Troubleshooting your fuel level sender

[Sinfidelity]

Damn it's weird!

So I cut out the fuel sender plugs n cable from an old loom to use as a test cable enabling me to read my running car and test in parallel, solo and in series to try figure this out.

All seemed to be good till I hit half tank while testing both senders parallel
Full 7-9ohm
3/4 55ohm
1/2 94 ohm
1/2 142 ohm left side only

Not sure what to follow here seeing as the right side isn't gonna change now till empty.

[JL1RX7]

Now since it is a averaging sensor I wonder if it only takes input from certain OHM readings. So say anything above 90 OHM it only reads from one side vs the other.

Most of the sensor's I use are for temperature across a large area. Not a split sensor system like this. But trying to see the logic of what they are trying to read, then using a sensor to read it. I could see it doing something like that.

[Sinfidelity]

Originally Posted by JL1RX7

Now since it is a averaging sensor I wonder if it only takes input from certain OHM readings. So say anything above 90 OHM it only reads from one side vs the other.

Most of the sensor's I use are for temperature across a large area. Not a split sensor system like this. But trying to see the logic of what they are trying to read, then using a sensor to read it. I could see it doing something like that.

I think I figured it.
The gauge cuts the circuit to the right sender once it gets below half. Much like how it activates the fuel light at near empty.
Then only reads level from the left.

My final readings were this

Full 7-9 together
3/4 55 together
1/2 142 left side only
1/4 200 left only
E 326 left only

I'm still not convinced it's 100% accurate but I think it's close enough to program a new single sender

[bleedingEdge]

Just want to relate my experience with the fuel level senders, for anyone else having this problem, and because this thread was the closest thing I could find from days of googling this.

Problem: fuel gauge doesn't fall below 1/2 - 1/3. Started abruptly, not going below 1/4 for a couple months, then got worse. 2004 RX8 MT, 260000 miles.

Cause: both fuel level senders' resistor cards became less accurate toward the bottom (tank empty) ends.

This really confounded me for a while because my brain really hates it when it seems like two distinctly different parts fail in exactly the same way at exactly the same time -- especially after so many years/miles. But it's true. I removed both senders and tested on the bench. They both returned 10Ω on the high (tank full) side, and 263Ω on the low (tank empty) side. Identical. It wasn't until I actually consulted the shop manual that I realized the low side is supposed to return ~325Ω.

I used a little pick tool and tiny wire brush to clean off the varnish from the contact strips, and sure enough, both senders low end results came back into spec. I have two new senders on order, but for now, these old ones are working fine again.

Moral of the story: there is a remote chance that, if your gauge isn't falling below 1/2, it could be both fuel senders are a little faulty.

On a side note, there is disappointingly little info about this coming back from my internet searching. I never found anyone discussing why this varnish thing matters or how to deal with it aside from replacing the part. I don't know much about electronics, but my guess is that the varnish is a pretty good conductor, therefore the more of it there is, the less the electricity is forced through the resisting bits, therefore the less resistance Ω gets added to the circuit?