Originally Posted by sherdep
when the tank is empty does it show full?
If that is the case then my guess is that they hooked the wires up to the wrong probes giving you the opposite indication which would be an easy fix - there are 3 wires on the outside of the Black water tank just need to swap the top and bottom wire to fix.
Good possibility. Get them manufacture and model number from the gauge, find the website for that manufacturer, and see if you can get a wiring diagram.
Does the gray tank and fresh water tank indicators work ??
Another possibility, is that even though you have liquid in the tank, it may not be reaching the 1st probe. A test would be to fill the tank and see what indicator lights come on.
BTW, I moved these posts to a new thread.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263