Fuel Gauge Troubleshooting Tips Please

[brndiar]

In my 1977 C with "J" Instument Cluster the Right Fuel Gauge sometimes reads 0 (Empty). Sometimes, in

flight, or after landing reads correctly again.

When it works, it reads correcty the rest of the day, even after refueling and master switch cycling.

But after some time in hangar  is again "dead".   The plane is always hangared with full tanks.

The respective tank was resealed receantly, but I do not suppose "mechanical" problem with Swimmer, because  If I remember

correcty, the problem was sporadic even before resealig. Last Time I tryed to shake wing, without succes.

Any tips for trobleshooting? 

 

Thanks in advance,

milos

[Fly Boomer]

9 minutes ago, brndiar said:

Any tips for trobleshooting? 

Sounds electrical.  I think you will have to start at one end or the other, and find the intermittent connection (either behind the instrument panel, or behind the cockpit sidewall).  It's a lot of work either way but, for me, the sidewall seems easier than poking around behind the instrument panel.  Maybe you will have luck, and find it immediately.  Others will be along with different opinions.

[47U]

8 hours ago, brndiar said:

Any tips for trobleshooting

When troubleshooting electrical problems, I’ve learned that the first step is to verify the ground.

[brndiar]

23 minutes ago, 47U said:

When troubleshooting electrical problems, I’ve learned that the first step is to verify the ground.

Thanks,

How can I do it practically here?

[carusoam]

Hey Milos!

The fuel level system is both simple, and is capable of having simple failures…

It has…

1) a resistance based fuel float…

2) a bunch of wires…

3) an ancient meter…

 

Hooking a resistance meter to the fuel float output may tell a lot… these can be OH’d… or upgraded with Ceis….

Checking to see if the float is free to move is also important… some get stuck….

Next is the meter on the instrument panel… a needle attached to a wire coil…. Another sticking point….  Tap, tap, tap….

Replacing the six pack of engine instruments is considered a sport around here….  
 

There are some low cost replacement indicators that look very similar… if you have the six individual meters… older Mooneys have one complex meter with six different needles in one housing

See if that helps….

After that… start shopping for a nice primary engine monitor…

PP thoughts only, not a mechanic…

Best regards,

-a-

[carusoam]

2 minutes ago, brndiar said:

Thanks,

How can I do it practically here?

Where grounds get lost…

the instrument panel has ground wires that some times get disconnected…

So check the grounds from the panel, to the airframe, to the fuel level sensor body…

Visually at first to know they are there… and a continuity meter to know they are connected….

 

PP thoughts only, not a mechanic…

Best regards,

-a-

[carusoam]

See if you can find a wiring diagram for the fuel level system…

It may go something like this…

1) 12V source… at the CB…

2) wire into the fuel level sensor…

3) variable resistor at the float… altering the voltage…

4) wire out of the fuel level sensor back to the instrument panel connected to the meter…. Meter reads the volts delivered, and moves the needle calibrated in gallons…

5) The meter is connected to the ground allowing the electrons to drain out as expected…

A common challenge with the variable resistor on the float… when it gets old and dirty enough… the resistance goes to infinity in some spots, thus stopping the flow of electricity… the gauge will probably go to zero as if you turned the master off….

If you are handy with the multimeter… checking the volts coming out of the FL sensor, while moving the float’s arm may tell the whole story…

If you see dead spots in the arm’s arc…. Cleaning and or OHing the float may be in order…

 

PP guesses only, not a mechanic…

Best regards,

-a-

[cliffy]

Does your A&P have any ideas?

I would remove both wires from the sender

Clean the connection areas on the sender with a small fine wire brush

Replace both wire end connectors with new AMP connectors  and reattach

But that is only one area where the problem might be. 

If that doesn't fix the issue you still have the sender itself or the wires at the other end of the system. 

It also does not eliminate the gauge itself. 

Its a simple system but takes time to track down an intermittent problem. 

Check all the wire connections first (including any splices in the wires between the gauge and the sender). 

 If the wires check out then my guess would be the sender in the wing. 

The sender has a wiper on a resistive coil that varies resistance with fuel level. Sometimes the wiper fails to make

contact with the coil and failure is the result.

You can put a 15 ohm high quality resistor across the leads at the sender and the gauge should show 1/2 tank full

If it shows 1/2 tank with the resistor in place,  solder the resistor to the wire ends  and fly it with the resistor in place to see if the 

1/2 tank reading changes. If it doesn't change then the problem is in the sender

If it changes like before you have issues with the wiring or the gauge. 

Could be a few hours of your A&Ps time and a few flights to track it down one item at a time. 

[Kelpro999]

On 10/17/2022 at 9:21 PM, brndiar said:

Thanks,

How can I do it practically here?

Perfect place to start a wiggle test. Wiggle till it works or not, master “on”

[Utah20Gflyer]

I had two INOP fuel gauges and decided that instead of spending money to troubleshoot and then repair 60 year old senders and gauges it made more sense to upgrade to CIES senders and an aerospace logic digital fuel gauge.  2k in parts and now I have much more accurate fuel gauges.   If it's a simple fix like cleaning up a ground then that's an obvious choice but otherwise I'd say it's worth a little extra money to modernize.  

