[s-cars] Fuel guage level - sending unit questions

[Dave Ellis]

I had this same problem on my '99.5 A4 recently (which seems to be much 
more common much sooner in the car's life on the newer Audis).  The 
problem is usually one of three culprits:

  1). frayed wire on sending arm
  2). dirty contacts on sending unit
  3). bad ground from sending unit

I haven't yet had the problem on my '93 S4, but I expect it to show up 
sooner or later as almost all the Audi models seem afflicted with some 
form of this problem eventually.

When I removed the sending unit the exposed contacts on the sending unit 
were quite filthy, but the actual problem was that the flexible wire 
braid connecting the sending arm to the rest of the unit had fatigued 
and broke.  After soldering on a new braided wire (from the back of an 
old blown woofer/speaker) and cleaning the contacts everything is back 
in order again.

A frayed wire will *usually* result in the fuel level reading fine until 
about three quarters to half a tank, at which point it will start to 
intermittently read empty for a short while (as the ends of the wire 
intermittently make contact as the fuel level arm lowers), and shortly 
after will always read empty as the ends of the broken wire loose 
contact as the arm lowers even further. 

Dirty contacts usually result in the fuel level consistently acting 
weird (read lower than it is or close to empty) at one (or several) 
points, with it often returning to the correct level once the fuel level 
goes down some more.  This cycle can often repeat itself and seems to 
differ quite a bit from car to car (model to model?), but it's tel-tale 
signature seems to be reading wrong consistently at the same fuel levels 
from tank to tankful of fuel.  Someone on the Audiworld A4 (B5) list 
wrote that he'd contacted AoA and was told that dirty contacts on the 
fuel sender was a common problem caused by a particular type of fuel 
additive found in North American fuel that was the culprit.  I'm not 
sure how accurate this is, but it would explain why very few Audi 
European owners have reported dirty contact problems.

If the problem is a bad ground, then the fuel readings indicated by the 
guage tend to be truly random and fluctuate quite a lot, or the quage 
just reads at or near empty most of the time.  There was a TSB regarding 
this problem a while back for our cars.  This problem seems to be much 
more rare than the first two however.

As Chris Chambers noted in his email, the fasted and easiest way to 
check is to pull the sending unit and take a look at the flexible wire.  
If it's broken, replace it.  While it's out clean the contacts for good 
measure.  Usually one of those two items resolves the problem.

There's a good write-up on this issue on the web somewhere for a B5 A4 
model (it's pretty much the same procedure for our S4/S6) though I can't 
seem to find it at the moment.  I'll post it if I find it, but perhaps 
another lister might know it also.

If you do need to repair or clean the sender, or even if you just wish 
to verify the operation of the sending unit separately from any other 
car wiring, you can check it's function by measuring the resistance 
across the two contacts of the sending unit (unplugged from the car of 
course) while moving the sending arm up and down with your hand. 

At one end of the scale the resistance should be around 50-60 Ohms and 
at the other near infinite.  Unfortunately I can't remember which end of 
the scale corresponds to which resistive value at the moment, but if 
you're in this range and the values change smoothly and consistently as 
you move the sending arm from stop to stop you should be good to go.  If 
after this test it checks out fine but the problem appears again after 
re-installation, then it's likely there's either a grounding problem, or 
the fuel guage itself is buggered.

Best of luck!
-Dave



Thompson wrote:

>Just took a 900+ mile drive from VT>PA>RI> back to VT and in
>that time of a few fill ups our //S6 decided to only show 3/4
>full on the guage after topping up with petrol. Drats. I
>thought that it might have been the folding beach chair that
>lays down perfectly in the luggage spot in the back that may
>have fritzed with the sending unit wires, but everything looks
>fine. Just turning 150k miles.
>
>Collective //SGeniuses, what should I look for?
>- Guage calibration?
>- New fuel sending unit?
>- How much and where to purchaseÐdealer only?
>- Should I install a new FP while I'm at it?
>
>I've got the Bentley CD so I'm going to dive into this tonight.
>Please advise.
>
>Thompson
>1995 //S6 (only 3/4 there...)
>1985 Porsche 944
>1996 Tacoma 4WD beast
>