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Fuel indicator light malfunction

2K views 10 replies 5 participants last post by  360modena 
Not sure what tests were performed, but it is common for the fuel level thermistor to crap out, uncommon for the gauge electronics to fail.

If you haven't already, you can try to artificially activate the low fuel light by grounding the R/B gauge wire, and/or activate the very low fuel light by grounding the B/Lg gauge wire.
 
...I'd like to test by grounding these wires as described above, but I'm unclear on the best spot to do so. Is it a matter of jumping two pins on a connector under the tank? Disconnecting wires from behind the cluster?
Any place you can access Black/LightGreen(empty), and/or Red/Black(low) is OK. Obvious locations are directly at the fuel pump connector, or at the gauge cluster.

If you ground the wires at the gauge connector, the plug must be installed while testing else the gauge won't see the signal. If you test here, the wires can be tapped by pushing a pin through the insulation then clip a jumper lead between selected pin and gnd (black/white).

If you test at the fuel pump connector, the plug may be removed while testing, just jump the appropriate pin to gnd. You may also just back-probe the connector.

It may help to know WHY this works: The bulb in the gauge has constant 12v on one side, the other side goes to the fuel thermistor. When the fuel thermistor is covered in fuel, it is high resistance (no connection, no light); when uncovered, it heats up it goes low resistance (makes a connection to ground), thereby lighting the bulb. Jumping the thermistor wire to gnd emulates an uncovered thermistor and should light the bulb is all else is OK.
 
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