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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[Jolls]

So I now have three fuel gauges; and none of them appear to work. All tests come up with the same result:

• Resistance between the pins (B+ and sender) on all three gauges is 54-59ohms
  
• 12V applied across the pins the needle moves to empty.
• 12V applied across the pins with a 15 ohm resistance between the gauge restance increases to 69-74 ohms. Needle moves to empty.
• 12V applied across the pins with a 30 ohm resistance between the gauge resistance increases to 73-89 ohms. Needle eedle moves to empty.
• 12V applied across the BAT pin and case case to earth needle moves to full in all three cases.

I pulled one of the gauges apart and measured resistance from B+ to where the second winding attaches. The resistance is 73-89 ohms (30 ohm increase) and when connected to earth the gauge moves to full. I expect that Rob is correct in that one of the windings is shunted to earth between the end of the coil and the sender unit connection. I now have three average gauges that are as useful as .. well you know.

Gaugeworks has advised that they can repair any of the gauges for $120 plus return post. I have only found one other tested and working gauge in the marketplace and it is going for $150 plus postage from WA. So a couple of questions:

• Does anyone have experience with Gaugeworks? If so can you please provide feedback on their services.
• I realise that repairing one of these gauges would be a right PITA. However, it is something that would be of interest to me. Has anyone successfully repaired a gauge with a similar fault? If so I would love to have a chat about where you sourced the coils and how you went about the repair
• Does anyone have a working fuel gauge that they would be prepared to part with for less than the repair cost of what I already have?

[ardiesse]

Craig,

You're not doing it quite right.  You're putting the resistance across the B+ and sender pins.  The resistance is supposed to connect from the sender pin, to ground, like this:

Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal, resistance, lead from resistance, battery (-), with fuel gauge case connected to battery (-).

Rob

[Jolls]

Thanks Rob,

Poorly explained; the resistance is on the sender side of the pins to B(-). The testing was executed as such:

Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal, to battery (-), with fuel gauge case connected to battery (-) = needle pointing to left hand dot (Empty)
Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal to 15 ohm resistance, lead from resistance, battery (-), with fuel gauge case connected to battery (-) = needle pointing to left hand dot (Empty)
Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal to 30 ohm resistance, lead from resistance, battery (-), with fuel gauge case connected to battery (-) = needle pointing to left hand dot (Empty)
Battery (+), lead to fuel gauge B+ terminal, sender terminal disconnected from battery (-), with fuel gauge case connected to battery (-) = full deflection to the right (Full)

My thoughts are that there is a short between the sender pin and the coil. To test my theory I borrowed two known working gauges from a friend. The results I have from these gauges are:

Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal, to battery (-), with fuel gauge case connected to battery (-). Reads Left Hand Dot (Empty)
Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal to 15 ohm resistance, lead from resistance, battery (-), with fuel gauge case connected to battery (-) = needle to 1/2 (gauge 1) and 3/4 (gauge 2)
Battery (+), lead to fuel gauge B+ terminal, lead from fuel gauge sender terminal to 30 ohm resistance, lead from resistance, battery (-), with fuel gauge case connected to battery (-) = needle at full defelction (Full)
Battery (+), lead to fuel gauge B+ terminal  sender terminal disconnected from battery (-), with fuel gauge case connected to battery (-) full deflection to the right

I'm not sure what the odds of pulling three gauges with the same/similar fault are, but I expect about as much as winning lotto.

I have done a deal with my mate to brew up some All Grain Beer and loan him my tap setup for a function he has coming up in exchange for the more accurate of the two working gauges. I will keep the others as spares and get one of them down to GaugeWorks if the one I have now falls over. I had our local signwriter make up some stickers to refurb the glass but I am thinking that it may be better to stick the surrounds on the back of the glass and use the airbrush to bring it the E, F and FUEL symbols back to life. I have a couple of sets of glass to play with so we will see how it turns out.

Thanks for your input Rob, appreciate it. I thought I was going a bit mad for a while there. So another problem solved - next issue to deal with is king pins!

Cheers n Beers