Fuel Sender Problem

[Rwsavory]

After newly overhauled fuel sender installed, it reads zero, even with 15-20 gal in tank.  The gauge reads full deflection when disconnected from the sender, which is as it should be.  

The sender was removed from the tank and connected to an ohmmeter, which indicated 0-30 ohms when moved through its range, which is correct.  When connected to the gauge it reads appropriately.  However, when re-installed in the tank it only reads empty.

 It seems like something is binding to prevent the float from moving when installed. Our A&P can think of nothing else to try except to remove tank inspection plate on the wing walk.  Any other ideas?

[takair]

So, open is full scale and connected reads empty. Could the terminal be shorting to the case when installed? Have your mechanic test the resistance between the stud and stud. Perhaps the terminal is somehow shorting to case. There is the chance that the arm is getting hung up, but it would have to be the wrong configuration. There is really not much to get hung up on.

[Rwsavory]

Thanks. We will give that a try. I don't understand why installing the sender would create a short, however.

[M20F-1968]

Check your installation and wiring.   One sender is isolated (and NOT grounded) the the airframe and the otheIS grounded.  As I remember the inboard sender is the one which is not grounded.

 

John Breda

[Rwsavory]

Check your installation and wiring.   One sender is isolated (and NOT grounded) the the airframe and the otheIS grounded.  As I remember the inboard sender is the one which is not grounded.

 

John Breda

 

Thanks.  We have only one sender per side in the C.  Both are grounded.

[neilpilot]

Resurrecting this topic since we still can't get right tank fuel sender to work.  My A&P and I are both stumped.

• This sender, part 5641991, was overhauled by Air Parts of Lock Haven [only one sender per tank in 65 M20C]
• With sender out of aircraft, it functions properly.  During manual float travel, the resistance moves from 0 up to 30 ohm.
• With sender installed, tank always reads empty, even when full.  The resistance at the sender is only 0-2 ohm irrespective of fuel level. 
• Panel gauge and wire to sender check out fine.  Gauge has worked properly using a decade box and also when connected to the sender (outside of aircraft). 
• The float does "float" when tested in water. 
• The insulating washer and nut on the sender has not been disturbed.

This sender has been in and out of the aircraft 3 times, always with the same result.  This seems to make no sense; the sender works fine on the bench but seems to be grounded (or the float is not moving up) when installed.   

 

Lunch to the first one who solves this problem. 

[carusoam]

(1)Power needs to get to the resistor...

(2) resistor needs to be attached to ground

(3) NJ guy is looking for free lunch...

(4) now going to check my drawings to see if there is anything else...

Best regards,

-a-

Drawing indicates the flow of electrons goes from the Cluster gage connector to the fuel quantity transmitter to ground.

Check to see if voltage is getting to the transmitter (float). Clean all the connector points.

[cliffy]

Are you installing the sender in the correct orientation with the tank and the float hanging down in the "empty" tank position? 

Just a thought :-)

[neilpilot]

#carusoam - afraid not on the free lunch.  Problem seems to be the opposite - for some reason the sender is going to ground when it should be showing resistant.  If the resistance at the sender was high (for example, due to a corroded connection) the fuel gauge would read high, but it reads empty.

 

#cliffy - good suggestion, except the float installs using 5 screws that are positioned such that the float will only install in the correct orientation.

 

Keep the suggestions coming!

[DonMuncy]

Have you taken the sender out near its position and hooked the wire onto it with the sender in mid-air. Use a jumper from the base of the sender to a good ground. Turn on the master and swing the float.   

[neilpilot]

Don - yes I have, and the sender works perfectly outside the tank, grounded to the aircraft.  When it's in the tank, it gives me 0-2 ohms with 50% fuel.  Outside the tank, I get 15 ohms with the float elevated halfway and 50% on my fuel gauge when connected to the sending wire and grounded to the aircraft.

 

Did I mention that this makes no sense?  That's why I posted, in the hope that someone has experienced this issue or can suggest a path forward.

[DonMuncy]

I don't really know of course; I'm just grasping here, trying to help.

Did you check the ohms from the ground you were using (in the above test), and that of the tank. How about resistance in the "screw to tank and sender". Maybe installing it and jumpering between the sender body and an independent ground.

[larryb]

I think there is a short in the sensor that is activated when the mounting screws are torqued. Or perhaps a screw is too long and shorts when installed. Can you fab a piece of aluminum scrap to mimic the tank mounting hole and mount the sensor to that? Another idea is that there is some sort of interference in the tank and that the arm does not rise when it's in the tank. Or maybe the float doesn't float anymore? You tested it in water but water is heavier than fuel. Sent from my iPad using Tapatalk

[neilpilot]

#Don - I appreciate your suggestions.  Unfortunately the issue is there's an unexplained change in resistant (too little) when the sender is installed.  See you at the Mooney Summit in 2 weeks.

 

#larryb - so far, your suggestions are most appropriate to the problem.  I also think there may be a short occurring internal to the sender when the mounting screws are torqued.  I tried to simulate this at home by putting hand pressure on the mounting flange, and saw no difference.  But in my mind this is the most likely problem.  

 

If one or more of the 5 mounting screws was too long, I don't see that that would cause any problem (and all the screws are OEM).  I also realize that density difference might cause the float to sink in fuel even though it floats in water, but Air Parts of Lock Haven did install a new float with the overhaul (still a possibility), and it's very buoyant in water.  As to interference in the tank, while I can't absolutely rule this out I don't feel any resistance when inserting the float, and the limited view I have in the sender hole while shining a light in the fuel filler looks fine.

