Thanks Joey. I wonder if there is a circuit diagram anywhere. I'm having trouble interpreting my measurements, and it would help if I knew how this thing works. What's the component in the plastic case, which sits in the fuel?

 

The AutoZone website may have info on wiring it the sites vehicle repair guides.

The fuel gauge sender unit has two parts.

One part works with the fuel gauge, the other part works with the low fuel light. These are two separate parts having separate wiring.

The part that works with the fuel gauge is a variable potentiometer, a device that can vary its ohm resistance linearly. In this case from 3 to 110 ohms.

It works much the same way as the old toilet tank water valves where a float attached to an arm moved up and down with the level of water. The arm was connected to a water valve. When the toilet was flushed water would drain out of the tank and the float (and arm) would drop opening up the valve. As the water refilled the tank the float would raise and slowly close the valve. The valve would eventually close completely.

In the case of the fuel gauge sender substitute the variable potentiometer for the valve.

When the gas tank is full, the float and arm are up and the potentiometer resistance is about 3 ohms. When the tank is empty the ohm resistance is about 110 ohms. Between full and empty the resistance varies linearly from 3 to 110 ohms.

In general most early model fuel gauges used a couple of electromagnets, one on each side of the fuel gauge needle. The electromagnets pull the needle one way or the other depending on their magnetic strength. If the magnetic force of both magnets is the same the needle will be centered.

The fuel sender’s potentiometer is wired in series with one of the electromagnets coils. One connection of the potentiometer goes to the magnet coil the other goes to ground.

Basically as the potentiometer resistance changes the magnetic force of the magnet also changes, the other magnet always has the same strength. Thus by varying the strength of one magnet the needle will move.

The electrical circuit design prevents any current from flowing through the potentiometer.

The sender is checked with an ohmmeter. The resistance should vary as the float (and arm) are moved.

The Yellow/Blue wire goes from one end of the potentiometer to the fuel gauge. The Brown from the other end potentiometer to ground.

The other part of the fuel sender is the low fuel warning light. This is a thermistor, a type of resistor where its resistance (in ohms) varies with temperature.

The dashboard has a light that has power applied to one connection of the light filament when the ign key is on. The other end of the light filament runs to the thermistor, the other end is connected to ground.

When the thermistor is immersed in fuel it is kept cool and as a result has a high ohm resistance. This prevents current flow from the light to ground and the light from illuminating.

When the fuel level lowers to a point the thermistor is exposed to air, the lack of fuel cooling it allows it to heat up. When this happens the thermistors ohm resistance drops substantially allowing current to now flow from through the light bulb to ground and illuminate the bulb.

The Yellow/Red wire runs from the thermistor to the light bulb. The White/Black from the thermistor to ground.

 

You previously confused me a bit when you said the lump (I'd referred to a lump of resin on top of the unit) was the thermistor. It makes sense that the thermistor is in the plastic case. But it seems that my car is wired differently, anyway. On my unit Y/Blue goes to the thermistor. I've reinstalled the sender now because I didn't want to leave the tank open. After my playing around with it, the gauge is now reading 3/4 full, not full as before. I'll monitor it and see if it moves.

 

The should be both a plastic float and a small lump. The lump would be the thermistor.

I only have the factory wiring info.

 

I have a 2009 Camry. Since my gas gauge was continuosly reading empty I took it to dealer. The dealer indicate the vehicle was in warranty until March 2012 but that because water dropplet were found in the tank the 'Sender' had burned out and was not sending the correct gas level to the dash's gauge and that it was not covered by the warranty.

I understand the the sender should have had some tolerance to some level of liquid humididy and should not have failed. This failure of sender should have been covered by the warranty.

They indicated that many pumps had gone bad by water being deposited in tanks of ther vehicles but they had no history of replacing senders. The pump works fine and the car runs great, but the guage in the dash was the only problem.

Should it have been repaired under warranty?

 

The question is whether the Sender affected by water in the tank? If condensation is common in every tank, then the Sender should have some tolerance, correct?