Fuel sending unit

[jetdriven]

Standard bladder installation is 54.7 gal. There is an add on kit for 10more gallons. 

[Bob_Belville]

Jerry, short bodies - Cs and Es mostly - were 52 from the factory. Mid bodies - Fs and Js were 64 originally.

The original O&N bladders were only 54.8 so Fs and Js lost capacity when they went to bladders.

Several years ago O&N started offering 4 cells per side totaling 64. And the offered an upgrade to 64 for plane that only got the 54.8. 

So, Fs and Js are no longer penalized and Cs and Es can carry more with bladders than original setup.

Got that? 

I don't know all the options for Ks and long bodies to say nothing about Monroy...

[LANCECASPER]

1 minute ago, Bob_Belville said:

I don't know all the options for Ks and long bodies to say nothing about Monroy...

http://www.monroyaero.com/

[1964-M20E]

Well my issue has been resolved.  Turns out ti was a unintentional ground at the inboard sending unit.  Things like this will drive you crazy.

 

Talking with the Griggs folks (thanks) my outboard sending unit was not in the best place so that is resolved as well.

@Stephen

I have not forgotten about you shipping fees on the way.

 

[DualRatedFlyer]

2 minutes ago, 1964-M20E said:

Well my issue has been resolved.  Turns out ti was a unintentional ground at the inboard sending unit.  Things like this will drive you crazy.

 

Talking with the Griggs folks (thanks) my outboard sending unit was not in the best place so that is resolved as well.

#Stephen I have not forgotten about you shipping fees on the way.

 

Can you elaborate a little on the unintentional ground?  My 74F left fuel tank gauge will only read 1/2 full even when topped off.

[1964-M20E]

Lets start at the beginning and if I get any o f the tech stuff wrong anyone is more than welcome to correct me.

We have a 60ohm resistance based fuel level system.  When the gauge sees 60 +/- ohms it shows full, 0+/- ohms it shows empty.  I you have an open circuit on the sending units it will peg the needle all the way past full and a short on the wire from the gauge to the inboard sending unit to ground and it will show completely empty.  A short to ground on the wire to the outboard sending unit and you will never see more than half full.

On the F and J models we have 2 sending units in each tank one inboard and one outboard each rated at 30+/- ohms with the float fully raised and 0 +/- ohms fully lowered.  The outboard unit is grounded at the wing rib.  These units are connected in series 30 ohms + 30 ohms = 60 ohms.  The inboard sending unit should be isolated from ground using the nylon spacers in the screw holes.  A single wire runs from center terminal of the outboard sending unit to the inboard sending unit where it is connected to the surface of the inboard sending unit with a special insulated connector that will keep it isolated from ground and from the screw that attaches it to sending unit but allow it to connect to the surface of the inboard sending unit.  The center terminal of the inboard sending unit is then connected to the fuel gauge.

Hopefully the attached schematics will explain things better.

Now if all you are reading is 1/2 full then the wire from the outboard sending unit to the inboard sending unit is making contact with ground thereby removing the outboard sending unit from the circuit, or the outboard sending unit has shorted out and is not adding any resistance to the circuit.  This is what was happening with mine.

 

Hopefully this makes everything clear as mud.

 

[AECOM] Scanned from a Xerox Multifunction Printer.tif

Scanned from a Xerox Multifunction Printer.pdf

[DualRatedFlyer]

48 minutes ago, 1964-M20E said:

Lets start at the beginning and if I get any o f the tech stuff wrong anyone is more than welcome to correct me.

We have a 60ohm resistance based fuel level system.  When the gauge sees 60 +/- ohms it shows full, 0+/- ohms it shows empty.  I you have an open circuit on the sending units it will peg the needle all the way past full and a short on the wire from the gauge to the inboard sending unit to ground and it will show completely empty.  A short to ground on the wire to the outboard sending unit and you will never see more than half full.

On the F and J models we have 2 sending units in each tank one inboard and one outboard each rated at 30+/- ohms with the float fully raised and 0 +/- ohms fully lowered.  The outboard unit is grounded at the wing rib.  These units are connected in series 30 ohms + 30 ohms = 60 ohms.  The inboard sending unit should be isolated from ground using the nylon spacers in the screw holes.  A single wire runs from center terminal of the outboard sending unit to the inboard sending unit where it is connected to the surface of the inboard sending unit with a special insulated connector that will keep it isolated from ground and from the screw that attaches it to sending unit but allow it to connect to the surface of the inboard sending unit.  The center terminal of the inboard sending unit is then connected to the fuel gauge.

Hopefully the attached schematics will explain things better.

Now if all you are reading is 1/2 full then the wire from the outboard sending unit to the inboard sending unit is making contact with ground thereby removing the outboard sending unit from the circuit, or the outboard sending unit has shorted out and is not adding any resistance to the circuit.  This is what was happening with mine.

 

Hopefully this makes everything clear as mud.

 

[AECOM] Scanned from a Xerox Multifunction Printer.tif

Scanned from a Xerox Multifunction Printer.pdf

Thanks for the detailed write-up.  I am going to try and check this out this weekend. 

So your issue was the result of bad nylon washers at the inboard unit?  Was this immediately visible.  Hoping mine is on the outboard unit so I dont have to remove the interior....again...

[1964-M20E]

Not very noticeable especially before I understood how the circuit was wired and works.  That makes a big difference.

 

 

[DonMuncy]

I have found that working on the "inner" unit was far easier than the outer, where you have to work up through an inspection panel.