I was hoping that it was only a bad ground connection on the instrument cluster. On my first repair attempt, I hit the top of the dash above the instrument cluster to jostle the connections and take out some frustration. When only the latter worked, I looked at my BMW repair CD.
To test the fuel gauge, the repair manual said to connect the brown-yellow wire on the fuel level sender to ground and to switch on the ignition momentarily. The gauge deflected to full, so I knew the gauge and grounds were OK. But that meant the sender was broke.
RealOEM.com lists the price of new sender at $261.40 (USD), so that was a good reason to try to fix it.
After I moved all the crap in the trunk to one side, I lifted the carpet and unscrewed the floor panel covering the gas tank.
The gas tank was full. When I tried pulling the sender out of the tank, (it’s a bayonet mount) gas poured out into the trunk. I decided to take a drive down Highway 1 a little south of Point Lobos to lower the fuel level.
The fuel sender is basically a variable resistor. The float moves up and down a guide rod and is also connected to a wire of known resistance. The change in resistance (by the position of the float) is translated by the fuel gauge as the range from full to empty.
One end of the sensor wire had broken off its terminal connection so the wire was now too short to connect to the terminal. I used a piece of 22 gauge solid copper wire (looped and soldered to the connector) to extend the connection so the sensor wire would reach its terminal. Since the sensor wire is a continuous run from one terminal to the other, I had to figure out the routing around the bottom of the sender. It seemed to just wrap around – at least that’s how I did it.
The repair manual says the resistance between the G terminal and minus should measure 3.2 & 73.7 ohms at the extreme positions of the float. I measured 3.4 and 84.7 ohms so at least my repair sorta worked.