Fuel Gauge not working : gauge or sender fault?

[P27]

The petrol gauge on a 1937 p2 is not working. When the ignition is switched on the needle moves very slightly ... but then stays there irrespective of amount of fuel in the tank.

I don't have a wiring diagram - and both manuals are out of stock.

How do I determine whether it's the gauge, wiring, or sender unit.

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[SHyslop]

I would guess at a stuck sender. You can check it before taking it out by disconnecting the cable at the right hand end of the sender unit and connecting that cable to a good earth (clean metal). The gauge should show empty or full when connected or disconnected. The arms stick at the pivot point and it can take quite a bit of penetrating oil and gentle pressure to get them to move without the arm breaking in the casting but it is worth saving them. If you undo the 6 screws holding the sender unit in, again carefully, because they can snap so use a sharp screwdriver that fits well and unscrew a turn or two, apply oil and then tighten then slacken, keep doing this until the screw comes out nicely. Of course, there is another obvious way to test the gauge on these cars and that is to press the oil level button and see what reading it shows!!

Once you have the sender unit out, check that the float is not leaking. If it is, new plastic ones can be bought to replace the originals. Once moving well, check the wiper arms in the resistance unit are making contact with the windings (remove four screws in the top cover, try not to break the gasket if possible or make a new one) and then if you earth the body of the unit to the car, and reconnect the cable to it , by moving the float up and down, the gauge should work. If is doesn't, it's probably wiring or the gauge but at the stage you are at, the sender unit would seem the place to start. There are compatibility issues between newer gauges and senders, and older gauges and senders. Later petrol gauges from the 60s onwards use a power supply that has been through a voltage stabiliser so the reading isn't instantaneous as on these but "smoothed out". Should that route be suggested to you because of components on the car that are beyond repair (we'll hope not) the one thing to note is that the majority of voltage stabilisers are negative earth but they are also made specifically for positive earth vehicles so just something to be aware of.

[P27]

Brilliant thanks.
But I'm confused with your " ... and that is to press the oil level button and see what reading it shows!!..."

Oil?, button? What/where - tell me more please.

[SHyslop]

On the dash beside the petrol gauge there should be a black button. When out (normal) the petrol gauge shows the petrol level in the tank. When pushed right in, the level of the oil in the sump is shown instead .The button is spring loaded to return to the petrol level after use. Provided it hasn't been disconnected or modified, the oil sender can work longer than the petrol one because the arm is well doused with oil (we hope!). It's the button furthest to the left on the instrument board and the gauge should be marked "Petrol and oil- Push Switch for Oil Reading".

[P27]

Thanks. I will look next time I visit the car (mid April)

[David2021]

I have a not dissimilar problem! But at the other end! I have no wires from the oil sump sender! And It is all VERY oily down by the sump.
Petrol gauge works fine!
I will clean the olly sender contacts, set up some cables to the push switch and see if I get any readings. Or how do I check it before doing this?

[SHyslop]

I think the following is what we know:
1) Your Petrol gauge works therefore the gauge and wiring to it all work .
2) When the gauge is connected to no earth, it will show full
3) When the gauge is connected fully to earth it shows empty.
Therefore:
4) When the gauge is connected to a working resistance it will show full when the resistance is fully in the circuit and empty when the resistance is least in the circuit.

If you disconnect the connection going to the petrol tank sender unit at the tank and then connect a long piece of wire to that lead, take it to the front of the car. Connect the end of the wire to an earth at the engine The gauge should show empty when switched on.
Now connect the wire to the sump sender terminal. It should give a reading which approximates to the level of oil on the dipstick. If it doesn't and the earth to the sump sender is good, there is a problem in either the sender or the wiring.
To eliminate the wiring from suspicion, connect the end of the wire you have been using to the detached sump sender cable. Go to the back of the car and now disconnect the connection you made to the petrol gauge cable and connect your long piece of cable to the petrol gauge sender.
Pressing the "push for oil" button should now give the same readings as you had with the petrol gauge to start with. If it doesn't, there is a fault in the wiring for the sump sender or on the connections on the changeover switch.
If the switch connections are A (to the gauge), B (petrol) and C (sump), mixing up A with B will cause C to not work.

[David2021]

That is really helpful!
I can’t get at it for a few days,but I will do just that…eliminate things one at a time
D