Erratic fuel gauges
 

Hello fellow sufferers,

Fuel gauges this time.

1973 T337G.

The fuel gauges lately have been erratic and erroneous. They flicker, they read wrong. They do it simultaneously, each mirroring the other. Sometimes, in flight they both read empty, even with full tanks.

Has anyone exprienced this problem. Has anyone solved it?

Thanks.
GJ Humphrey

The first place I would look would be to pull the wing panels just outboard of the fuselage, the round ones right at the leading edge. In those holes you will find a big white plastic plug . I would clean all the pins with contact cleaner, look for corroded pins. Sometimes the pins are worn and don't make good contact. With the plugs connected, power on the plane and wiggle the wires at these plugs watching for erratic guage movement. 9 times out of 10 thats where the problem is on our airplanes.
Hope this helps
Kim

Should have read your problem a little better. Might be a bad connection behind the guages there is a long plug that slides on and it may be loose Check the wiring at the box in the overhead.
Kim

Check all the plugs on the signal conditioner which is mounted in the roof,center panel, port side. the small white single conectors are a problem.
If not looks like the signal conditioner has an internal fault. Maybe take the case off and look for any wires off.

Pete

left tank guage fluxuates between correct reading to empty once in awhile so would your suggestions be suitable for this indication? thanks rick

Hi Rick
That looks like signal conditioner problem.
Try moving the wiring to it while watching the the gauge, or tap the conditioner to see what happens.

Pete

that is the one in the roof on the right side,? being stupid, what am i looking for, just wires, or what?

Rick
The signal conditioner is mounted in the roof center panel port side. The box has some adjusters for full and empty on it.
This is for the 337G and on only.

Pete

thank you, have any conversation with jim in yuma? rick

Yes Rick
Sent him a email but waiting for a reply.

Pete

Is this a 337 with the resistive senders? If so, you will note they are notoriously erroneous. I just replaced 3 ($590 each). (Makes me soooo mad too. Silly SW units that look IDENTICAL to my $25 SW unit made for MG’s)

They tend to get more inaccurate as they reach the limits of the throws on the armature. Lead and time make contact with the windings less. Not to mention the sloppy rivet that the arm pivots on.

There is a guy in Canada that rebuilds them for around $100 a piece. If someone is interested I can post his information?

I have a similar problem with my Left gauge. I have narrowed it down to the molex connector on the conditioner box. I can flex the wires there, and the needle will go from where it should be to full. I was hoping it was the connector that plugs into the box, and had that replaced. But it's still doing it. So I guess the connector on the box will have to be replaced (or one of the pins needs to be resoldered inside or something).

I too suffer from erratic fuel gauges on my 73 with capacitance-type senders feeding a signal conditioner (P/N C668004-0102). Because typically the guages go to zero together, we started out by tackling the signal conditioner. Found all connectors OK so we opened the box and found a component whose discoloration suggests that it's burnt (see figure below). Hard to believe that this is it, since 30-40% of the time the guages read accurately, but we'd like to change out the component. Problem is, we can't read what it is. If anyone has a schematic or knows what the component is, please let me know. Meanwhile, we're going to look at the other suggestions, namely the inboard wing connectors and the long plug behind the guages.

Thanks,
Ernie

Ernie
That is a high wattage resistor, which if you have a DVM you should be able to measure the resistance of.
I will have a look through my books and see if i can find a circuit diagram for the conditioner.

Regards
Pete

Hi All
The big trouble with trouble shooting the capacitance system is that with out a breakout box it is also impossiable to pin down the fault.
Cessna once did a box which covered all the aircraft up to the 421, however this was taken over by a Bendix test box.

Find a test box it will save you hours and hours of taking the system apart to fix the fault!

Pete

I agree with Pete. It appears to be a resistor. It is higher wattage than others on the board. This is evidenced by its larger size.

However, don't try to replace it with a resistor based on the value you measure with a volt-ohm meter. You will measure it's current value which is likely to be much higher than its original value. (It may even be totally open - though it is rare, it may be intermittently open - this usually only happens with wire wound resistors).

This resistor is not marked with color bands indicating its value. You may possibly be able to read its value based on numbers printed on it. The burned area may prevent this, but you may be able to get some information from the side of the resistor closest to the circuit board.

Good luck. Hope Pete can find some information from other documentation.

How to Fix Poor Conductance of Long Connector Behing Gauge?
 

On my erratic fuel gauges, we've made some progress but some problems persist. The earlier thought that it might be due to the discolored (burnt?) resistor found in the signal-conditioning box has been abandoned after further testing and the fact that sometimes one gauge will work and the other one won't. Connectors on both the signal-conditioning box and at the wing root seem fine.

We now believe that it's poor conductance in the long connector behind the gauge. After disconnecting and cleaning both male and female sides, there was an improvement but we can't get a consistent signal. We tested and all was well. Did it again and no signal on either gauge. Jiggled it and we get a signal. We then lightly sanded the circuit-board male side and re-cleaned it, and now we have consistent reading of the right tank, with the left tank reading depending on jiggling the connector.

