Fuel gauge reads zero

[Glen Davis]

My friend has a 1985 M20K. The analog fuel gauges read correctly when the airplane is on the ground. Once the airplane departs the ground and is either in climb, cruise or descent the left gauge reads zero while the right gauge reads correctly. This happens no matter which position the fuel selector switch is in. Once the airplane is back on the ground both gauges read correctly once again. Any idea what might be causing this anomaly?

[N201MKTurbo]

It is probably a bad sender. Does it start reading again after some fuel is used? If the sender opens, it will read zero and since they are in series either sender can do it. It could be caused by a little piece of crud on the variable resistor. If so it may resolve itself after a while.

[Pinecone]

Is that correct?  I thought they read Zero when grounded.  Full when 30 ohms per sender in series.

[N201MKTurbo]

23 minutes ago, Pinecone said:

Is that correct?  I thought they read Zero when grounded.  Full when 30 ohms per sender in series.

Maybe. It has been a while since I messed with them. I have a set of senders at the hangar. I’ll measure them next time I’m out there.

[EKG]

To check, what is the OHM range for M20K 231 fuel sender? Buying new gauge, but I see that OHMs can very.

[Pinecone]

0 - 30

[Fix]

Is it possible to access the fuel senders (connection side) from any inspection panel?
Or you need to open fuel tank ?

My Left Fuel gauge is not working as it should.

 

[PT20J]

Inboard senders are accessed from the cabin. Outboard senders are accessed through a removable panel outboard of the tank. No need to unseal the tank. 

[KSMooniac]

Skip is correct... you'll want to empty the tank before pulling the sender(s) but no need to breach the sealed access panels inside the tank.  For the outboard one, just open the lower inspection panel just outboard of the fuel filler cap and it should be on the tank boundary rib with 5 screws holding it to the outboard side of the rib.

I just installed CiES senders and will be listing my OEM's for sale shortly.  I did measure DC resistance and got `~1.5-32/33 OHMS on my multimeter.  

[ArtVandelay]

0 - 30

There’s 2 senders, in series, so the gauge will see 0-60.

[Pinecone]

True, but each sender is 0 - 30 for testing.

[A64Pilot]

If I understand correctly then you should be able to ohm out either sender and determine if there is a short? 

Then disconnect one and that should tell you where the short is?

My Right seems to work, but my left indicates full regardless of level, so I likely have a short in the left?

How often do senders short? I would think an open would be far more likely?

So reading full is zero or 60 Ohms?

I have bladders but assume the senders are located in the same place

Edited October 17, 2024 by A64Pilot

[ArtVandelay]

For those with digital gauges be aware that a sender can be erratic and is ok for analog gauges, but not digital. So when checking individual senders, slowly move arm and make sure the readings don’t jump.

[A64Pilot]

1 hour ago, ArtVandelay said:

For those with digital gauges be aware that a sender can be erratic and is ok for analog gauges, but not digital. So when checking individual senders, slowly move arm and make sure the readings don’t jump.

You know that’s true for old analog gauges too, I’ve seen senders with dead spots that when the sender hits that dead spot the gauge goes full range whichever way an open indicates, that was for tanks with one sender, I assume for those with two it would go to half way from correct reading? Either way it’s very erratic. As fuel is burned off it starts working again because it’s off of the dead spot. I believe most senders are simply a brush that travels up and down a wound resistive wire so the resistance varies with position?

Many digital gauges aren’t digital, just the display is, as such they aren’t any more or less accurate than needles, that by the way can actually be pure digital. I think nearly all automobile gauges are run off of the bus so they are digital, as is the speedometer even if it has a needle.

Just about everything digital on the A model Apache really wasn’t, it was analog “stuff” connected to a DLTU, a device that converted analog to digital. So why we thought we had accuracy to the .1 because the gauge displayed that we really didn’t. D model was pure digital, there was no B or C model fielded.

