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# Why does my fuel gauge always read full. - Cars & Trucks

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• The float in the Gas tank sending unit may be stuck. Read this:-
• The Sending Unit The sending unit is located in the fuel tank of thecar. It consists of a float, usually made of foam, connected to a thin, metal rod. The end of the rod is mounted to a variable resistor. A resistor is an electrical device that resists the flow of electricity. The more resistance there is, the less current will flow. In a fuel tank, the variable resistor consists of a strip of resistive material connected on one side to the ground. A wiper connected to the gauge slides along this strip of material, conducting the current from the gauge to the resistor. If the wiper is close to the grounded side of the strip, there is less resistive material in the path of the current, so the resistance is small. If the wiper is at the other end of the strip, there is more resistive material in the current's path, so the resistance is large.
In the sending unit, the fuel has to drop below a certain level before the float starts to drop.When the float is near the top of the tank, the wiper on the variable resistor rests close to the grounded (negative) side, which means that the resistance is small and a relatively large amount of current passes through the sending unit back to the fuel gauge. As the level in the tank drops, the float sinks, the wiper moves, the resistance increases and the amount of current sent back to the gauge decreases.
This mechanism is one reason for the inaccuracy of fuel gauges. You may have noticed how your gauge tends to stay on full for quite a while after filling up. When your tank is full, the float is at its maximum raised position -- its upward movement is limited either by the rod it's connected to or by the top of the tank. This means that the float is submerged, and it won't start to sink until the fuel level drops to almost the bottom of the float. The needle on the gauge won't start to move until the float starts to sink.
Something similar can happen when the float nears the bottom of the tank. Often, the range of motion does not extend to the very bottom, so the float can reach the bottom of its travel while there is still fuel in the tank. This is why, on most cars, the needle goes below empty and eventually stops moving while there is still gas left in the tank.
Another possible cause of inaccuracy is the shape of the fuel tanks. Fuel tanks on cars today are made from plastic, molded to fit into very tight spaces on the cars. Often, the tank may be shaped to fit around pieces of the car body or frame. This means that when the float reaches the halfway point on the tank, there may be more or less than half of the fuel left in the tank, depending on its shape.
Now let's see how the gauge works.
The Gauge The gauge is also a simple device. The current from the sender passes through a resistor that either wraps around or is located near a bimetallic strip. The bimetallic strip is hooked up to the needle of the gauge through a linkage.

As resistance increases, less current passes through the heating coil, so the bimetallic strip cools. As the strip cools, it straightens out, pulling the gauge from full to empty.The bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they are heated or cooled. Each type of metal has its own particular rate of expansion. The two metals that make up the strip are chosen so that the rates of expansion and contraction are different.
When the strip is heated, one metal expands less than the other, so the strip curves, with the metal that expands more on the outside. This bending action is what moves the needle.
Some newer cars, instead of sending the current directly to the gauge, use a microprocessor that reads the output of the resistor and communicates with the dashboard. These systems actually help improve the accuracy of the gauge.

Posted on Apr 04, 2015

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Depending on your system, either the float is stuck or the sending unit is bad in the tank, or your gauge is bad. Rarely is it the wiring in-between, but that is also a possibility.

Posted on Apr 04, 2015

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### 1989 Saab 900 low level fuel light

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur

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