Incidentally, let me note that the design supplies a low voltage to the gauge, especially for low fuel quantities (for a 1/4 full tank it is a fraction of one volt). Anyhow, we're considering replacing the female connector but, because that connector looks good, we're concerned that the problem is really the circuit-board male side, where the copper strips seem worn and may be causing the bad connection. Has anyone run into this? Kim, Pete or anyone else, is there a way of "refurbishing" the circuit-board male side in situ? Any other thoughts?

BTW, day before yesterday, when I went to the airport to work on this, I found that the airplane had been hit by some vehicle (e.g., motorcycle, golf cart) along the front-left side, roughly from below the center of the front-engine left cowling to a point below where the circuit-breakers panel sits. While there is no structural damage, there is considerable skin damage. Oh, did I mention that I have no insurance and the culprit ran away? Anyhow, my attention has shifted to getting this repaired -- getting labor quotes and finding the necessary parts. I will be posting some pictures on a separate thread but, on the matter of parts, let me mention how fortunate we all are that Don Nieser of Commodore Aerospace exists. My thanks to him.

Ernie

Hi Ernie
Thanks for the info.
I had to remove the whole cluster to clean one up, also found that the connections to the various indicators from the printed ciruit not very good and had to remove and clean these. The cluster does come out easy once the pipes are removed.

Pete

Hi Pete,

My name is Ed Asmus and after reading this thread it sounds like the problem I have in my C337G 1973 model. Erratic fuel guage readings began on my last flight last week and my shop is 99% certain the problem is in the "Fuel Control Monitor" also known as the "Signal Conditioner" part number C668004-0102. My mechanic re-soldered the female plug that connects to the control monitor with no improvment. Is it possible to re-solder the pins on the male plug that is attached to the control monitor board? My mechanic thought it could be done seeing as overhauled ones can be purchased so somebody knows how to do this. It looks like to replace the entire control monitor unit would be $1200+ which would make me sad knowing that somewhere there is a simple lose connection. How do you remove the male plug from the control monitor board? Any suggestions?

Thanks Ed

If you read my post above Pete's you'll see that I've also been struggling with the problem. My best advise to you is to make certain that it's the box before you spend the big bucks. Because of the small voltages, the system is sensitive to the aging effects of corrosion and wear on the connectors, resulting in increased impedance and erratic readings. It would not be inaccurate to say that I've kind of given up on the problem, and I now get good readings if it's mostly full and erratic ones under 1/2 full. So I never take off without climbing on the wings and doing a visual check.

Ernie

Ernie,
Aero-Mach Labs in Wichita can fix these things. They fixed mine and it works great now.

www.aeromach.com

Thanks Ernie...I did read the entire thread and I was aware of your heartache over this issue. I agree that I certainly do not require super accurate readings to know how much fuel is on board but it does make for a nice cross check. I suppose more importantly, fuel gages would be my only line of defense should I spring a leak during flight and fuel was exiting the wings without going through the engines. My problem appears to be in the aforementioned fuel control monitor, C668004-0102. My gages indicate quite accurately when they receive electricity but it seems to be an all or nothing affair. If you wiggle the connection on the box, the gages come back to life. Jeff Axel has given me a lead on a possible repair facility to fix my existing box so I'll see where that leads in the morning. Thanks! Ed

Ed
I have replaced that connector, but you need to know what you are doing. You need a proper desoldering rig to do it. As I have said on this thread, the only way to fault find and calibrate this system is with a capacitance test box, it makes the job so easy, saving you time and money at the end of the day. Once calibrated this system is very accurate

Pete

Thanks Pete....I'm going to contact Aeromach today and hope for the best! Thanks!

erratic fuel gages
 

HI GJ,

I was reading your thread on your erratic fuel gage problem in your 73 C337. I also have a 73 with the exact problem you were describing. The gages bounce up and down together mirroring each other exactly. Sometimes they do read zero and sometimes they work fine but what ever they are indicating, they always do it together. They look like wind shield wipers going back and fourth. Did you ever actually find the problem and if so what was it? I have had my signal conditioner repaired by Aero Mach labs in Wichita but I think I need to send it back because I think the problem is in that 9 pin plug on the conditioner. I'm certain we have the same problem. I'm mostly curious if you solved it!!!

Thanks Ed Asmus

GJ,

Same problem here. Been to the shop, more times than I can remember. Replaced each part of that system at least once. Still no hope. Now I ignore the needles and fly by weight/amount of fuel alone. Never less than 3 hrs worth in the tanks for contingency.

So, I am waiting for the AuRACLE CRM 2120 to be released (it is a primary replacement - yahoo!!), and then install it. Good riddens to Cessna fuel gauges.

a simple solution and very accurate instrument.....buy a Shadin FF guage :)

Simple and accurate -- but expensive.

Ernie

expensive, and won't give you any clue if you have a fuel leak. All a fuel flow tells you is what you have burned, not what you have left. I like having both a fuel flow and working fuel guages. If they agree, great, if not, you'd better figure out why....before you have a very quiet Skymaster!!

