Originally Posted by Vulcanbike
2001 MS 390
Water gauge has crapped out. Tank 1 won't read at all. Tank 2 shows 1/4 full when tanks are actually 3/4 full. Not sure to check sending units (which appear to be glued in), or gauge unit?
The link vacser sent is likely the correct replacement unit; if not, the S5U might be. I think the only real difference is in the threads. In any case, my receipt for a replacement unit says S3U.
The unit you have is a prior product (I can tell from the year of your boat; ours was 2002 with the same older style sensors), since replaced with the newer stainless screw-in/out unit.
The new units are easier to attach to the tank; IIRC with the new mounting adapter ... probably the FL-2WE/FBR Mounting Adapter (from my receipts)... there's a shaped backing plate you can fit inside the tank, and then screw a top fitting to... and then the S3/S3U sensor screws into that.
The WEMA/KUS folks have been very helpful with details on what products to use, what lengths, mounting adapters, etc.
That said, each sensor will likely also have an inline fuse, probably 1-amp IIRC, somewhere in your system... so before replacing the sensors, you might want to just check fuses first.
As to calibration... it is what it is. Use some Mark I* Standard Eye Ball on the tank at various fill levels, make comparisons to what the gauges say at those known levels, work from there.