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Author Topic:   Voltage supply to fuel tank sender
kwik_wurk posted 05-11-2010 04:55 PM ET (US)   Profile for kwik_wurk   Send Email to kwik_wurk  
What should the standard supply voltage be at a fuel tank sender unit when powered? I measured my unit (disconnected from sender) and got ~7.5 VDC. I figured I would get 12 VDC (or whatever the battery was at).

[I didn't have time to go to the back of the fuel gauge and measure locally, but that is going to be the next step.]

I am in the process of diagnosing a bad fuel tank sender. After checking the resistance on the sender terminals I got a resistance that was out of range for the fuel level. I inserted a turn pot resistor (disconnected from sender) and ran through the resistance ranges 33-240 Ohm, and got proper fuel level readings on fuel gauge.

jimh posted 05-12-2010 09:15 AM ET (US)     Profile for jimh  Send Email to jimh     
My understanding of the usual arrangement for a remote reading electric fuel gauge is as show below:

Battery positive (+12.0-V)--->meter--->sender--->battery negative (0.0-V)

The sender consists of a variable resistor that controls how much current flows in the circuit. The meter is arranged to show a tank level based on how much current flows in the circuit or how the voltage divides between the meter and sender.

If you break the circuit between the meter and sender, you should not measure any voltage at the sender.

Based on your narrative, it sounds like the problem in your case is in the tank sender. If you disconnect the meter circuit from the sender, you should be able to measure the resistance of the sender. It typically will be in the range of 33 to 240-ohms, depending on the tank level. If the sender resistance is not in that range, there is something wrong with the sender.

kwik_wurk posted 05-12-2010 12:42 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
Yes I know that my sender is bad, and verified this.

However the I surprised to find the supplied voltage at the sender to be ~7.5 VDC and not ~12 VDC.

And I wanted to know if the fuel gauge (or meter as JimH calls it) is suppose to output a ~7.5 VDC to the fuel sender. If I am suppose to get ~12 VDC, then I have another problem to sort through.

Benn posted 05-12-2010 07:38 PM ET (US)     Profile for Benn  Send Email to Benn     
The positive side of the supply is likely the gauge side, which has an external or internal voltage regulator, most likely internal built into the fuel gauge. The negative side is hooked up to ground (or is grounded through the tank in some applications, not likely with a plastic tank). The fuel gauge reads the flow of current that passes through the voltage regulator into the gauge and then through the resistance coil on the sending unit and back to the battery.

So the 7.5 volts you are getting is after the regulator. Without the regulator, if your fuel gauge worked at all, the gauge would move around as the system voltage when the battery was low, the engine revved, etc. I don't know what the spec should be, but 7.5 makes sense as it would need to be a good bit below the "normal" range of low system voltage swings (say 10 volts or so) but high enough to push enough current through to work the fuel gauge and drown out the effect of stray currents from other electrical system components.

If you have a good digital voltmeter you can diaganose this a bit more. You won't likely see this with an analog meter.....

With the electrical system off, if you measure between the battery negative terminal (ground) and the downside (not the gauge side) of the sending unit you should read roughly zero point zero volts if things are hooked up correctly as there should be very little resistance between that side of the sending unit and ground. If you measure between battery neutral and the gauge side of the sending unit, there should be a voltage difference in the range of millivolts and it likely will dance a round a bit, but not too much. This is a measurement of the potential difference created by the resistor in the sending unit. If it reads roughly zero volts and it is a solid reading....there is likely a short in the sending unit. If there is a big difference and/or the reading dances around a lot (with the ignition off) the resistor is also likely bad, but it is discontinuous and not shorted....a more likely scenario, but still it is bad.

tmann45 posted 05-12-2010 07:49 PM ET (US)     Profile for tmann45  Send Email to tmann45     
I inserted a turn pot resistor (disconnected from sender) and ran through the resistance ranges 33-240 Ohm, and got proper fuel level readings on fuel gauge.

This says your fuel gauge is good and therefore the 7.5 V is probably is what it is supposed to be. A new sender should make you gauge read correctly.

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