I have started to give thought to alternate means of determining fuel quantity but I am not ready to give up yet on the Cessna gauges. Mine have worked very well until about 3 weeks ago and we are still working the problem. I will have an update in a few days. I do agree with Jeff in that knowing the quantity or at least close to it is important. I have an expectation when I fly as to what the gauges should say and if I see something different, I want to know why. Especially if one gauge is dropping quickly. I feel it is the only way to know that fuel is not exiting the airplane through a hole in the wing. As for knowing precise fuel flow, I am not ready to make that leap yet. As long as I can "guesstimate" within a 2 or 3 gallons at the pump what the airplane should take erroring on the side of caution and confirm that fuel is not leaving the wing through a hole, I feel that is an acceptable level of safety. Thank you for the input.

And on the subject of fuel exiting the airplane through a hole in the wing, you should know enough about your fuel system to decide what to do if that happens. Specifically, once all the fuel is gone from that wing, it's often preferable to shut off the engine fed by that wing and operate on a single engine, rather than leave that engine on and cross-feed it from the other wing. Why? Because in the cross-feed mode, roughly half of the fuel coming from the other wing ends up in the leaky tank and will go out the wing.

In fact, if fuel conservation is critical (say, over a wide expanse of water), then one should consider doing the opposite cross-feed -- so both both engines are fed from the leaky tank -- as soon as you confirm that fuel is going out the wing. Not only are you now feeding both engines from fuel that will soon be gone, but you are actually moving fuel from the leaky tank to the good one (obviously, this accumulation in the good tank only happens if there is space in that tank, because you've already used some). And in that mode where both engines are fed from the leaky tank, it may be wise to use up as much of the fuel from the leaky tank as possible before it's gone, by climbing to a higher altitude at maximum climb rate. In a utopian scenario, fuel from that tank runs out when you've reached maximum altitude and regained cruising speed. At the slightest sound of fuel starvation -- and it will be very noticeable because both engines will starve -- promptly switch the cross-fed engine to its regular (non-leaking) tank (if you do it quickly, you should not even need to use the boost pump to regain full power) and shut off the other engine.

Disclaimer: These are my procedures based on my understanding of the fuel system, and are not intended as recommendations. My only recommendation is that you learn how your fuel system works and use that knowledge if you encounter a fuel starvation problem.

Ernie

Point well taken! This does bring up and interesting question. Let's assume we have had an engine stop running in cruise and there has been no apparent loss of fuel from the wings. Years ago during my multi training one of the trouble shooting steps to attempt to regain the engine would be to attempt to x-feed fuel to the engine not producing power (try to get fuel from another tank). I have a 1973 C337G (no turbo, no pressure) and this configuration is not addressed anywhere in my manual. The manual does not suggest it as a trouble shooting step or pohibit it. I do however have in my posession an old "Air Facts" magazine from 1973 where a review was written by Leighton Collins (father of Richard Colllins, Flying magazine editor for 1000 years) of the brand new 1973 P337 (yes, pressurized model). In this extensive column, Leighton writes that under no circumstance should both engines be run from the same wing at the same time as this can result in double engine failure. He does not however explain why. I was always curious if this was true and if it applied to my non-pressure model. I have asked my mechanic about this and he cannot see why this would be the case. I have tried running on the ground with both engines running from the same wing with no problems but I have not had the desire to attempt it in flight. Can any of you 1973 P-model guys confirm this and explain why. I am not familiar with the P-model fuel system. Is it different then the non-pressure model of the same year?

I do have a theory but I do not know if it is true. Is it possible that with both engines running from the same wing that fuel could be drawn from the boom tank faster than it can be replaced? It looks as if that small boom tank only holds about a gallon or two of gas though I have never actually messured this? Any feedback!

Thanks Ed

I have never heard that you should not feed both engines from the same tank, and I thought I knew a bit about the subject -- see the Fuel Supply Management Page in my "backup" site at www.SkymasterUS.com

It's not recommended that you take-off or land in that configuration, for obvious reasons, but why would the option of cross-feeding an engine be included if it should not be used?

Because most of my flying is over water, I routinely excercise the cross-feed function and operate at cruise with both engines feeding from the same tank, and it's never been a problem. In fact, I was once coming back from a lengthy Search and Rescue mission on my '69 with four separate tanks and, for reasons I can't recall, I was running both engines out of the same tank when that tank ran out of fuel. Behind me were two passengers who had begged me and my co-pilot to search for their family members lost at sea. You should have seen their faces when both engines quit. I knew where my fuel was and got both engines restarted in seconds.

And on your first subject, an engine stopping, I would certainly consider as one of my options re-starting the engine with fuel from the other tank. I would excercise caution by making sure I understand the problem before I try the opposite-tank restart. For instance, if the engine died from lack of fuel because of a leak in the tank, then (as explained in my earlier message) I would continue with a single engine.

Ernie

Thanks Ernie. I am not at all familiar with all the different fuel systems on all the different skymasters. This article from 1973 that I referenced made this statement about the 73 p-model and it would be interesting to know why. Do you and I have the same model? I have the 73 G model, no-pressure, no-turbo. Which do you have?

Exactly the same -- 73 G model, no-pressure, no-turbo.

Ernie