My fuel sender is stuck at the 1/4 mark. Shorted out the terminals and the gauge reads full scale. This tells me problem is with the sender. Question: Do I need to empty the tank to remove the sender unit? I see some old posts saying something about a chamber but don’t know what that is.
Pat H
The sender is on the side of the tank. So as long as the fuel level is below the sender, you should be OK.
Hmmm…you may well be right about the sender but if the number on your gauge starts with FG or X shorting out the sender connections should make the gauge read below zero, not full scale. With the connection to the sender open circuit, this type of gauge will read above full.
However, if what you did was to short the green/black and green/yellow wires you haven’t actually shorted the sender connections because there are two separate circuits, one for the gauge and one for the low level warning light. Shorting the green/black wire to earth, effectively short circuits the sender and if working correctly the pointer should go hard to the empty end stop. Shorting the green yellow wire to earth should light the low fuel lamp. If you connect them together, you’re just open circuiting the gauge since both rely on an earth return so neither circuit is complete.
Depending on the age of the sender, you may find the float has some fuel in it or the arm isn’t free to move its full range for some reason.
Understood. I shorted the two terminals on the sender thus making its resistance zero. The point of the test being to determine if I had a wiring issue (no) or a gauge issue (no). The gauge pegged right with a red light.
Pat H
Hi Pat,
The sender float arm can get frozen: rust, gum, etc. I have on one occasion fished it with coat hanger wire from the filler. Once it moved it freed up and has been fine since.
Regards,
Clive.
With this type of gauge, the lower the resistance the less the gauge should read. I know it’s counter intuitive but that’s how it works. Zero ohms results in a sub empty reading. If what you did was to connect the two terminals on the sender together, you actually connected one end of each of two completely separate circuits. In the sender there is a variable resistance, one end of which is connected to earth via the case and the other comes to a terminal that has the green/black wire. There is also a completely separate simple switch one side of which is again connected to earth via the case and the other goes to the other terminal on the sender. The contacts of the switch are closed when the float arm gets down to a predetermined level.
A lot of confusion arises because it’s easy to think that the two terminals are both part of the gauge ie both ends of the rheostat but this is not right.
Pat,
Just to add a bit more explanation, the reason that the gauge reads full and the low fuel lamp lights is because shorting the two connectors on the sender makes a circuit between the 12 volts that is applied to the lamp through the bulb, then through your short circuit, back to the T connection on the gauge and then to earth through the Full side coil in the gauge. Although you still have the normal 12 volt that goes from the B terminal to the Empty side coil in the gauge, the magnetic field will be stronger on the Full side because of the difference in the resistance of the coils allowing more current to flow on the Full side.
If this is too much information, I suppose you could say that you got the right answer for the wrong reason.
No. Thank you very much for the information. I would never have thought something as simple as a fuel gauge would be so complicated. At least I know the problem is with the sender. Need to pump out some gas and remove it for r&r.
Pat H
Here is the best explanation I have seen describing the Smiths fuel gauge operation: http://mgaguru.com/mgtech/electric/fg_05.htm#fg_diag.
That site has other pages detailing repair of sender unit and gauges, plus lots more besides!
Regards,
Clive.