Fuel "Control monitor" or "Signal conditioner"
 

Anyone out there know where I may find a fuel control monitor, Cessna part number C668004-0102 for a 1973 C337G model (no-turbo, no-pressure)? I have quotes from Cessna and Preferred Airparts for well over $1000 for new or used and a quote in the works from KRN Inc. I also found an old post from years ago on this site that actually listed the part I need from a parted out Skymaster. Hopefully, he will respond. I will go through the part locator flow chart I have seen on this site but I thought I would throw this out there in case anyone happen to have one in hand they don't want! Also, I am open for suggestions as to anyone who may be able to repair the one I have. My shop said this may be possible in the right hands. Apparently the plug that is on the circuit board has some lose pins and they need to be reworked or replaced. Thanks in advance! :o)

you might give Aero-Mach Labs a call, they can repair that part.
www.aeromach.com
316-682-4678
they fixed mine and it works just fine...

Awesome...thanks Jeff, I'll give them a try in the morning.

Any recollection as to the cost of that repair? Thanks!

they do time and materials, so my fix might not be the same as yours. It wasn't unreasonable though.....of course, what is reasonable for me might not be for you!! : )

Thanks...I'll keep my fingures crossed! Believe it or not, after 20 years of airplane ownership I am rarely taken back by maintenance issues. This is just a crazy expensive hobby and that's all there is to it! Not for the faint of heart, that's for sure.

Well, the part is getting shipped to Aero-Mach Labs today and it sounds promising. Thanks again and I'll keep everyone posted on the results. I will be out of town for a week or so but I will have updates as they become available.

Well the signal conditioner was returned from Aero-Mach with apparently several items repaired that were "not per factory specs." The bill from Aero-Mach was $470 plus some shipping charges which seemed reasonable compared to any other options. My shop installed the unit last Friday ($140 to them) and calibrated the unit with the fuel gages. I got in the airplane yesterday (Saturday) and after visually inspecting the fuel tanks and seeing they were full, I turned on the master switch to see the fuel gages indicating EMPTY!!! :o( %^&*(#@!$%^&%^$

After engine starts the gages came to life although occasionally still wiggling back and forth until reaching cruise at which they worked fine and indicated correctly as fuel was being used. I suppose something is loose somewhere so we will dig some more and I will share the info as it becomes available.

That sounds good for the repair, but now down to finding out what is really wrong with the system.
Firstly replace the white connector on the signal conditioner.
Check the wing break plugs for any signs of corrosion.

The pins on both of these plugs and sockets suffer from a build up of corrosion and the only way to cure this is to replace them.

The plug and socket on the back of the instrument cluster again suffers from the above and needs to be cleaned.

The fuel indicating system on the 337G - H requires some looking after and any corrosion etc will deem the system useless.

Did your shop use a capacitance test box to calibrate the system??

Good luck
Pete

There's another thread on this with lots of info. I've checked everything and still get erratic readings. The signal level, especially at smaller fuel loads, is quite small, so any corrosion or other impedance screws up the reading. I've given up, take the readings when they're there (I never get a positive but inaccurate reading -- it's either correct or zero) and never take off without visual confirmation of fuel in the tanks.

Ernie

Ernie
What you have to remember is that these aircraft are getting old now, you live down in Miami and The Bahamas and the sea air does take its toll on the electrics. The pins and sockets used on the white connector do suffer and do need inspecting, this affects not only the Cessna but all other makes as well. So even unplugging a connector cleaning and reconnecting may cure the problem. I have seen it over here with Aztecs/Navajo and a lot of Cessna 150/152/172 based at sea side airfields.

See you
Cheers
Pete

I usually find the problem is the connection in the wing root, the pins are usually corroded or loose. I'll replace them or clean them then try to close the female pins a little. I have also found the probe wiring inside the tank worn through from years of sloshing around and rubbing on the tank floor.
Hope this helps
Kim

Thanks much for all the responses. I'll be back to the shop by Tuesday with a print out of all your input and we will get to work. I'll keep you posted. Thanks again!!! The help is greatly appreciated!

