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Old 11-18-2002, 11:38 AM
dpetryk's Avatar
dpetryk dpetryk is offline
Electrons can do anything
Join Date: Sep 2001
Location: Houston, Texas
Posts: 1,072
Once you have an understanding of how these things work you will understand how they fail and how to fix them.

Here is how they work:

The sensor functions as a simple variable resistance element. The resistance is determined by the length of a resistor wire. The length of this wire determines the resistance of the circuit. The length of the wire is changed by a floating shorting device which moved up and down over the length of the resistance wire. The length of the resistance circuit is determined by the position of the float. As the float moves along up and down the shorting device provides a short circuit thus changing the restisance at the terminals of the sensor. As this resistance changes the fuel guage moves as well.

Here is how they usually fail:

The shorting device on the float usually fails to make good contact with the resistance wires. When this happens we see the "dancing guage" phenomonen. The guage will bounce from empty to some other level. This problem is usually due to a deposit or small corrosion forming on the resistance wires. They can fail where the wires are broken and in this situation you cannot repair them. But in my limited experience, I have never seen that.

How to fix them:

Using steel wool (very fine grade) carefully remove any deposits that may be on the resistance wires. One must be carefull not to break the wires since they are very fragile.

In the case of the sensor shown here which its from a 1987 420SEL, the resistance varies from about 85 ohms when the tank is empty to 3.6 ohms when it is full. I assume that your sensor operates of the same principal and is probably very similar to mine.

So check your sensor with a good meter and substitute the sensor with a resistance source to check your guage and isolate the problem.
Attached Thumbnails
Fuel sender or gauge?-sensor.jpg  
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Last edited by dpetryk; 11-18-2002 at 11:45 AM.
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