Because Owning And Flying Your Beechcraft Can Be Done Safely AND For Less Money!
  Beechcraft Fuel Sensor Mysteries Revealed

 

 

 

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 different.

 

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 solvent.

 

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 HERE

 

Hyland's tester is used on:

 

F33A E772 & up

 

V35B D10120 & up

 

A36TC EA1-EA272

 

F33C CJ149 & up

 

A36 E1111-E2110

 

B36TC EA273-EA319 EA321-EA388

 

See Birks Aviation for detailed info on the in cockpit fuel gauge "modules" and their replacement modules.
 


 

 

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?

 

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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

 

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