Fuel tank indicating empty when there is plenty of fuel in the tank.

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silver4444uk
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Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by silver4444uk »

My daughter's 1.0 AUC 2000 Polo is reading empty at all times on the Fuel guage. the amber low level petrol pump symbol is not lit.

I have tested it with Vagcom and it shows the following fault

DTC 00771 - Fuel Level Sensor (G)
30-00 Open or Short to Plus

i have carried out some wiring checks using a Haynes Manual ( I hope the diagram is right ) and found the following:

0.3V supply at pin 2 on the fuel guage sender when the ignition is switched on.

When a resistance check is carried out between pin 3 on the fuel guage sensor and pin 7 on the blue plug to the instrument cluster there is a high resistance of 4 Megaohms.
However when a resistance check is carried out between pin 2 on the coolant temperature sensor connector and pin 7 on the same blue plug to the instrument cluster the resistance is 1.2 ohms.
Also when a resistance check is carried out between pin 2 of the expansion tank coolant level sensor and again pin 7 of the same blue plug to the instrument cluster the resistance is also about 1.2 ohms

Does anybody know where the three brown and white earth wires are connected / spliced together into one brown and white earth wire as shown in the Haynes manual ???
As I suspect there is a high resistance between pin 3 on the fuel guage sender connector and wherever they are connected together.

Any help / directions / corrections would be much appreciated.

Rgds.

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silver4444uk
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Re: Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by silver4444uk »

Also if someone has access to the proper VW wiring diagrams
I would appreciate it if they can check the wiring checks I have carried out so far.
As I have been caught out by Haynes manuals before.

Thanks

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silver4444uk
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Re: Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by silver4444uk »

To prove the Haynes manual wiring, I disconnected the connector from the tank sensor unit.
Put a wiring link between pin 2 on the connector and pin 2 on the tank sensor unit.
I then connected pin 3 on the tank sensor unit to battery earth.
Turned the ignition to "ON" and the FUEL guage indicated correctly.
So the Haynes manual is correct.

So if anyone does know where the three earth wires from the fuel guage sender, coolant temp sensor and expansion tank coolant level sensor
are spliced or connected together into one earth wire that goes to pin 7 on the blue plug on the instrument cluster, I would be much obliged.

Otherwise I will have to connect it to a local earth as a temporary measure.
The guage will work but I would like to do the job properly and find the fault.

Rgds.

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937carrera
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Drives: 2000 Polo GTi
Location: North Yorkshire

Re: Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by 937carrera »

I spent some time trying to diagnose a potential fuel gauge problem on one of my GTi's.

It turned out that the resistor wires in the tank were a little contaminated giving false readings, there was no issue with the wiring..

What you should do before going any further is to do the instrument check in VAGCOM, just to make sure that it isn't the gauge that is at fault. Take a look here

http://forums.vwvortex.com/showthread.p ... -a-VAG-COM

The method for testing the operation of the low level sensor involves putting 6 litres of fuel in the tank and checking the resistance which I think should be around 100 ohms (but don't take my word on this). You can adjust the position of the needle / warning light in Vagcom if you wish
silver4444uk
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Re: Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by silver4444uk »

937Carrera,

Thanks for the reply.
I could not find the splice.
I eventually got it fixed by putting in a temporary earth.

Rgds.

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chill
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Drives: 2000 6N2 1.0 MPI
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Re: Fuel tank indicating empty when there is plenty of fuel in the tank.

Post by chill »

I know this is an old thread, but I thought I'd post my solution to this problem.

I tested the sender earth at the fuel pump plug, and it measured approx 53 ohms to a good body earth. So this meant whatever the sender resistance (between about 70 ohms empty and 300 ohms full), the wiring circuit was adding 53 ohms, meaning the gauge was always reading a lot higher than it should.

I remembered reading this thread, so I didn't bother trying to trace the sender wiring. The pump earth measured virtually zero ohms to a good body earth, so I removed the pump and bridged the pump earth to the sender earth with a small soldered link, as per the photo. The pump earth is a fatter cable than the sender earth, and since the sender circuit only carries a couple of hundred milliamps at most, I figured the pump earth could cope with that extra current. The gauge now properly reflects the float level when tested out of the car.

Image

I left a little note on the pump cover plate, so that the next person who has to change the pump knows about this mod, and knows that there's still an unresolved problem with the sender earth wire in the vehicle loom.
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