I have a Datcon 100437 fuel sending unit with Ohm readings of 10.5 to 75.1 when testing with an Ohm meter.  this should work with my 56 Mercury.  I have the gas tank out and have hooked the sending unit to the wire that runs to the gauge and grounded the unit.  If I put the float adjusting arm at the empty position the gauge reads just past the 3/4 full mark.  If I put the adjusting arm at the full mark the gauge peg the needle.  If I put the adjusting arm at the mid point it again reads just past the 3/4 full mark.  If anyone knows what could be the problem please let me know.  The gauge goes to full when it is grounded.  I read somewhere on this site of an explanation of the sending unit but was unable to find the article.  Any help would be appreciated.  Thanks Larry

Larry

Does the gauge go to E when key off?  On my 55-converted to 12 volt- the sending unit reads 88 ohms when tank is empty.  You may need second person to watch gauge while you move float arm up and down to watch gauge travel.  If it doesn't go any lower than 3/4 thru full travel of float arm, I would suspect gauge is bad or needle is bent not allowing it to drop below 3/4.

Ben HessTulsa,OK

The gauge goes to E when the key is off.  The Datcon unit is new so I am not sure where to go next.  Thanks Larry

Larry

The original '56 Merc fuel tank sending unit has a set of points operated by a thermal heating coil built in. An over simplified explanation is that it acts as a voltage regulator that results in an average voltage around 6 volts or so. Without voltage regulation the dash gauge will read high. If it were me I would try to find a good working original '56 sending unit.
Wade