How It Works:
The AE86 measures the fuel level with a mechanical float in your gas tank. The float has a arm connected to a floating plastic box that goes up and down depending on the level of fuel in the tank. As the arm goes up or down it moves a variable resistor arm. When the float is all the way down, the resistor is at maximum resistance (gauge reads empty). When it's all the way up your tank will have minimum resistance (tank reads full).
Float all the way down (empty tank), about 111 ohms.
Float all the way up (full tank), almost 0 ohms.
float down, about
Why Yours Doesn't Work:
Our cars are pretty old. The electrical connections in the trunk or hatchback of your car can become corroded if they are exposed to moisture, and the variable resistor connected to the float arm can become covered with fuel varnish and corrosion causing it to have a poor connection. The wiper arm on the variable resistor itself may have bent slightly after twenty+ years of going up and down and may not be making a good contact. A wire in your trunk may have been broken or pulled out while piling in cargo or working on your car.
My fuel level sender after pulling it from the tank, it's covered with white residue and rust. The little can on the bottom of the other metal rod is for your empty fuel light.
How to troubleshoot it:
Although many people will jump right into purchasing parts on Ebay or ordering them from Toyota, I have found on 3 of the 4 AE86's I have owned that the problem can be fixed without spending money on a new fuel sender, I have also seen people spend money on a new one only to install it and find that it still doesn't work due to a bad connection.
The first step it to locate your fuel level sender, it's under a small round plate in the trunk or hatchback of your car.
Hatchback panel, it's the round plate in the bottom right of the trunk.
Next you need to check your electrical connectors, make sure the wires going to the plate are connected and not broken. If they are, go ahead and disconnect them and have a look at the pins. If they are covered with green or white corrosion and not shiny metal you should clean them. I do this with a product called DeoxIT, Q-tips, and small pieces of high grit sandpaper.
Now go ahead and pull out the fuel level sender, be careful about sparks if your tank is full of gas.
NOTE: now is a GREAT time to shine a flashlight into your tank and make sure it's not rusted out on the inside.
Fuel sender, Removed.
Now that you have it out have a good look at it. Move the float arm up and down. Make sure it's not stuck.
Carefully remove the metal cover from your variable resistor. It's got a few metal tabs holding it on. Gently bend them with needlenose pliers.
The problem areas on this wiper arm are either the little copper arm is not in contact with the resistor, or the arm doesn't make contact with the resistor because it's covered with corrosion.
first simply spray the resistor down with the contact cleaner, gently wipe away any fuel varnish or rusty residue.
Next look at the arm while you gently move the float up and down. Make sure the arm is in contact with the resistor the entire time it moves up and down:
IF YOUR FUEL METER WORKS "SOMETIMES" THIS IS THE PROBLEM:
If the wiper arm comes out of contact slightly then it needs more tension to keep it in place. I have found a trick for this.
This is a body clip from an RC car.
Place it in between the wiper arm pivot and the metal housing to cinch down the wiper arm a little bit and hopefully it will now make a good connection. Wipe it back and forth again to check. Use a multimeter between the copper arm and the end of the resistor while wiping it back and fourth to make sure it does not read O/L (overload) or open circuit.
At this point if your connections and wiper arm are all making good contact you should be able to button up everything and now have a functional fuel meter:
If your fuel sender has a broken wire or cannot be fixed you can either order a new one or look for a junkyard replacement. Although the SR5 and GTS AE86 use different fuel pumps, the fuel level senders are interchangeable.
The Toyota part number for a new replacement fuel sender is: 83320-19715
If you want to fix your low fuel warning light, a nice writeup is on AEU86: