Fuel sending units and gauge

[all4thekidz]

I have rebuilt sending units that are 30ohms.  The F model has sending units on the inboard and outboard of the fuel tanks.  I could not figure out why the gauge (NEW) was only increasing to 1/4 of a tank when the floats where moved to the up position.  After further investigation it appears the previous owner had Mitchell fuel gauges installed bought from Spruce and the Ohms on the gauge according to the box is 0-200 ohms.  I believe this is the problem! 

I have been told that the sending units are cumulative and I need a 0-60 ohms gauge?

I am not so sure that the Ohms would be cumulative according the wiring diagram in the maintenance manual? 

I have found 30 ohm gauges on spruce for 105 dollars but I don't want to make another mistake as the previous owner had. 

Any insights would be appreciated.

Todd

M20F 1967

[carusoam]

1) you have long tanks...

2) two additive sensors make sense to average the level of fuel as it can move and change the level, erroneously, in flight...

3) if you add two 30 ohm resistors, you get 60 ohms of resistance...

4) sounds like you would need a fuel gauge to match 0 to 60...

5) where this logic goes awry...

• You have a certified plane.
• the fuel instruments are listed in the POH on the MEL...
• They are required for flight... minimum equipment list... they need to be there, ‘they’ are defined by the parts manual...
• Buying stuff on the internet because it might work and be low cost, is best for the experimental crowd...
• Specific part numbers for the fuel senders are used because our tanks are different than most...
• Even the Ceis fuel float guy has given details on the differences between various Mooneys, and what gets in the way... of the float’s arc from full to empty....

6) Many MSers have had their fuel senders rebuilt to work like new, and the gauges can be OH’d as well....

7) Others have gone with Ceis digital fuel floats and use a JPI or similar....

Use extra caution when requesting parts for certified planes... to get what your plane needs.

Sorry for delivering what might be unwanted news... 

Best regards,

-a-

[bob865]

10 hours ago, carusoam said:

3) if you add two 30 ohm resistors, you get 60 ohms of resistance...

Unless you wire them in parallel, then it's 15 ohms. 

[carusoam]

A meeting of the experimental airplane society...   ^^^^

-a-

[wburger1]

I also have a 1967 M20F.  It does have 4 sending units as I opened up the inspection panels and looked.  I removed the connection on the back of the gauge and it measured 30 ohms from the plug to ground for the right tank that was 1/2 full and 20 ohms on the left tank which was about a third full so the gauge must read 0 - 60 ohms. Next I will remove the cab wire from the sender and measure to ground through a 60 ohm resistor and it should read empty.   I checked because my gauge does not go to empty when master is off but is does seem to read correctly when master is on.