Fuel gauge

[Reedy1]

Can I cure an erratic fuel gauge? Would it be a fuel sender jn tank or the gauge at fault? Early Series 11

[mike1864]

Yes, possibly and possibly. Or the voltage regulator. Or the wiring.
Can you expand on "erratic"; just wobbly, or flicking from one end to the other? Or giving different steady readings at different times?

[geoberni]

Early Series 11

Early cars, up until the Black face Speedo circa 1964, the fuel gauge was a Moving Iron type. Don't worry about the technical aspects, but the key features are that it
A - reads fuel level immediately
B - due to point A, it can move around 'erratically' especially at lower fuel levels, as it responds to the float moving up and down when fuel sloshes around in the tank.
C - It is fed directly with 12v, or whatever the battery/dynamo/alternator is providing. There is NO Voltage Stabiliser.

Important Note: People in the Classic Car world often mix up the terms Voltage Regulator and Voltage Stabiliser which can get confusing. The first is the Bakelite Box under the bonnet controlling the Dynamo, the second is a small device, looking like a Relay Can, attached to the back of the Speedo (post '64) which provides stabilised 10v required by latter Smiths gauges.
So I'm not sure which Mike is referring to here, but I suspect he means Stabiliser, especially if he has a post '64 with no experience of earlier cars.

Or the voltage regulator.

The later gauge post 1964 is a Bimetal type gauge, which will slowly creep up from zero when switched on and will not respond to fuel sloshing around because the gauge is dependant on the working heating and cooling in response to the varying voltage.

[jaekl]

If the reported flicking is drastic such as actual to empty and back as opposed to a few degrees showing the sloshing of the fuel is most likely the sender unit on the first design of gauge/sender. It could be just a matter of dirty spots on the coil or the contact fingers. Try cleaning the sender.

I think the later design of gauge/sender was introduced just before the black face speedo, mid 64. I think it's related to the introduction of tenths in the odometer. The speedo face was more silverish, definitely not black. It was still 64 but earlier than the black instruments.

[mike1864]

So I'm not sure which Mike is referring to here, but I suspect he means Stabiliser, especially if he has a post '64 with no experience of earlier cars.
OK, "stabiliser". I have a mixture of speedos and cars. But it's the OP's symptoms we still need to see. And - like me - has he or a previous owner changed the speedo? Incidentally, can someone confirm whether the sender module is the same, regardless of the gauge technology? I vaguely recall the resistance change works in opposite directions?

[geoberni]

So I'm not sure which Mike is referring to here, but I suspect he means Stabiliser, especially if he has a post '64 with no experience of earlier cars.
OK, "stabiliser". I have a mixture of speedos and cars. But it's the OP's symptoms we still need to see. And - like me - has he or a previous owner changed the speedo? Incidentally, can someone confirm whether the sender module is the same, regardless of the gauge technology? I vaguely recall the resistance change works in opposite directions?

I wish you had 'quoted', simply because I didn't get notification that you had.
The sender is different, and as you say the variation in resistance is the opposite.
As I recall, the original ones were something like Empty 3 Ohm, Full 75 Ohms.
The Bimetal ones are Empty (element cold) 260 Ohm and Full (element hot) 18 ohm.

[mike1864]

When the OP gets back to us to detail the nature of the erratic readings, can he/she indicate whether they have access to a Digital Voltmeter (DVM). And the knowledge to use it! A "yes" to both will help inform the diagnostic path.