Belly Open and Troubleshooting 70's Tank Sensors

[CA_Tallguy]

I've got my belly pan open and am trying to troubleshoot my black tank sensors, which always show half full.

Swapping the connection to the gray tank I get the same behavior so it seems the reading is coming past the wiring harness connector to the tank (two wires come off and split into the various wires for the different sensors).

I can see the tank is empty by looking down through the toilet with a flashlight.

Removing the half tank wire, the reading goes to quarter tank... So it seems like either both quarter and half tank sensors are fouled or my trailer is sitting in a way that there is leftover fluid that won't drain at those two sensors. I cant imagine that any such leftover fluid is hitting the half tank sensor, however.

Or could I have a wiring issue in that segment of the harness where the wires come together? Can someone guide me in testing with a multimeter? I've never done resistance testing with one before, only voltage.

Or, should I simply unscrew the sensors from the tank? It looks like there are some tabs to help unscrew them so I'm thinking about trying that. They don't seem finger tight so the next step would be using lock jaw pliers.

Searching the forums, all the info I find about trying to fix this seems to be tricks for when the belly is closed up, like ice in the tank. Since I have access, I feel like I should be able to do more troubleshooting.

[4RXLA]

I've got the same problem and look forward to hearing your resolution.

[CA_Tallguy]

I pulled one of the sensors out of the tank and there was indeed a lot of crud on it. Cleaned it and put it back.. I think it is fixed. Still need to go out and clean the other to see if it is the same deal.

It probably would have resolved with some heavy flushing and driving along with maybe the ice in the toilet trick. Search other threads for info on tank wands and such.

This probably occurred as a result of having the tanks open to the air while my valves were off while I was redoing them maybe some crud was stuck to them then dried, then got soggy again once I finished the plumbing. I dunno.

Also, my trailer has been stationary for awhile. Which is a bit unusual. So no sloshing to clear them off.