Fuel Level Sender Unit

1. I first checked the wiring at the gauge.
   1. The gauge was getting power (12.3 Volts as measured by my trusty Multi-Meter).
   2. The sending wire to the sending unit was getting power (5.4 Volts).
   3. The sending wire at the tank end was getting power (5.4 Volts).
   4. No corrosion, broken wires, or detached wire ends. All looks good!
2. I suspect the sender must be bad. Let’s test it.
   1. I detached the wires from the sender unit, unscrewed the holding screws and removed the unit. It’s one of those cheap float-type units and it looked quite corroded.
      1. This type of sender uses a swing-arm with a float at the end. As the tank level changes, the angle of the swing-arm changes. The pivot point of the swing-arm is attached to a reostat that changes the level of resistance. So the 5.4V I measured at the supply point should read 5.4V when full, decreasing to zero as the arm moves to empty.
   2. Using the multi-meter in Resistance Mode with the red point on the positive and the black point on the negative, the Resistance read “Infinite.” Unfortunately, no matter what angle the swing arm was at, no voltage could get through.
   3. With all the corrosion in and around the reostat, I’m sure that’s the problem.
   4. I dissassembled and soaked the unit in CokaCola for 12 hours. It came out shiny and almost new looking! After washing thoroughly with water and set to dry, I tested it again.
   5. This time the resistance level were better, but still very high. The resistance should read between 35 Ohms and 240 Ohms. Mine was reading in the 1000s.
   6. Yep. Bad sending unit.
3. I bought a new Moeller Marine Universal sending unit from West Marine (MOELLER 03572510, MOE03572510) for $40.
   1. The bolt pattern is standardized, so I simply screwed it back into place where the old one sat. Easy.