What does the fuel gauge do when truck is off?

[Accelo]

I am attempting to track down an issue with my fuel gauge.
What does the fuel gauge do when truck is off?
Mine reads one quarter tank, the same if the ignition is off or on.
I know the fuel gauge is pretty darn reliable.
However, my truck sat for years.
I just need to rule out the obvious.
Cheers.

[Steeveedee]

The fuel gauge measures current. If the power supply is off, nothing happens. Some gauges have some sort of bias (hysteresis) that causes them to drift to some other value when there is no power to them. That is an extremely small effect, so when the power is on, those gauges will go to the area of the dial that corresponds to the current, probably with some small amount of error, which I wouldn't worry about, personally.

What is your concern?

[Accelo]

I am attempting to prove the wiring and tank sender are functional. This has not worked since I installed the rear fuel tank. Right now I just fill up often!
I am assuming the tank sender is 90 ohms. Unsure if that if that is for full or empty.
Assuming empty and 0 ohms for full? So when I ground the wire at the sender the gauge should read full. Is that correct?

[cj847]

My understanding is that the fuel gauge reads when the key is on, then stays at that reading until the next time the key is on (ie if you have 1/2 tank, turn off the key, siphon the tank, the gauge will still read 1/2 tank until the key is turned on).

But it sounds like your gauge is not changing when you add gas?? So, you need to determine whether your wire is bad or your sender?

Here is where you can check to see if the wire is carrying a signal.

[Accelo]

Today my friend came by and the two of us were able to track down the fuel gauge issue. First we proved the wire and grounds were all good. Then grounded the sensor at the tank with no changes on the dash fuel gauge. I had another unrestored gauge cluster so I plugged it in and the gauge worked just fine. By the way when you ground the fuel level sensor, at the tank, the gauge returns to empty.
The test fuel gauge went past the full mark when connected with a full tank of fuel. It works but not as closely calibrated as a factory sensor. I can live with that. I need to repaint the needle for the spare fuel gauge and change it out. One more thing off the list.
Cheers.

[57taskforce]

Glad you got it fixed. I had a few issues with mine when I first got it back on the road 2 years ago. My problem was the gauge as well, more specifically the resistor on the back of the gauge was bad. I had an extra I swapped in and good to go since.

[Accelo]

57taskforce
Thanks for the note. I will check the resister before condemning the fuel gauge.
Might save me some work.
Cheers.

[yuccales]

Originally Posted by Accelo

What does the fuel gauge do when truck is off?

Takes a break from it's constant right to left travels.

[Accelo]

yuccales; Now that's funny!

On the serious side, well maybe!
Maybe someone can tell me what purpose of the resister(s). Seems most of the gauges have them. I know the amp gauge has one installed.

[Steeveedee]

The gauges don't always necessarily read correctly when manufactured, and need a bit of calibration. It's all about tolerances and speed down the assembly line, whether it's a whole vehicle, or just the gauges.

[pjmoreland]

The resistor is connected to 12V on one end, and the other end is connected to the tank sender. The tank sender is connected to ground. The variable resistance of the sender that's connected to the fixed resistor together make up what's called a voltage divider. The voltage of the junction of these two resistors changes depending on the position of the sender.

There are two sets of windings inside the gauge. They are both connected to the same voltage divider junction at one end. The other end of one winding is connected to 12V and the other winding to ground. This causes the windings to swap in terms of dominance of one over the other depending on whether the junction voltage is high or low. One winding pushes the gauge toward FULL, and the other toward EMPTY.

If the resistor fails open, then the balancing act of the voltage divider system is messed up.

The temperature gauge works the same way.

[pjmoreland]

By the way, the gauge clusters out of early '80s Chevy vans have these same 90-Ohm ceramic resistors in them. Vans like that are fairly easy to find in wrecking yards these days.