Fuel gauge

[TiggerToo]

How can I test whether my fuel gauge is working properly? It shows "full-scale" (actually it is well past "full") and I want to know whether it is a problem with the gauge or with the sensor in the tank?
It is a VDO unit.

 

[FullCircle]

I did this to mine after my ran out of fuel fiasco earlier this year.

When they replaced the sender unit under warranty, they did not adjust it to my tank, and therefore I ran out whilst still showing 1/2 full.
I removed the sender and noted that the float on the extended wire was too loong and therefore hit the bottom of the tank before reaching 1/2 way on the sender column.
The only remedy I could effect was to shorten the wire as much as possible, and bend it up so it only touches bottom of the tank when fully deflected on the sender column.
The fact is that my tank is not deep enough for the sender unit, so I had to compromise, see below....

As my tank was nearly empty when I did this (just the emergency 5 litres in from the running out fiasco), I went to the fuel pontoon. I checked the reading - 0 / 0
Putting in 5 litres at a time, I stopped, noted the fuel gauge reading and carried on.
Thus I now know that 10 litres is 1/4
20 litres is 1/2
40 litres is when the gauge starts to show 1/1 - i.e. Full.

My tank is 126 litres, so I only know that when my gauge dips below 1/1 then I have 40 litres or less to get me home.

As I said, a bit of a compromise, but it is just another job I will get round to when I fit the second fuel tank.

 

[TiggerToo]

I am sorry if I did not make this clear: but how can I tell if the gauge is working properly or not? I mean the thingy at the instrument panel, not the actual emasuring device in the tank.

 

[FullCircle]

tigger,
does it go to zero with the electric switched off, or is it full time deflection?

I have a spare VDO unit we could plumb in to try if it all goes wrong.....

 

[pvb]

The problem could also be a break in the wiring (or poor connection) between sender and gauge - this could cause a full-scale deflection on your gauge. To check the gauge itself, if you disconnect the sender input and put a 100ohm resistor between the sender input terminal and the negative (earth) terminal, you should see the needle read about midway.

 

[alan]

My friend had this on his boat and it proved to be the sender unit which had the wire "coil" open circuit. I made up a little variable resistor which I connected to the gauge (meter) in place of the sender and with this I was able to prove the fault was in the sender. It is on my boat in the tool box and I cannot remember the value of the resistor but I got the info off the VDO web site regards the value and the connections. If you want I can check the value later today ........... boat is only ten mins walk away!
The effect of the sender being faulty (open circuit) is to give a full scale reading (FULL) on the gauge !!!!!!
Also as PVB says it could be just a poor connection.

Alan.

 

[TiggerToo]

if it is switched off, it goes to zero.

 

[alan]

Following-on from my earlier post, I used a 0 to 470 ohm variable resistor to replace the sender in order to check the gauge. Wires were connected to centre pin and one of the outer pins on the variable resistor. The var resistor was obtained from my local electronics shop and you should be able to get one from Maplins or similar. You could use a fixed resistor of around mid range (e.g. 270 ohms, 1 watt) which will give you an on scale reading.

Alan.

 

[pvb]

[ QUOTE ]
Following-on from my earlier post, I used a 0 to 470 ohm variable resistor to replace the sender in order to check the gauge. Wires were connected to centre pin and one of the outer pins on the variable resistor. The var resistor was obtained from my local electronics shop and you should be able to get one from Maplins or similar. You could use a fixed resistor of around mid range (e.g. 270 ohms, 1 watt) which will give you an on scale reading.

[/ QUOTE ]Most VDO senders give a resistance of around 10ohms at empty, rising to around 180ohms at full, so you'd need something between those two extremes to get a reading mid-scale. That's why I'd suggested 100ohms.