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Fuel quantity sender help


chuck459

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The right fuel quantity gage in the instrument cluster of my 1979 M20J has developed a problem.  It will not move below a half-full indication.  My shop has swapped gages and found both to be working properly.  The outboard fuel quantity transmitter also seems to be working properly.  That indicates a problem with the inboard fuel quantity transmitter which is extremely difficult to access.  My shop is not a Mooney Service Center and has never accessed this part.  They were more than happy at my decision to continue flying the airplane for a while with the hope that the sender may “unstick” and began working again.  I realize this is simply putting off a problem which is going to have to be fixed eventually.  To help me better understand what is involved in accessing this fuel sender, could someone explain the correct procedure of reaching this component.  Can the fuel sender be accessed from below the wing, or will the access plate under the wing-walk have to be opened?  Since the closest MSC is over two hours away, it would be nice to fix this problem locally.  Any ideas would be appreciated.

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The right fuel quantity gage in the instrument cluster of my 1979 M20J has developed a problem.  It will not move below a half-full indication.  My shop has swapped gages and found both to be working properly.  The outboard fuel quantity transmitter also seems to be working properly.  That indicates a problem with the inboard fuel quantity transmitter which is extremely difficult to access.  My shop is not a Mooney Service Center and has never accessed this part.  They were more than happy at my decision to continue flying the airplane for a while with the hope that the sender may “unstick” and began working again.  I realize this is simply putting off a problem which is going to have to be fixed eventually.  To help me better understand what is involved in accessing this fuel sender, could someone explain the correct procedure of reaching this component.  Can the fuel sender be accessed from below the wing, or will the access plate under the wing-walk have to be opened?  Since the closest MSC is over two hours away, it would be nice to fix this problem locally.  Any ideas would be appreciated.


As Don mentioned, the inboard fuel senders are behind the carpeted lower panels. Removed them and you will find they look like this:

9276568b2471c5a6323d0bd67448565c.jpg

When removed, the sender will look like this:

aba25a0bdcce5c8bd319dcdd0abfc2bb.jpg

You should replace the cork gasket when you put another one in. Depending on which inboard it is, it will have a different bend to it.

de92a33fa58d376c6e7c8be0a1497905.jpg


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Take note of those plastic washers. The wire coming from the gauge goes to the terminal which goes through the coil in the sender to the sender body. The body is not grounded on the inboard end. The wire then goes from the body to the outboard sender, through the coil and to the body which is grounded. Thus the senders are electrically "in series", reading the total resistance through both coils. The end result is the gauge reads the average of the two senders.

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Take note of those plastic washers. The wire coming from the gauge goes to the terminal which goes through the coil in the sender to the sender body. The body is not grounded on the inboard end. The wire then goes from the body to the outboard sender, through the coil and to the body which is grounded. Thus the senders are electrically "in series", reading the total resistance through both coils. The end result is the gauge reads the average of the two senders.

So what is the resistance when full and then at empty?
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41 minutes ago, teejayevans said:


So what is the resistance when full and then at empty?

Unfortunately, I don't remember which way they go nor the actual numbers. I saw somewhere that Don Maxwell cited the resistance values. I have a used one at the hangar, but it might not be accurate. I could check it if it would help anyone.

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My airport is in such turmoil due to the fires in So. Cal. I am not in the mood to start working on this problem right now.  I've taken notes on everything posted here and definitely appreciate the helpful advise.  At least I know how to proceed.  Thanks again. 

SZPFire12-12-17.thumb.jpg.4a61b6c4c8785f6b4b4b09c73f4e2fe9.jpg

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I once asked Don Maxwell himself how to make the fuel gauges in my aircraft indicate whether there was any fuel in my aircraft.  The reaction from the audience suggested I was far from alone in this problem.

Put another way:

Fuel Gauges!  We don't need no stinkin' fuel gauges!

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I once asked Don Maxwell himself how to make the fuel gauges in my aircraft indicate whether there was any fuel in my aircraft.  The reaction from the audience suggested I was far from alone in this problem.
Put another way:
Fuel Gauges!  We don't need no stinkin' fuel gauges!

I have 2 words for you: tomato Flames
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My old sender arms are shaped somewhat differently from Marauder's. See pic.

If the arm pivots near the bulkhead of the tank, as with the top unit in the pic, it is possible, in fact it happened with mine, that the bent rod hangs up on the coated nut on the inside of the tanks. In my case, the gauge indicates full until the actual level drops by 10 gallons or more. Fortunately in my case the gauge was quite accurate from there to empty.

N943RW fuel transducers.jpg

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