 

I will consult with Air Parts of Lock Haven this week, and promise to post the resolution when (if?) the problem is solved.

[takair]

I like Larryb's idea. One more thought, any chance the wire is chaffing when connected. Perhaps it is under tension when installed. It is not the best routing and after a couple of repairs the wire could be too short (no pun intended). I think someone else mentioned this. Install the sender but not the wire. Use a jumper with alligator clips to connect the wire to the stud. This way you can manipulate the wire to look for a short and you can also test the theory of the torque issue. It is also possible that the stud is turning and then shorting, when torqued. You can test this in the plane or on the bench.

[neilpilot]

I like Larryb's idea. One more thought, any chance the wire is chaffing when connected. Perhaps it is under tension when installed. It is not the best routing and after a couple of repairs the wire could be too short (no pun intended). I think someone else mentioned this. Install the sender but not the wire. Use a jumper with alligator clips to connect the wire to the stud. This way you can manipulate the wire to look for a short and you can also test the theory of the torque issue. It is also possible that the stud is turning and then shorting, when torqued. You can test this in the plane or on the bench.

As I indicated above, when mounted in the tank the sender resistance is not increasing with fuel level.  This measurement is taken with the wire disconnected, and has nothing to do with the condition of the wire.

 

Tomorrow I plan to bolt the sender to a Piper’s fuel tank my A&P has out of the aircraft, to determine if there’s an internal short to ground.  If the sender checks out good (runs to 30 ohms when the float is moved up), I will consider Don Maxwell’s suggestion to open my right tank panel so I can move the sender by hand with it installed (something I really don’t want to do).

[N201MKTurbo]

Tie a string to the float and route it through the fuel cap. You should be able to get about 1/3 deflection when you pull on it.

Use the string to actuate the sender while you tighten the screws and see if you can figure it out.

Fine print:

Remove string before flying plane....

[DrBill]

Are there left and right senders ??  Sounds to me like the sender is in the wrong orientation when installed in the tank.

BILL
 

[Awful_Charlie]

Has there been any work done inside the tank recently? I had a case where the float would sometimes 'stick' to the sealant on the bottom of the tank.  A good thump on the bottom of the wing where the float settled would then make it spring up.  If the sealant isn't at all tacky, then check to make sure the float and arm are clear of the internal metalwork - there's not a lot of clearance (on the outboard Bravo ones anyway), and a judicious slight bend of the arm stops it fouling on the tank internals.

 

Good luck!

 

Ben

[neilpilot]

Waiting to hear how the sender does when bolted on a Piper tank (see post #16).

 

#N201MKTurbo - finally something that makes sense, that I may be able to try.  Not sure I can fish a string through as you suggest, but if the sender checks out on the Piper tank that's what I may try before we remove a tank inspection panel.

 

#DrBill - there are L & R senders, but I've triple checked the part number against the Mooney parts listing.  I also took a photo when the senders were removed for OH, and the correct sender is in the R tank.

#Aw_Charlie - stock tanks with no work done for many years.  No ready to try bending the float wire yet.

 

So far larryb (post #13) and N201MKTurbo  (post #17) seem to be the front runners for that free lunch.  Updates to follow.

[larryb]

I have a video inspection camera. It's a little camera on the end of a 36" flexible wand. There is a LCD screen in the handle. I got it on amazon, and they can be found at Home Depot. You can use it through the filler neck to see what's going around the sender after it's installed.

Sent from my iPad using Tapatalk

[N201MKTurbo]

If you determine that float is sticking to the bottom of the tank, you can probably clean the tank where float lands by taking some pieces of shop rag and stapling them to a paint stir stick, saturating them with MEK and clean the spot through the hole. I recall that the place where the float lands is just sheet metal and any sealant there can be removed. I would suspect if it is sticking that it is sticking to the red stuff and not the polysulfide. The red stuff comes off easily with MEK.

[neilpilot]

My A&P reports that the sender worked fine (0-30 ohm) when bolted on a Piper tank and moved by hand.  Friday I plan to first take a look in the tank using a borescope or inspection camera, and then go from there.  Will report on my findings this weekend.

[neilpilot]

Here's my update.  With the sender removed used a boro scope to look for any obstructions; none found.  Then fished a string through the fuel filler and wing spar and tied it to the float, as N201MKTurbo suggested.  Pulled on the string to lift the (installed) float about halfway up and the panel reading moved smoothly between zero and half full.  The boro scope once again confirmed nothing holding the float down. 

 

So with the string removed (thanks N201MKTurbo!) we proceeded to add 15 gal of fuel, in 5 gallon increments, and the panel stayed on empty.  The same problem we reported earlier, even though there's no logical reason why the float would not float.  At that point, we decided to give up since we couldn't think of anything else to try.

 

After leaving the maintenance hangar, as I taxied to SS fuel the reading remained at empty.  Once topped, on the way to our hangar the panel read FULL, something it hadn't done since the O/Hed sender was first installed.  Since no actual repair was done, I'm not optimistic that the system is now functional.  If the weather cooperates, will test the system next weekend on the flight to Panama City's Mooney Summit.

 

N201MKTurbo gets the lunch (Larryb was a close runner up).  Thanks to all for your input.

 

woops - edited for correct winner (string-on-float idea was the winner)

[Marauder]

If it does still not work, I am voting for a leaking float.

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