The Game Is Afoot

[Dark61]

1957 petrol. Assuming (never assume) now negative earth, as earth strap goes to block, positive off to starter.

At some point someone has changed the gauge e to f and vice versa. Notwithstanding, the needle sits on “e” constantly, whether the power is on or not. Manipulation of the sender makes no difference.

I dont see how changing from positive to negative earth would have changed the way the fuel gauge would operate, but I am no electrician.

Any comments gratefully received as usual.

cheers,

D

[Xtreme]

I'm not an electrician either but have you checked the electrical connection/s from the sender to the gauge? Maybe a short circuit somewhere.

[Dark61]

Thanks Roger. I have eyeballed it only. It starts off yellow and ends up pink by the time it gets to the gauge! I should at least do a continuity check I suppose. I’ll dig out some extensions for my multimeter. I ought to teach myself how the thing works as well, that might be helpful!

cheers,

d

[JohnboyLandy]

If your multi-meter has a resistance measurement, then maybe pull the sender out of the tank, connect one multi-meter lead to the connector and one to the metal flange, and move the float arm, you should see some change of resistance.

The gauge is just a coil wrapped around a metal strip, and when current runs through the coil the metal strip bends and moves the needle. So you can measure the resistance of the fuel gauge, and from memory it should be about 40ohms. Polarity doesn't matter.

Also, not sure about a '57 model, but on later 2/2a/3 the fuel gauge (and temp) gauge run off 10V from the "Voltage Stabiliser". The voltage stabliser needs a load (fuel or temp gauge) to operate, so you can only measure that voltage with one or two gauges connected.

It might also be worthwhile disassembling the gauge, and gently check the needle movement.

The only reason I can think that the E and F were swapped over is that someone has replaced the original internals of the fuel gauge with another gauge internals, temp perhaps ?

Cheers,
John

[Dark61]

Thanks John. Interesting re the e f amendment.

i took the binnacle off this evening and the wire supplying the current? to the gauge exists for only about 2 cm or so from the terminal at the back of the gauge, its been chopped. Looking at the wiring diagram it needs to be connected up to a fuse somewhere, i think shared by the windscreen wiper. I dont see a voltage regulator.

Thanks for the tip re the sender and resistance.

It sounds iffy to me putting a current to something that sits in petrol though?!

cheers,
d

[Aaron IIA]

The gauge in question is not a heated bi-metallic strip design. It is an opposing coil design, used in a SI and early SIIa (metal spoke steering wheel, amp gauge). This is the gauge that moves instantly. At rest, gravity pulls the needle to the bottom. When supplied with power, one coil has full battery voltage applied, lifting the needle to the top. The second coil is earthed in series through the rheostat in the fuel tank. When the tank is full, resistance is high. This results in minimal current in the second coil, so the needle goes high. When the tank is low, resistance is low. This results in maximum current in the second coil, which pulls the needle back down again.

To test the gauge, supply it with power, with the wire to the sender unit disconnected. The needle should go high. Supply it with power with the wire to the sender unit earthed out and the needle should remain low. The gauge needs to be connected to earth at all times.

With regard to the sender unit, it will show a high (but not infinite) resistance between it's terminal and earth when the tank is full, and nearly zero resistance when the tank is empty.

There is no voltage stabiliser in this system. It does not matter whether it is positive or negative earth. As the gauge works by two opposing coils, it remains accurate as battery voltage changes.

The gauge in a late SIIa (plastic spoke steering wheel, temperature gauge instead of amp gauge) is the heated bi-metallic strip design. This is the gauge that takes a few seconds to move up to it's position when energised. The heater wires in this gauge are in series with the rheostat in the sender unit in the fuel tank. When the tank is low, resistance is high. Minimal current flows through the circuit, leaving the needle low. When the tank is high, resistance is lower. More current flows through the circuit, warming the bi-metallic strip and raising the needle. Do not test this gauge by earthing out the sender unit wire for more than half a second, as you could burn out the gauge in a short period of time.

This gauge system is dependent on a specific voltage to be accurate, so uses a voltage stabiliser unit.

The two different gauge systems use opposite style sender units. If your gauge has had the full and empty symbols reversed, it could be possible that you have a sender unit that is meant for a heated bi-metallic strip gauge used on an opposed coil gauge. Test your sender unit to see which style you have.

Aaron

[JohnboyLandy]

Good to know Aaron, I'm used to the bi-metallic strip design, and also good to know about the lack of "Voltage Stabiliser" on these older gauges.

From what you said, the senders for these two types of gauges work in an opposite manner to each other i.e. tank full=resistance high vs tank full=resistance low.

After reading your description about the two coil gauge, I had the same though about Dark's E/F gauge modification being due to the wrong sender type !!

Cheers,
John

[Aaron IIA]

The senders for the two styles of gauges do indeed work in the opposite manner to each other.

The two coil style gauge is more accurate and more precise, but it does bounce around a bit as the fuel sloshes around. It is a more expensive and delicate gauge to manufacture.

In contrast, the bi-metallic strip gauge does not bounce around, as it relies on the slow acting bi-metallic strip to heat and cool. It is affected by changing voltage and temperature. A voltage stabiliser will look after a changing voltage. If it is an old style mechanical voltage stabiliser, it will to some extent compensate for temperature change too. It is an easier and cheaper gauge to manufacture.

Aaron

[Dark61]

Is it possible for one of the coils to become magnetized? I say thus as when at rest/ no power, the needle is at the top.

if i apply power with the sender lead disconnectedat the tank the needle hits the bottom of the gauge.

if i apply power with the sender lead not connected to the tank but earthed, the needle stays at the top, unless you give the gauge a few flicks, and then it falls to the bottom. Turn the power off and the needle hits the top again. Turn the power back on, the needle hits the bottom.

Apply power with the sender lead connected to the sender at the tank and the needle hits the bottom of the gauge.

Manipulation of the sender does nothing to the gauge.

The body of the gauge is earthed.

I need to teach myself how the OHMs setting works as on my first attempt, the reading was flying about.
cheers,

D

[Dark61]

Taught myself to use the multimeter on OHMs over the weekend. Getting no reading on the old sender at all. Tried on a new boxed i bought for the diesel but never fitted and got 25 “empty “ and 0 “full” .
i still dont understand why i’m getting no reading from the sender or why the gauge sits on “full” even though there is no power to it , but there you go.

Cheers,

d