Faulty fuel gauge diagnosis

[MisterC]

The fuel gauge on my SIII started acting up, one minute 1/2 full (which the tank was) next minute empty, fluctuating in between on occasion.  It's now fixed firmly at the bottom.  The temp gauge is working fine, so I'm assuming the voltage stabilizer is OK, since that feeds both gauges.  I used my multimeter to check for voltage to the fuel sender supply, with it disconnected from the sender, and this read a constant 11.7ish volts with ignition on.  When I connected it to the tank fuel sender the reading varied wildly from 0 to 9.5ish volts and everywhere in between.  Can I assume from this that the fuel tank sender is at fault?  Thanks in advance.

[dave_2A_2.25Turbo]

Sounds like a sound diagnosis to me - the readings should be constant at any given position of the sender

[spy]

On almost the same subject, my temp guage hasnt worked since some swine broke in and ragged the wiring behind the dials.  Whats this about a shared voltage stabiliser?  

Any chance someone can give me a monkeys guide to the wires on the back of the temp guage?  :D

[dave_2A_2.25Turbo]

Not off-hand.  Mine didn't come with one fitted at all, so I bodged, sorry, modified a capilliary type to fit.

Haynes manual?  If not I've got the wiring diagram on a CD somewhere

[hobbit]

After some problems with heating in the disco and thehybrid, the disco was solved and confirmed after fitting a mechanical gauge, to confirm the electrical one was working

On the hybrid, with the s3 electrics, more fun, the fuel gauge is shall I say not very reliable, I take to checking the tank after so many miles. The temp guage the same.

I have now had a new rad in, and the nice man at the rad place kindly fitted a different size thread union in the bleed plug hole, (tdi rad), that now has a mechanical sensor in it, so I can now get a true reading on the machine, feel a lot better, the thought of screwing up a 300tdi engine for the sake of a few quids of temp gauge.

The wiring on these machines are a pain to say the least, the voltages that control the fuel and temp gauges have always been hit or miss, I prefer not to rely on them if possible, I'm sure there are some after market tank measuring devices, one of which could be fitted

[Neil Jones]

I lost my fuel gauge also, I took out the old gauge, dismantled the sender and discovered a mash of wires.  The new sender is great, although I don't like the rate at which its going down at the moment!

[hobbit]

The temp on the series 2 1/4 engines if anything are very efficient, one of mine has no fan at all, and I only switch the fan in when running low speed, as soon as I get on open road over 40 the flaming thing cools right down, even when it was summer, in this weather it needs a rad cover to keep it warm, bit like me really

[Miniman]

I had alot of problems with my fuel guage and it all came down to a bad Earth. Check its still connected and strong. Also if you have done this and its still the same sounds like all the wires in the fuel sender have perished and needs a new one. Take the sender out of the tank and do a risitance check on it then slowly move the fuel float see what happens if its all over the place then its the tiny coil of wires in the sender that are stuffed. If it gives a good reading clean all the way then we may need to think about another solution to your prob....

[jimthelandyman]

I have the same problem with my fuel gauge but it flickers on and off when it gets below the half tank mark, its so annoying.

[MisterC]

Thanks for all the replies and advice.  I put £20 quids worth of juice in today and the fuel gauge registered around half full!  On todays rate that's in the right ball park.  When I first noticed the problem it did appear to manifest itself once it got below half full, like jimthelandyman.  Bad earth seems a likely cause (thanks Miniman), where was your bad earth, as the route to earth appears to be the fuel sander unit itself via the fuel tank?  Thanks in advance.

[Miniman]

No the two wires from the sender are both + feeds one is for the low fuel light and the other is for the sender itself. That is where I went wrong. The unit earths via the tank. The earth for the guage is a black wire which fastens to the back of the guages with the round finger nuts which also hold it in..... I will have a look for a pick for ya...

[Miniman]

Sorry I cant find a pic. But there is a chain of black wires that connect each gauge via the body of them. Check and see if this is ok. If not take an earth and twist it round one of the threads which hold the gauge in and put it to a good earth then go for a drive see if its fixed or even just pull out the sender and manualy move the arm.

[MisterC]

Thanks Miniman.  Having had the dash off several times, I know the black leads you're talking about.  Incidentally my tank sender only has one feed, it doesn't have the low fuel warning light, which (just putting the hood up on my anorak) I believe was optional.  Being colour blind, taking the dash off was a bit of a challenge when re-assembling, hopefully I can find a passing gynaecologist to get his/her fingers inside my dash without disconnecting too many of those wires (whatever colour they are).

[hobbit]

Safest way is to take some pics as soon as you can when popping the unit away, at least it will give you some idea what is what.

The last person who worked on the dash on the hybrid used white masking tape and felt tip to mark some of the wires  :?