A failed fuel gauge may have several causes. Here’s is some troubleshooting tips from past postings on the C320IA Discussion List.
First off, has the gauge ever worked correctly? The fuel “sender” is simply a variable resistor attached to the fuel sender arm. If the arm were installed incorrectly (rotated 90 degrees in the tank) it would not be stuck on full, instead the gauge would not be able to reach full as the pivot arm would bind inside the tank.
If the gauge is reading above ‘Full’ all of the time, the most likely culprit is a short to ground in the positive lead (pink) from the tank to the gauge. Conversely, if the gauge reads below ‘Empty’ all the time, the most likely cause is loose wiring. Check the top of the tank where the fuel hoses enter and verify that the wires connecting to the tank sender are in good shape. Next, remove the wheel and open the engine pod, and check the fuel gauge. Ensure that the connections are in good shape Clean if needed with a wire brush and some WD-40, tuner lube, or dielectric grease. Follow the sender wire(s) from the back of the gauge to the "terminal block" (where the wires come up through the tube from below). Frequently this junction block is a problem. Watch for loose wires and other wires that are clamped on their insulation not bare wire. On the back of the gauge, there is a power wire and a ground wire (may be connected to the frames of all the gauges.) This powers the backlighting of the gauges.
Since the fuel gauge is a simple ohmmeter, it may be tested as follows:
- Disconnect Pink Sender Wire from Gauge and turn key ON. The Fuel Gauge should read ZERO Scale (below Empty)
- Short the Gauge Sender Terminal to Ground. The Gauge should read FULL Scale (above Full). If both these checks pass, the gauge is not defective.
- Measure resistance of Pink Sender Wire to Ground: should read approx. 240 Ohms (Tank Empty), 103 Ohms (½ Full), & 33 Ohms (Full)
- Disconnect Pink Wire from Sender at Fuel Tank and Test continuity of Pink Wire: should be Zero Ohms from Tank End to Gauge End. If not, repair or replace the wire.
- Measure Resistance from Sender Terminal (on Tank) to Ground on Tank.
Should read as per (3) above - If not, replace Sender.
Replacing the sender is not a big deal, but here are a few tips:
- Try not to do this with a full tank! The emptier the better as you'll be able to diagnose whether it's really fixed or not.
- Important: Before Removing the plate on the top of the tank holding the sender, take a black marker and make a mark on orientation of the plate and the top of the fuel tank so that you get it oriented the same way during reassembly. It only fits ONE way. Any other orientation, you won't be able to get all 5 screws to align with their holes. Close, but not quite. It took me a while to figure out that the holes weren't symmetrically placed, presumably to ensure that the sender goes in properly.
- Use a Phillips screwdriver bit on a socket wrench to give you 90 degrees access to the screws holding the cover and it's Easy. Use a regular screwdriver and you'll fight with it. Don't lose the screws!
- Note the direction that the sender arm swings when removing it.
- Keep some rags nearby to protect against drips, clean the seal really well before putting it back together.