connect them to the sender connector that leads to the gauge.  That way, you will be able to vary the resistance in multiples of 10 ohms and see if the gauge behaves linearly or not.

 

That would actually be a very useful piece of information - what do the white lines correspond to in terms of ohms.

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