Are you 100% sure the diagrams you have are for your exact year? The connectors do change from year to year, frustratingly.

I'd pull the rear seat bottom and test between the gauge cluster (that pinout is reliable through the years) and the connector for the sender at the tank.

If the tank is reading full all the time, that means the resistance is low - so more likely shorted to ground rather than missing ground.

-Charlie

 

I think there is a voltage reducer for the fuel gauge and temperature gauge. If it is malfunctioning then both gauges would read higher than the really are supposed to read. Not sure if the open test would still read low or empty in that case. Some of the older ones were in the dash, which is where the reducer (or resistor) is in my brothers 81 truck, but it may be too much voltage versus other possibilities.

 

The temp and fuel gauges are completely separate, as far as I have been able to tell from the diagrams and what I remember from looking at the back of the cluster.

An unplugged fuel sender would show empty with no low fuel light. Since the low fuel light works, at least the in-tank thermistor is still working. Since the gauge is reading full, it is reading a lower resistance to ground than it should.

-Charlie

 

I have looked at some more wires. I'm satisfied this is the same brown wire and yellow-lightgreen wire under the dash.

What confuses me is why the wire colors kind of criss-cross at the plug near the tank, and why the black wire instead of brown? I checked and re-checked that it wasn't somehow turned around backwards.

Makes it a little less certain when probing pins 3 and 4.

Have not had a chance to do thorough electrical tests.

 

I have looked at some more wires. I'm satisfied this is the same brown wire and yellow-lightgreen wire under the dash.

What confuses me is why the wire colors kind of criss-cross at the plug near the tank, and why the black wire instead of brown? I checked and re-checked that it wasn't somehow turned around backwards.

Are you sure the wire is yellow-lightgreen and not yellow with blue stripe? Y-L in Toyota lingo is yellow with blue stripe.

The wiring schematic show connector H1 and F.

Don't know why the color change at the plug near the tank, ask Toyota. Going by the picture, wire colors, and pin numbers the right half of the connector (in the pic) is correct. Pin 1 = white with black stripe, pin 2 = yellow with red stripe, pin 3 = yellow with blue stripe, pin 4 = brown.

The yellow connector in your picture is wrong. That is not H1; the wire colors may match, but the pin locations are wrong. There is no way 2 wires next to each other can be pin 13 and pin 16, the pin numbers go in order. Your picture shows H1 pin 13 = brown wire and H1 pin 16 = yellow with blue stripe wire.

You need the correct wiring diagrams for your car, before you start probing things. The wiring diagram for the gauge cluster shows how things are wired to the fuel sender, fuel gauge, and low fuel light.