Fuel sender wiring diagram

[M20Kid]

I just got all 4 fuel senders overhauled for my M20K-231.  My mechanic reinstalled them, but there appears to be a wiring issue.  He doesn't have the wiring diagram and we putting it back together from memory; which appears to be flawed.  Does anyone have the wiring diagram for the fuel system that they could send?  Mine is a 1981 M20K-231, if it matters.

Thank you in advance!

[M20Kid]

I just found the M20K electrical system schematics for M20K planes posted in the Download section.  I downloaded that and sent it to my installer.  If there's something easier to use, I would appreciate that, too.

[Pinecone]

They are pretty easy.  A wire comes from the gauge to the inboard sender.  A wire goes from the inboard sender to the outboard sender.  The outboard sender is connected to ground.

[M20Kid]

I thought it should be pretty easy, as well.  But my mechanic figured out how to do it wrong.  I tried to calibrate a new gauge after having the senders overhauled and found the senders were not responding to fuel when added to the tank.  Sigh ...

[PT20J]

Rebuilt senders and a new gauge? Sounds like there is more to the story. No output sounds like an open circuit somewhere. I don’t think there is any way to connect the wires incorrectly at the senders themselves if you used the factory wiring. 

[M20Kid]

The gauge shows output at about the level I would have expected based on previous attempts to calibrate the gauge, it just doesn't change as fuel is added.  Both sides respond the same - flat.

No new wiring was added; only original wires reattached after installation.

[Pinecone]

Sounds like the floats are stuck.

[PT20J]

2 hours ago, M20Kid said:

The gauge shows output at about the level I would have expected based on previous attempts to calibrate the gauge, it just doesn't change as fuel is added.  Both sides respond the same - flat.

No new wiring was added; only original wires reattached after installation.

This isn't clear. I think you are saying that you have some fuel in the tank and the gauge reads correctly but when you add additional fuel the gauge reading doesn't change. Since it happens in both tanks, I would suspect an installation problem on the outboard senders restricting their motion. 

[M20Kid]

The fuel tanks were drained completely.  After adding unusable fuel, the new gauge give a resistance reading.  After adding fuel, that resistance level should increase - it doesn't.  It reads as if the tanks are empty regardless of how much fuel is added.  The outboard senders don't move until the tanks have over 25 gallons and right now there is only about 3 or 4 gallons in each side.  The outboard senders could have a grounding or other issue, but their lack of motion could not be a factor with this small amount of gas.

[PT20J]

Since the gauge is new and the senders are rebuilt, I would isolate the gauge from the senders for troubleshooting. Mooney is pretty good about not using the airframe for grounding but the fuel gauges are an exception. Since the outboard gauge is grounded to the airframe, it is important that there be a good ground to the airframe at the gauge. If that's not the problem, I would stick a flexible borescope into the empty tank and make sure that the floats are moving freely. If that looks good, I'd disconnect the inboard sender from the gauge making sure that there is continuity between the inboard and outboard senders and that the outboard sender is grounded. Then I would measure the resistance between the inboard sender and ground as I added fuel to verify that the senders are working properly.

 

 

[M20Kid]

Thank you!  That seems a logical place to start.  We'll be at the hangar on Saturday trying to bring some life to this thing.

[MikeOH]

In all this excitement I kinda lost track of how many gallons are actually in each tank...I've never had only 3 to 4 gallons in a tank.  Will that amount even register/cause the float to move?  Can you use a mirror/borescope to see if the fuel level is even up to the float?

[M20Kid]

14 minutes ago, MikeOH said:

In all this excitement I kinda lost track of how many gallons are actually in each tank...I've never had only 3 to 4 gallons in a tank.  Will that amount even register/cause the float to move?  Can you use a mirror/borescope to see if the fuel level is even up to the float?

Mike, the older planes needed more gas.  Mine is supposed to be accurate down to 1.5 gallons, which is considered unusable.  I wouldn't plan to fly with such a low fuel level, but I'm doing the gauge calibration and you only put in small amounts at a time.  When the gauge didn't respond, I quit putting in gas.  37.5 gallons per side would be full in my plane.

[laytonl]

Attached is a simplified diagram I drew several years ago when troubleshooting my gauges.  Lee

[M20Kid]

On 12/5/2024 at 7:32 PM, laytonl said:

Attached is a simplified diagram I drew several years ago when troubleshooting my gauges.  Lee

Thank you, Lee - that's exactly what I was hoping to find!

[M20Kid]

I finally got the wiring done correctly by calling Paul at Weep-No-More.  I've had him reseal the tanks on my Mooney in the past and know he's an expert at Mooney fuel tanks and systems.  His advice was straight forward and easy to follow.  Now my gauge is working property.  Thank you to all who helped make this happen.

In short, the outer sender is installed as a ground and the wire from that sender to the inner must be attached around one of the isolaters so it doesn't get grounded, too.  Pretty easy when you know what to do.

[DCarlton]

44 minutes ago, M20Kid said:

I finally got the wiring done correctly by calling Paul at Weep-No-More.  I've had him reseal the tanks on my Mooney in the past and know he's an expert at Mooney fuel tanks and systems.  His advice was straight forward and easy to follow.  Now my gauge is working property.  Thank you to all who helped make this happen.

In short, the outer sender is installed as a ground and the wire from that sender to the inner must be attached around one of the isolaters so it doesn't get grounded, too.  Pretty easy when you know what to do.

"must be attached around one of the isolators"?   I'm getting ready to install overhauled fuel senders next month.