Petrol gauge

[Hobbo]

Hi my petrol gauge is reading empty, even when petrol is added. Have removed wire from sender unit on tank and earthed it on chassis, the gauge should have I thought flicked over to full no movement at all. Removed petrol gauge from facia and found gauge had three wires to it, one on central fixing pin, one to tank and one to I presume live. The car has been changed to negative earth before I got it would this make any difference or do I need a new gauge. Thanks Mike

[GK5268]

if yours is of the "moving coil" type set-up, instructions on how to test both the gauge & sender are here

[Derek Sheldon]

A nice piece of hard wood stick works wonders. I also use the mileage gauge to good use, bugger when the speedo broke on my last trip to southern France.

[Hobbo]

Thanks for the answers so far, forgot to mention the vehicle is a Big 7.

[Colin Wilks]

Not sure if this is relevant to Big 7, but the Cornwall Austin Seven Club has a good article here:
http://www.austin7.org/Technical%20Artic...20Working/

[John Cornforth]

Hi Hobbo

I am pretty sure that your car uses the Smiths moving-iron type gauge, as used in the Austin Seven.  Within the gauge there are two fixed electromagnet coils wired in series.  Battery voltage is applied to one end (B Terminal), and Earth to the other end (E Terminal). The junction between the two coils is brought out to the Tank sender (T terminal). So there are three terminals.  Changing from Negative to Positive Earth doesn't affect the operation.

In use, the tank sender acts as a variable resistance to ground in parallel with one of the coils.  When it has a low resistance, it increases current in one of the gauge coils, and decreases it in the other.  The change in magnetic fields causes a piece of soft iron attached to the needle to swing round and thereby change the reading.  The sender resistance range is likely to be from a few ohms at one end of its travel to some tens of ohms at the other end. It is likely that maximum resistance corresponds to a full tank, but I can't be 100% sure of this.

I suggest you check that the gauge Earth terminal is indeed connected to earth, and the Battery terminal is getting battery voltage.  Once this is OK, disconnect the wire to the Tank sender at the gauge end, and try the gauge with both an open circuit and earth on its T terminal.  This should make the needle swing from one end to the other. Normally gauges are reliable (unless they have been "got at") and it is the Tank sender and its wiring which give trouble.

Cheers, John.

[Hobbo]

Hi thought I would give an update, tested and put live to the B terminal with T terminal with no wire reading full all the time, must need another gauge. Thanks for all the valued help

[Douglas Alderson]

When I got my special the fuel gauge did not work, I discovered that the sender unit was stuck, this was replaced with a working one and the gauge replaced as this was also faulty, I also installed a resistor as the car is on 12v.  So 5 years later the fuel gauge is reading empty all the time. 

Checked the sender unit, this is working ok.
Checked the wiring this is all ok.
Resistor is ok.

So I guess the gauge is dead although in testing the gauge it will shoot up to full.  Any ideas?