On our H340, the holding tank gauge quit reading properly - it always read empty. I decided to put on my dirty jobs T-shirt and see if I could figure out what was wrong. First task was to pump out the tank, and unload the starboard aft lazarette which is where the batteries and battery boxes are located, in order to remove the floor panel giving access to the holding tank.

How to repair a non-functional holding tank gauge

This picture was actually taken when I went to reinstall the sending unit. I had covered up the hole with duct tape while the sending unit was out.
The sending unit mounted with six sheet metal type screws that went into but not all the way through a reinforced ring on the top of the tank. Electically there are two wires, and I found an existing splice about 18 inches away from the sending unit, and cut the wires there.

The holding tank

I was actually surprised that the smells out of the tank were not too bad. I had the tank pumped out, and waited until a cool day. The screws were covered with what appeared to be a silicone sealant, but they backed out pretty easily. I ran a razor knive under the mounting flange to break the seal with the tank. The sending unit was pretty nasty. Lots of dried poo, but it was obvious that the float was stuck. The sending unit has a black plastic cylinder with lots of 1/8 inch holes. Most of those were plugged up as well. I took it home and soaked it in warm water with some detergent. It cleaned up pretty easily and the float inside the cylinder started moving again. I checked the resistance across the wires and got an indication that the resistance changed as expected as the float moved.

After the sending unit was cleaned up

After I was satisfied that I had cleaned it as well as I could, I took the unit back to the boat. There I connected the wires and verified that the gauge moved properly as I propped the sending unit upside down to make the float go to the top. Then, I mounted the sending unit back in the tank with some more silicone sealant. Not as big a job as I feared, and hopefully a success. Well find out for sure this spring!

Back together again

I took the time to figure out the routing of the hoses. This picture is of the black hose at the bottom to the pumpout and the macerator (which is in place but not electrically connected as we are on an inland lake). The hose is connected to a fitting installed at the starboard aft bottom part of the tank, and goes through a PVC T fitting to split the flow to the pumpout and the macerator. The inlet hose (not shown) is on the inboard side of the tank, at the top of course. The vent (white hose) is on the top of the tank as shown.

Just for reference