Getting to know the fuel gauge sender

[SpyderMike]

As I was troubleshooting my car not starting issue, I determined it was due to being out of fuel. My fuel gauge was sending mixed messages of 1/2 full but the low level light being on. So I pulled the sender to see if I could figure it out.

There are three electrical contacts for the outside wiring connector - W, G, 31. Looking from the top of the installed sender, W is the one at about the 1 o'clock position, 31 has the copper ground connection at 7 o'clock and G is at 9 o'clock in this photo:



Pulling the unit proved to be a little difficult as the rubber gasket to the tank is firmly sealing against the sender tube and the sender had been in there for many years. Opening the unit up requires removing the small nut on the bottom after bending the locking washer out of the way. There is a tab to the small washer that orients into a hole in the bottom of the sender. Once this washer and nut are removed the tube slides off.

Be careful not to damage the very thin internal sender wires....there are three of them. Two are silver in color and one is copper in color. The copper wire is the 31 wire for ground. The two silver wires are actually a single wire looped down to the bottom of the sender and then back up. You can see the loop here at the bottom:



It is this loop that is giving the fuel level resistance to the gauge. As the float slides up and down this assembly, the wire travels through two contact arms on each side of the float subassembly that connect to the two silver wires (the loop). Notice that on each side, one contact arm is on one side of the wire and the other is opposed...to insure wire contact as the float slides up and down due to fuel level. This float subassembly shortens the electrical loop and reduces the resistance as the float goes up. At full tank (per Bentley Manual), the resistance is 3.2 +/- 0.7 Ohms. With the float at the bottom and the longest electrical wire loop the reading should be 71.7 +/- 2.3. For my car, I wasn't seeing this range, so my gauge is off.







The shiny disk at the bottom of the float subassembly is to short out two contacts at the bottom of the sender. My disk had some buildup making it not so shiny. I cleaned it somewhat with contact cleaner:



As the float hits bottom this disk shorts the W and 31 contact points shown here, making the low level light on the gauge turn on. You can see that the bottom contact has an electrical connection going to the threaded central rod. This rod is actually tied to the W connection.



Looking under the top of the sending unit, you can see the connections going to the outside connector. The center rod is W and connects to the silver looking plate and contact at the 11 o'clock position. The copper wire is connected at the 1 o'clock position and the silver wire loop connects at the 3 o'clock and 10 o'clock positions. You can further see that the 10 o'clock silver wire is connected through copper to the copper wire connection at the 1 o'clock position. Therefore, the silver wire connection at the 3 o'clock is the G contact and the copper wire connection at 1 o'clock is the 31 connection. The resistance between G and 31 drive the fuel level gauge reading.



I hope this all makes sense. It is pretty basic, and for my car anyway, pretty reliable. What I realized is that the fitting of the tube to sender top cover is a bit finicky. The float fits rather snugly in the tube with little clearance. If the tube is even slightly out of alignment, then the float will easily get stuck and not read properly.