[carusoam]

+1 for this idea…

1) Fight the good fight trying to get the old system to work… all the old bits can be OH’d or replaced…

2) Stop before you spend too much just troubleshooting…

3) Improve the whole system to better than new accuracy using Utah’s post above/below…

-a-

10 minutes ago, Utah20Gflyer said:

I had two INOP fuel gauges and decided that instead of spending money to troubleshoot and then repair 60 year old senders and gauges it made more sense to upgrade to CIES senders and an aerospace logic digital fuel gauge.  2k in parts and now I have much more accurate fuel gauges.   If it's a simple fix like cleaning up a ground then that's an obvious choice but otherwise I'd say it's worth a little extra money to modernize.  

 

[brndiar]

Where can I buy (or let overhaul it)  the analog, resistor-based inboard fuel sender  for my 1977 C Mooney?

Thanks

[0TreeLemur]

18 minutes ago, brndiar said:

Where can I buy (or let overhaul it)  the analog, resistor-based inboard fuel sender  for my 1977 C Mooney?

Thanks

Some years ago, I had one overhauled by Air Parts of Lock Haven, located in Pennsylvania, for a bit less than US$200: http://www.airpartsoflockhaven.com/.

 

[cliffy]

There are (were) actually 2 places in/around Lock Haven to get them repaired

It takes a little looking to find both. 

[David Lloyd]

What others have said, check the connection at the sender is corrosion free, same at the gauge.  If you aren't having other gauge problems, it probably is not the ground.  I did have an amp gauge that would not work most of the time.  Checked everything, finally removed the gauge for replacement and found the needle rubbed the plexi cover.  No, the needle actually rubbed the gauge face.  A little tweak and it works perfect.

[Yetti]

Here let me help.   The gauges will read full with 12volts...    So what that means you are losing the circuit on the right gauge.   Probably not a panel ground lest the other one would do the same.   I would first check the sender connections.   One is on the outboard panel   the other remove the cabin kick panel near the rear seat.   Now that you have it open.  Do continuity tests with the sender wire.

 

[brndiar]

Thanks you for Tips.

Checked Mooney Parts Catalog (rev. June 1977).

Does the late M20C have an Inboard Fuel Senders only ?  (Part Number 610242-001).

Outboard Fuel Senders (No 15 in Catalog, Picture C) are related to F model, but not to C model ?

That s how I understood the Catalog.

At the time I would like to replace not working sender only and keep the stock gauges. There some units to find at aftermarket.

Does someone to know, If Ceis sending units can be used with the stock gauges? Has anyone done this?

Was reading through their install manuals and it looks like the gauges with resistive analog outputs and with digital

outputs are two different parts.

Digital Gauges with Engine monitor will be my next Project. 

 

m

[N201MKTurbo]

2 hours ago, brndiar said:

Thanks you for Tips.

Checked Mooney Parts Catalog (rev. June 1977).

Does the late M20C have an Inboard Fuel Senders only ?  (Part Number 610242-001).

Outboard Fuel Senders (No 15 in Catalog, Picture C) are related to F model, but not to C model ?

That s how I understood the Catalog.

At the time I would like to replace not working sender only and keep the stock gauges. There some units to find at aftermarket.

Does someone to know, If Ceis sending units can be used with the stock gauges? Has anyone done this?

Was reading through their install manuals and it looks like the gauges with resistive analog outputs and with digital

outputs are two different parts.

Digital Gauges with Engine monitor will be my next Project. 

 

m

From the picture you sent earlier, I think you have two senders. I say that because you had two wires going to the inboard sender. The planes with two senders have the inboard sender isolated from ground with insulating washers and the wire from the outboard sender goes to the sender flange. The easiest way to know for sure is to remove the inspection panel just outboard of the fuel tank and see if there is a sender in there. This system can give no reading if any of the connections is bad or either sender is open, usually intermittently. You can see which sender is bad by grounding the wire that goes to the outboard sender, if you get a reading it is the outboard sender if you don't it is probably the inboard sender. Check all the connections first. Also, unhook the wire going to the inboard sender flange and use an ohmmeter to make sure that flange isn't grounded. If it is and the inboard sender works, it will never go below 1/2 tank.

The old senders are riveted and soldered together. You can take them apart by drilling out the rivets, heating them up with a propane torch until the solder melts and they will come apart. After cleaning them up and getting them working again, reassemble with brass screws and nuts and solder it all back together, including the screws and nuts. Make sure there are no voids in your soldering or they will leak. 

This isn't like electronic soldering, it is like plumbing soldering.

[EricJ]

3 hours ago, brndiar said:

Thanks you for Tips.

Checked Mooney Parts Catalog (rev. June 1977).

Does the late M20C have an Inboard Fuel Senders only ?  (Part Number 610242-001).

Outboard Fuel Senders (No 15 in Catalog, Picture C) are related to F model, but not to C model ?

That s how I understood the Catalog.

At the time I would like to replace not working sender only and keep the stock gauges. There some units to find at aftermarket.

Does someone to know, If Ceis sending units can be used with the stock gauges? Has anyone done this?

Was reading through their install manuals and it looks like the gauges with resistive analog outputs and with digital

outputs are two different parts.

Digital Gauges with Engine monitor will be my next Project. 

 

m

I think whether you need one or two senders depends on whether you have 52 or 64 gallon tanks.   The larger tanks require the outboard sender.

My M20J shows the same part numbers for the inboard senders, 610242-001, and outboard senders 610243-001, -003.   I have all of the stock senders from my airplane since I upgraded to CiES senders a few years ago when I had a JPI monitor installed.   They're available if you need cores for rebuild or just replacements.   One of them had a float that doesn't float any more (sinks instead), and I don't remember which it was, but I think it was one of the outboard units.