Update on signal conditioner. At the end of last week my IA and myself checked every possible connection we could and all was well. My IA still felt the problem was in the signal conditioner. So I called Aero-Mach labs and told them of the problem still randomly occurring. They were very understanding and said they would be happy to take another look at my signal conditioner. So back it went for a second time to Wichita. I just spoke with the technician a few minutes ago and THANKFULLY, the first time he hooked the box to his machine, the box failed. YIPPEE! He has found loose capacitors and who knows what else (the technical talk is beyond me) but he feels these are certainly items that can cause the problems I have been having. He is continuing to work the problem until he is confident the signal conditioner is stable. He has found that the problem is affected by temperature as well as by vibration. Unfortunately the first time he had the box he did not do any temp or vibration checks. When we sent it back the second time, we specifically asked for these tests. As for the cost, my initial repair was $470. The worst case scenario is $702 if the box gets a complete overhaul. I'll keep you posted.

Ed Asmus

Ed
Thats good news, however if you certify a part as serviceable the unit should have been checked in accordance with with the manufactures spec and clearly it was not.
It has cost you money to go through the wiring, plugs and to find out it is the box you first started out suspecting.

Not good.

Regards
Pete

Pete, in theory I'm with you. But how many repair facilities have a shaker table to simulate vibration? For that matter, how many subject a repaired item to temperature limits to verify specifications? The issue here is going to be money. If Aero-Mach recognizes that it had an obligation to verify conformance across the spectrum of environmental conditions and fixes the unit without additional cost, I think Ed would be doing well.

Ernie

I hear what both of you are saying. I do struggle with this stuff all the time when it comes to maintenance bills. In a perfect world I would walk into the shop, tell of the problem, and they would fix it correctly the first time. Unfortunately it does not work that way all the time based on the experience I have had over 18+ years of airplane ownership and 7 years of Skymaster ownership. In the real world it becomes a process of elimination starting with the most likely cause and working from there. Inevitably something else gets broken in the process (which I just experienced with a fuel pump wire while trying to repair a leak in the pitot/static system). What I do look for in people is a good faith effort in trying to resolve the problem. Certainly I expect competence but not perfection. It becomes a balance between overall service and occasional frustrations, a balance between working with my home shop versus the time and expense of flying the plane around to different shops. My shop has been in the airplane business for 40+ years and my mechanic has been there since day one. They have been a Cessna repair station forever and I believe they have seen everything there is to see yet sometimes they still get fooled. I do pay for it, a lot, but looking at the big picture, they get most of it right. I wish I could do all this stuff myself but I can't, not even close. So as difficult as it can be, I have to put my faith and money in other people, encourage them to do their best, be a friend, not a jerk, and hope for the best. If I was truly dissatisfied, I would go somewhere else. As for Aero-Mach, they have been always polite on the phone and seem to be doing all they can to fix my signal conditioner. As for the cost, I will pay their bill knowing it was still less than getting a new one from Cessna and chalk it up as another chunk of money to owning an airplane. When the day comes that I don't want to pay anymore, I won't, and the airplane will be somewhere else. This forum is a great way to hopefully take a piece of that cost and put it back in our wallets (and use it for 100LL instead). It's all good!

Couldn't agree more. Great way to look at things.

Ernie

Absolutely a great attitude!

I too have had reoccuring problems with my left fuel guage, and noticed something a few weeks ago when working on a fuel leak caused by a dried cork . If I played with the wires that go between the outboard sensor, and the inboard, it caused the fuel guage to spike, and at times work. This even if I isolated (taped down) the connections that went through the tank caps.

I now beleive that I may have a crack in the wire itself. Why else when I move them around while isolating the connections, should I get what amounts to different impendence or continuity .

