01-09-2005 Ask Mike! Archive
Fuel gauges and voltage

I have been looking at the Beech wiring diagrams I was sent by another bonanza owner, off and on most of the day. It appears that my Serial number D-1099 does have the resistor (adapter) in the Gauge or rather on the back of it. My confusion comes from the statement that the serial numbers D-1 through D-3998 used 6 volt Gauges with a 30 ohm resistor. The only difference I can see in the diagram from serials D-1 to D-300 & Serial D-301 (including mine D-1099) to D-6561 is the "wing break connectors." I noticed the diagram does not mention any part numbers. I would be curious to know if the part numbers for the fuel sending units in the tanks are different by serial number? I am not an electrical wizard, but it appears that a 30 ohm resistor would not be near enough to reduce 12 volts to 6 volts. It would have to be more like a 300 ohm or larger unit wouldn't it? I wonder if it is a typo? Is there any place I can go to get the answers? Do you know the answer? 
As I mentioned above (Sending unit numbers by serial number & Resister Ohms verification)? I know you may not have this, It is apparent the ABS does not know it as well as I have been waiting for an answer from their tech support for three weeks. All this stems from my desire to have two independent fuel gauge systems. It sure would be better to have just two independent instruments or fuel gauges, After all I already have two independent gauges for my wing tip tanks, seems only logical to do the same with other other. I am curious to know if I can use my current sending units with a new set of fuel gauges. Do you know the resistance numbers on a 1947 Bonanza fuel sending unit? Can they handle the full 12 volts or must they be 6 volt gauges only? 
 
Thank you
Keith

Keith;

Don't be pulling your hair out just yet!
 
First thing to keep in mind.  These gauges were originally designed for 6 volt use.  As part of their design, they were a 30 ohm system.  Ohms law tells us that if we double to voltage, we end up with twice the current (amps) - which the windings in the gauge can't handle; therefore, to keep the current the same, we double the resistance (add 30 more ohms), hence, the ballast resistor.
 
Parts book for early Bonanzas shows same fuel quantity transmitter, left and right sides, all serials D-1 thru D-4865.  same parts book shows gauge consistent from D-1 thru D-4391 (F), G's using a different number.
 
This will most likely take a field approval (discuss with your local FSDO before doing it):  If your transmitters are OK (no dead spots, etc), they can take the 12 volts.  All they are is a variable resistor actuated by a cork float.  Get two 30-ohm / 12-volt fuel gauges; possible sources would include Mitchell or Westberg.  The gauges generally come with wiring diagrams also. 
 
But, if panel space (for 2 more instruments) is a concern, consider this:  You mentioned you have tip tanks with gauges---  If they are the earlier type Brittan's or Osborne's, they use a 30 ohm system, as do the D'Shannon's.  Use the same gauges, but with a switch to move them between senders (tanks).  If your tips are direct feed (to engine), you would want to use a toggle switch to be able to consistently monitor level of selected tank in flight.  If your tips are transfer, then use a momentary switch (like a push to talk switch) adjacent to the gauge, press the button for tip quantity, otherwise the gauge reads the main all the time.  This is the way, along with an indicator light (to show the pump is operating) I have set several of them up, and will be doing my F that way.
 
Gear Green,
Mike