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LH Fuel gauge shows only 18gal when filled 25

NicoN

By NicoN
in Modern Mooney Discussion

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NicoN Rising Star

NicoN

Posted
Posted

We have a 1980 M20K with 72gal tank capacity.

One or 2 years ago the tanks were re-sealed due to fuel leak.

The other co-owners tell me that since then the left tank is showing 18gal max., even when filled with more.

my own experience is similar. We always fill both tanks with 25gal.

The RH fuel gauge is showing 25gal as expected, while the left is showing - surprise -  18 gals, which is simply half of the complete capcity.

In flight, I think the indication for LH is quite fast moving  to lower numbers, but I did not take the time to Monitor thsi in accordance with fuel flow. I also did not try what happens when the fuel gauge says "0". Still Fuel for about 45min in that tank?

As I have a maintenance Manual for the m20K, I know that there are 2 fuel senders on each side and that they are wired in series.

But the Manual does not give substantial Information on typical failure situations.

Can the above described symptoms be tracked down to a specific fuel Sender and what Kind of Problem can be the reason? Is it the iB or OB fuel Sender?

broken wire? Or simply a fuel arm which stucks to the bottom of the tank?

steingar Grand Master

steingar

Posted
Posted

One year at Oshkosh I asked Don Maxwell what to do to make the fuel gauges show that there was actually fuel aboard the aircraft.  Judging from the reaction from the audience I surmised that I wasn't the only one with that particular problem.

NicoN Rising Star

NicoN

Posted
  • Author
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You have a distinct opinion whta causes the Problem.

Please excuse my stupid question; I am not english native speaking.

I think, "insulator" is meant electrically while "gasket" means something different.

Is there a drawing available where I can see what is needed ? I guess this means one wire goes to the fuel Sender while the Body should be isolated from the aircraft Body?

is there a part I have to order?

DualRatedFlyer Rising Star

DualRatedFlyer

Posted
Posted
20 minutes ago, N201MKTurbo said:

They didn’t install the inboard fuel sender correctly. It needs insulators to isolate it from ground. If they ground the sender it will do what you describe.

Is this true only for the inboard, our could the inverse situation apply -- i.e. outboard is grounded and inboard is not.  Just curious as to why you think its the inboard - i have a similar issue.

N201MKTurbo Grand Master

N201MKTurbo

Posted
Posted

You should search for previous posts. It has been discussed quite a bit.

The fuel senders are wired in series. The outboard sensor is grounded. Its wire is connected to the inboard sender's mounting flange. the inboard sender is insulated from ground with the gasket and insulating washers on the mounting screws. If they neglect to install the insulating washers, it grounds the inboard sender and shorts out the outboard sender. This will show that the outboard sender is empty.

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NicoN Rising Star

NicoN

Posted
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This sounds doable.

I do not have a Manual at the Moment.

What size of screws are used? I think, what I Need are "shoulder washers", right ? Size ?

Another question: Is it possible to do the work without emptying the fuel tank? I mean, put the insulating washer in screw after screw without removing the fuel Sender and gasket.

ArtVandelay Grand Master

ArtVandelay

Posted
Posted

These are the insulators specified in the parts manual.
 

I had a problem when tightened down, the insulators pushing through allowing contact, I had to add an additional plastic washer.
N201MKTurbo Grand Master

N201MKTurbo

Posted
Posted
3 hours ago, teejayevans said:


I had a problem when tightened down, the insulators pushing through allowing contact, I had to add an additional plastic washer.

It has been a while sense I installed some new ones, but I think they are a bit long and I had to trim the ends a bit.

1964-M20E Grand Master

1964-M20E

Posted
Posted

go look at this topic and look at the schematic I attached.   I just solved a similar issue.  Your outboard sending unit wire connected to the inboard sending unit is grounded somewhere. 

Something on the inboard sending unit housing is grounded.

Has the inboard sending unit been removed recently?

Has the wire from the outboard sending unit been disconnected and reconnected recently?

C

 

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  • 2 months later...
NicoN Rising Star

NicoN

Posted
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To bring this Topic to a positive end, I can Report "Mission completed" :)

I had to remove the whole lower interior side Panel including the air vent in the foot room (correct?) to get Access to the fuel Sender.

And indeed: All but one screws had the insulating washer installed. The last had not!.

After isolating the screw my simple "continuity tester" still showed me a Connection between the housing and my screws.

But that effect came from the nearly empty tank. Using an Ohmmeter gave me about 9 Ohms.

Now the fuel gauge Shows the full range!

I am happy!

Thank you for all your help!

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