The above picture is the typical Beechcraft
fuel level sensor removed from the wing. Most Bonanza/Baron aircraft are
configured with an OUTER level sensor that deals with about the first half of
the tank and then an INNER sensor that deals with the second or bottom half of
the tank level.
IA and E55 Baron operator Stuart S. offers: Barons with
interconnected tanks, all model 58 and 1974 and newer model 55s have two 90 ohm
senders in the 'main' (40 gallon) tank and those with outboard leading edge
tanks 172 gallon capacity, an additional 19 ohm sender in that outboard tank.
The 30 gallon box tank does not have any senders.
Resistance increases with fuel quantity, but since the 19 ohm
sender is installed from the bottom of its tank the internal wiring is
The same system was placed into the Bonanza around 1970 or so.
You can see why Model and SN are key when seeking fuel system gauge help.
HERE is an
article by electrical and avionics guru John Collins, which describes the fuel
system sending units and their functions.
Here is what you will see when you open the
sensor up. It is nothing more than a wire wound resistor, like a rheostat that
is used to change the resistance in the circuit. The fuel gauges are essentially
voltmeters measuring the drop in voltage caused by the change in resistance as
the sensor moves up and down on the wire wound resistor element.
The problem creeps in when this wire wound
resistor develops corrosion or a break in the wiring. Many owners have found
good results in restoring accurate gauge readings by disassembling their sensors
and giving them a good cleaning with fine sandpaper and/or contact cleaner
There is no seal on the sensor housing, it
functions immersed in the tank.
Since the resistor is
grounded, the resistance to ground is reduced by the
slider. There is always a constant path to ground so there is NO arcing.
There is play in the shaft to bushing
interface. Air containing dust and normal pollutants enters the tank through the
vents as fuel is displaced. These contaminants over the decades are believed to
cause the corrosion and coating of the wire wound resistor that results in
erratic or no readings at all from that particular sensor.
The fuel gauge system wiring varied greatly
over the years depending on SN. So please research the wiring diagram from your
shop manual for your particular serial number.
Do you have fuel gauge "modules" with
adjusting pots on the back of them? Suspect problems there? Check with Beech
Talker Hyland Lyle to rent or buy his "tester" for your gauges. His tester
instructions and contact info are
Hyland's tester is used on:
F33A E772 & up
V35B D10120 & up
F33C CJ149 &
Birks Aviation for
detailed info on the in cockpit fuel gauge "modules" and their replacement
Here is a picture of what an erratic fuel
gauge looks like in a 1949 Bonanza. Gee, I can't understand what was going
on....Can you see what could possibly be the problem?
Clearly the ground cable is nearly severed and
the felt washer looks like it was just unearthed from King Tut's tomb. And check
out the safety wire job! Come on people, I know we can do better than this, and
the vast majority of Beech owners do, but Geez, this is embarasing! This is the
kind of thing that can cause an owner or casual operator of the aircraft to
believe they have fuel when in fact they don't. A fuel exhaustion "glider
rating" is somewhere in their future!
One can combat this kind of thing by liberal doses of Corrosion X or your
favorite anticorrosion spray on these connections in the sending unit at annual.
These connections send critical resistance measurements to the gauge which is
just displaying the reading it gets from the sending unit.
Pictures courtesy of Beechcraft owner, David
T and Kevin O.
See a few great sources of fuel gauge repairs
and overhauls HERE