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Author Topic: How do you test for faulty fuel gauge/sending unit??  (Read 217 times)

smoulding

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How do you test for faulty fuel gauge/sending unit??
« on: September 16, 2017, 08:00:49 PM »

On my '04 carbureted engine, non digital fuel gauge, always reads full. I think the sending unit in the tank is bad, but can't find test info in the manual to put a meter on it. Does anybody have test numbers for my ohm/volt pocket meter? Thank you!
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Christmas

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Re: How do you test for faulty fuel gauge/sending unit??
« Reply #1 on: September 17, 2017, 07:48:27 AM »

I literally have no understanding of the electronic side of things re the fuel gauge but if you can pull it out see if it moves up and down freely because my understanding is that it's a float and if it's not floating or is not lowering then it will always read full.

Also check the wiring to make sure it hasn't decayed because it will always show full when not plugged in to read otherwise.

More importantly.. Don't trust the bastard anyway and use the odometer trip meter.
« Last Edit: September 17, 2017, 07:50:14 AM by Christmas »
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mrcrum

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Re: How do you test for faulty fuel gauge/sending unit??
« Reply #2 on: September 17, 2017, 04:50:02 PM »

It sounds like Christmas is right on.  In most cases the sensing unit is usually a float connected to a potentiometer or rheostat (a variable resistor) inside the tank.  Sometimes, as in many cars, it’s a printed circuit board potentiometer which actually uses resistive ink painted on the board and a wiper drags across it.  Newer ones might use magnets.  As the tank empties, the float drops and slides a moving contact along the resistor, increasing or decreasing its resistance depending on the design.  I would assume that it decreases in this case since an open circuit would read full. The indicator measures the current flowing through the resistor.  The manual doesn’t seem to say what the resistance should be but you can put an ohmmeter across the sender and see if it changes when the float level changes (either by removing the sender and manually moving it or by filling or emptying the tank).  You could also check the voltage drop across the sender while it's hooked up and see if it changes when the tank level changes.An interesting note is that this works while submerged in liquid because gas, like water, is non conductive.  Water is only conductive due to the impurities that are in it.  The dirtier the water the more conductive it is.  Distilled water has very low conductance, tap water or rain water has more.
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Hylife

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Re: How do you test for faulty fuel gauge/sending unit??
« Reply #3 on: September 20, 2017, 10:56:18 AM »

The same fuel gauge has been used for many years across carb and EFI models. It is a float adjusted potentiometer.
It is removable from under the tank being held in place with 2 screws.

Once removed and cleaned the resistances should be as follows:-
When full, resistance between the terminals should read approx. 90 - 105 ohms.
When half, resistance between the terminals should read approx. 38 ohms.
When empty, resistance between the terminals should read approx. 4 - 10 ohms.
Replace with a new one if out of spec.
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