Warning: Fuel is a hazardous chemical, take appropriate precautions
Recommended Tools: Allen key, pliers, multimeter
The fuel level sender on the Porsche 924 Turbo is, as you might expect, located in the gas tank. It is better not to have much fuel in the tank when doing this (if you are repairing a broken sender, it might be difficult to know how much is in there!). To gain access, lift the carpet in the trunk and pry off the round plastic cover to see the top of the sender unit held in place by 5 x Allen head bolts.
In the picture above the wires are not connected properly, if you are confident the wiring is correct, take a picture so you know how to connect it back up. I sometimes don’t bother with the picture and take the opportunity to check the connections against the wiring diagram. Disconnect the wires and undo the 5 x Allen bolts, the sender unit should then slide out but it is a fairly tight fit and mine required some wiggling – be careful here as there is probably still going to be fuel in the tube and there is a bleed hole near the bottom, have a bowl and rags ready.
Once the unit is out and the fuel has been drained, you can start dis-assembly by removing the nut on the bottom. I carefully gripped the nut with some pliers and a rag and was able to undo it without damage. With the nut off, the washer should lift off and the base with channels in the bottom can be unscrewed and removed to allow the outer tube to slide off, WARNING: There are some very fine wires inside running the length of the tube, take care when removing the tube.
Now you can see the float with three wires running through slots down the side. The two thin wires are actually one wire that runs from terminal G in the top down through one side of the float to the plastic 3 pronged base, back through the opposite side of the float to ground. It is routed through the plastic base such that it does not make contact with anything else that would cause a short. The float is connected to this wire by small fingers on each side that press on the wire while still allowing the wire to slide through. These fingers are connected to each other causing an intentional short circuit to form a variable resistor. As the float moves down the rod, it increases the effective length of the wire and thus the resistance – from about 3Ω to 75Ω. Terminal G is connected to the gauge in the dashboard that moves the needle based on the resistance.
When the float reaches the base where the resistance is at maximum, the metal ring seen on the bottom of the float makes contact with two pins in the plastic base. One of these pins is connected to the third wire that runs through an empty slot in the float to terminal W in the top and the other is connected to ground. When this connection is made and terminal W is connected to ground, the low fuel light on the dash is illuminated.
Both of these circuits (gauge and low fuel light) were broken on my unit, there was no connection at all between any of the terminals on the top. The resistance wire for the gauge had good connection with terminal G but the other end was not connected to ground. I struggled to get solder to stick to the aluminum so I pressed a metal tab between the copper plate that the wire is soldered to, and the top plate to make a connection.
After lots of testing I found that one of the pins on the plastic base for the low fuel light had a broken connection. While I was poking around, a small tab fell off one of the pins, it was probably cracked causing the break in connectivity. The float needs clearance in this area to ensure that it will reach the bottom of its travel and make good connection with the pins. I was able to solder a small wire in place to repair the broken connection.
Now with the float at the top, I get about 3Ω resistance between terminal G and ground and no connection between terminal W and ground. With the float at the bottom, I get 74Ω between terminal G and ground and terminal W shows connection to ground. With all the readings looking good, the unit was re-assembled and installed back in the tank. Now I have a working fuel gauge, I’m confident that the low fuel light will work but can’t test until I can burn some fuel!