My personal experience is unless your fuel quantity indication system is capacitive don’t trust it, spend your money on a good fuel burn measuring system preferably one you set the quantity and it decrements from there or do as I do and keep a list and subtract fuel burned from the tank each time you switch tanks, that will have you switching tanks a lot less often. Due I think to the thickness or lack thereof of aircraft fuel tanks you will never reach to accuracy if an automobile without capacitive probes, I don’t think. By that I mean full to empty is 6 inches or so? 

Having said that my little C-140 with its direct reading magnetic gauge stuck in the wing root like some Mooney’s have in the wings is dead nuts accurate, but gen it’s only a 1k gl tank and as it’s a simple mechanical gauge there isn’t much that can break. I have bladders so no magnetic wing gauges but I’d like to have them.

Edited October 17, 2024 by A64Pilot

[Fix]

Anyone has some good pictures or description how to access the inboard fuel senders?

Also anyone that know the difference between 610243-003 vs 610243-001 and 610242-003 vs 610242-001.
 

[PT20J]

You don't really need a picture -- the inboard senders are behind the lower trim panels. Once you get those removed the senders are obvious. 

[Fix]

1 hour ago, PT20J said:

You don't really need a picture -- the inboard senders are behind the lower trim panels. Once you get those removed the senders are obvious. 

Thanks, will check tomorrow.

Are they easy to remove.
Do I need to remove seats etc?

[hammdo]

Its much easier if you remove the copilots…

-Don

[PT20J]

16 hours ago, Fix said:

Thanks, will check tomorrow.

Are they easy to remove.
Do I need to remove seats etc?

I don’t know which interior you have. In mine the lower panels are carpeted and installed first before the side panels so it requires taking the side panels off first. 

[Ragsf15e]

1 hour ago, PT20J said:

I don’t know which interior you have. In mine the lower panels are carpeted and installed first before the side panels so it requires taking the side panels off first. 

Mine is the same.  While not terribly difficult, it will require removing the seat on that side, side panel, lower carpet panel.  Since you’re going to try to get the sender back on and sealed properly, it’s well worth taking the time to get a good working space available.  You don’t want a fuel leak in the cockpit or to have to go back in later.

[Fix]

I found the fuel transmitter behind the lower carpet panel.
I could fold the panel gently to access it, it was done previously too...

For a swap, I think a full removal of panel and seat is preferred to gain space and better access.

I grounded outboard sender, so it's 0 Ohm.
With 27 gallon in the tank, fuel gauge is almost full.
So either the inboard gauge is more than 30ohm or the fuel gauge is not working as it should.
When I grounded inboard sender, fuel gauge went down to zero.
I think I've ruled out the fuel gauge as faulty.

Will measure Ohm tomorrow on Inboard sender.

Will order 2x 30Ohm Potentiometers, then I can quickly test the fuel gauge.

 

[Fix]

Anyone that know the difference between 610243-003 vs 610243-001 and 610242-003 vs 610242-001.

 

Is -001 a working substitute for -003 ?!

[hoot777]

I think I wrote about this before but with all the fuel used gages out there - I have a  JPI - they still will not help if a leak developed in flight. You will not know the quantity in your tank. My dad had a fuel drain break off in flight in a V35 Bo. Made a precautionary landing Chattanooga . All the high tech stuff doesn’t measure quantity. The FAA should be ashamed of the Ricky dink systems they’ve allowed in GA over the years. My GMC Sierra is accurate to 0.1 of a gallon. My M20 G reads so so when it gets low in quantity. I stick my tanks on the pre flight.

[Yetti]

For new fuel gauge sender gaskets https://www.brownaircraft.com/aviation-seals-s/22.htm

Should probably replace the rubber fuel line that is in there while you are at it.

[jamicozzi]

We've learned on ours that the fuel gauges are useless. Read full or almost full all the time. Was running on fumes on the right tank and showing more than 3/4. 

Tricky because the outboards seem to have been added after, and still trying to determine at what point the inboards start to spillover into the outboards. Seems like right around 20 gal.

Looks like we'll be doing an avionics upgrade early spring which will include new senders. Sure will be nice to have a reference beyond constantly having to do the math to keep track of fuel in the tanks...UGH