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  How to bench test a fuel quantity gauge?

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Author Topic:   How to bench test a fuel quantity gauge?
pcrussell50 posted 05-01-2013 11:32 PM ET (US)   Profile for pcrussell50   Send Email to pcrussell50  
I was always told that when it comes to instrumentation, and old saw:
"AC lies, DC dies", which means that AC powered gauges remain at their current reading when power is removed, and DC powered gauges, like we have on small boats, "die" or go to zero when power is removed. Well I have this fuel tank quantity gauge on an old boat I bought that is reading a little less than half a tank, even when it's fully removed from the panel, giving lie to the old adage.

So, before I invest too much time wiring it into a twin tank system I have for it, I was wanting to bench test it, while I have it out of the panel.

How do I bench test a fuel level gauge? I think it's a pretty standard design, with "I" for positive, "S" for sender, "G" for ground.

-Peter

jimh posted 05-02-2013 12:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Connect +12-Volts to the instrument's terminal-I. Connect a resistor of any value between about 50 and 300-Ohms to terminal-S. Connect the other end of the resistor to 0-Volts. The meter should read some intermediate fuel tank level.

Most electric fuel gauges use a standarized resistive sender. The sender resistance varies in proportion to tank level. A current passes though the gauge and the sender. The gauge indicates tank level in proportion to the current flowing through the sender. There are only four elements in the circuit:

--the source of the current, usually the battery via some sort of secondary power distribution through the engine, and via an ignition key switch;

--the meter

--the sender

--the wires that connect the devices

You can generally test the meter by connecting to battery negative the pole of the meter that normally connects to the sender, but insert a series resistor of about 33-ohm to protect the meter movement. This should cause the meter to read full scale, typically.

The sender can be checked by disconnecting all wiring to it and measuring its resistance.

The wires that connect the devices can be checked for continuity and shorts.

The source of the current can be checked with a multimeter.

jimh posted 05-02-2013 12:59 AM ET (US)     Profile for jimh  Send Email to jimh     
Your particular instrument and the tank level sender may have their own calibration and resistance, but the typical configuration is to use a more or less standard 240-ohm to 30-ohm variable resistor or rheostat sensor.
jimh posted 05-03-2013 12:30 PM ET (US)     Profile for jimh  Send Email to jimh     
quote:
...when it comes to instrumentation...and old saw:
"AC lies, DC dies", which means that AC powered gauges remain at their current reading when power is removed, and DC powered gauges, like we have on small boats, "die" or go to zero when power is removed.

Most meter movements are Weston meter movements, a modern implementation of the D'Arsonval meter movement, and they are really direct-current meters. Meters with scale graduations in units of alternating current or alternating current voltage typically contain a rectifier which converts the alternating current into direct current to flow through the meter movement.

Another type of meter is the iron-vane movement, but this is an older meter technology. I have not seen an iron-vane movement meter in decades.

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