Fuel Gauge - odd readings

[Jolls]

Hi Team,

Just started the restoration of my FC Ute and my evening tasks include cleaning and testing all of the electrical components. I'm a sparky by trade so understand the DC fundamentals but lack depth on the instrumentation side of things.

Any way I have pulled and cleaned the fuel gauge and searched the forum to find the supporting onformation needed to test the gauge. In short positive to the threaded side of the unit, via a resistance to replicate the sender, and the bullet style termination to earth. The resitance values I have replicated is 0 ohms - empty; 15 ohms half a tank and 30 ohms full tank.

The readings I am getting are quite odd. O ohms - Empty but as the ristance goes up the needle indicates "more empty" until I get to 150 ohms where there is no needle deflection (up agianst a hard stop I expect). So instead of the needle moving from empty to full it continues to move left as I step up the resistance.

Thought I would reverse the polarity to see if that made a difference - no discernable change so it is definitley not a polarity issue.

Wiring and coils look good - thoughts on where to look next?

Cheers
Jolls

[Errol62]

It is usually the sender at fault and not hard to understand why. RDS will be on here in due course and sort you out.


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[Dr_Terry]

It sounds like the terminals at the gauge are mixed up.

The gauge is made up of 2 coils, one opposing the other. So one coil has constant magnetism & the other varies to swing the gauge.

Just double check the 3 connections at the gauge. I'll have a look at one for you later in the day & check.

Dr Terry

[Jolls]

Hi Dr Terry,

Yep 100%. It is definitely a balanced coil type gauge. The needle can make a full sweep so there is no impediment to movement. When power is applied at 5 ohms the needle instantly responds to where the E symbol would be, but as the resistance is increased the needle moves below the E. I have switched the resistance to the power and ground side of the gauge as well with no discernable change. As the gauge moved in the wrong direction as the resistance increased I also changed the polarity to see if I had that incorrect and there was no discernable change in the system.

My understanding is that balancing coil gauges use competing magnetic pull from the two electrical coils to move the needle one way or the other. The difference in current flow and; therefore, magnetic pull, between the two coils is determined by how much resistance is created by the sending unit in the fuel tank. I expect that one of the coils is faulty so that when there is 0 resistance it moves to the E market but as the resistance is increased gravity is taking over and pulling the needle below E as opposed to pulling the needle to F as the field strenghts change due to the increase in resistance.

[Jolls]

I have done some more investigating and found that the circuit across the coils is complete and has a 54 ohm resistance. I will see what I can dig out in realation ot this gauge to see if this is within tolerance. I could tell from the plier marks on the chrome trim that the gauge has been apart before. The needle also looks as though it has been "aligned" previously. I have it doing a reasonable sweep now and pointing in pretty much the right direction under the influence of gravity. I have identified the two screws that hold the coild in place. I expect that if I adjust those I should be able to promote movement in the gauge.

Pending advice tothe contrary my plan is to grab a 15 and 30 ohm resistor and see if I can adjust the coil location to read full, half and empty using 30, 15 and 0 ohm resistances. I expect if I can a full reading at 30 ohms the rest should, touch wood, simply fall into place.

Cheers n Beers
Jolls

[Errol62]

Where’s Rob??


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