Roger
You have the resistor type system on your T337D, now this system requires quite a bit of maintenance, the grounding of the tank unit has to be 'spot on' if you have any corrosion under the tank units or the ground strap is not clean this system will not work correctly.
On the resistor type system a open circuit from the gauge to the the tank unit will cause full scale deflection of of the fuel gauge, and a short would cause the opposite effect.

Easy check for volts at the tank unit, also check that the tank unit is grounded and no corrosion on any of the connections.

Regards
Pete

Update: I spoke with Aero-Mach a few hours ago and they are sending my signal conditioner back and they feel they have it working per spec under all conditions now. They found several things wrong and feel they have the unit repaired. I should have it by Monday so I will keep you all posted as to the results. They did say however if I still have problems, they would not attempt another repair but would only sell me an overhauled unit. Aero-Mach did not charge me any additional amount for the second round of repairs so kudos to them. I hope it works when we get it all back together. Stay tuned!

The signal conditioner arrived today and I promptly went out to the hanger to hook it up. It PROMPTLY failed. My IA got the same intermittent behavior on the gauges by wiggling the 9 pin plug that is attached to the signal conditioner. Aero-Mach is sending an overhauled unit tomorrow to see if I get the same behavior with that one. This is fun.

I was kind of unhappy when I gave up on the system and now rely on fueling supervision, visual confirmation and time. After hearing what you're going through -- no longer unhappy.

Ernie

hahahahaha.....Thanks Ernie, that was funny!

I received the over-hauled signal conditioner on Friday and plugged it in today (Monday). It has the same erratic behavior as my original one.

Does anybody know what it means, as far as the fuel gauges are concerned, when turning the master switch on causes the gauges to indicate negative or in other words, below empty. When the gauges work, they work fine. When they don't, they indicate below empty. If you turn the master switch off when the gauges are indicating negative, they rise slightly and indicate empty. When the master switch is turned on and the gauges are not working, they get "sucked" below empty. It is probably a short somewhere. Anybody have any theories?

OK Dave.

1. Was thew plug replaced on the signal conditioner?

2. Inspect the plug on the back of the instrument cluster, that does mean removing the plug and checking it.

3. Has your engineers got the proper test box for this system, if not it will make fault finding almost impossible.

I keep on about having a test box for this system, however you choose to ignore me, you cannot maintain the capacitance system without it. I have kept quiet for a while on this one as I just wanted to see what other owners do, and it does seem to me 'not a lot' perhaps you will listen now.

As regard the test box, Cessna made one years ago to cover all the aircraft with this system, this included all the connectors for the signal conditioners. Bendix did one as well and you could use the Cessna connecters.

Let me know how you get on

Pete

Pete,

I am printing your response and taking it to my shop. You cannot believe how frustrated I am over this issue. This is one of those times when I am simply trusting the people who supposedly know how to fix airplanes. I am an air traffic controller not an electrical tech or maintenance tech. If these people can't fix it, then I lose! Simple as that.

Ed

What I can tell you Pete is that Aero-Mach has given me two signal conditioners. My "repaired" one and an "overhauled" one. I have them both and the system behaves the same way with either signal conditioner hooked up. That would lead me to believe that the problem is not with the signal conditioner but somewhere else. Maybe that is a bad assumption. I really have no idea anymore.

Ed

Two points.

For Pete: Where does one find, or who would have, such a box? How does one find a shop that has one? Dealers only?

For Ed: From my experience, it's understandable to be frustrated, but not at the people. I went through it when my mechanic (who I rate highly) and we looked at every item and tried everything. It's just a very elusive system, especially without the right tools. We inspected every connector and lightly sanded them to remove any corrosion and opened pins and closed females for more solid contacts, including the ones behind the cluster mentioned by Pete (there's a thread in this Message Board about it). Remember that the system relies on very tiny signal levels of capacitance, so any minor impairments in the lines can affect readings. I think Pete is correct that it's virtually impossible to troubleshoot the system without the specialized box.

Ernie

Sorry Ed I put Dave just me having a senior moment.

Ed
I am only trying to help and of course try to save you time and money, it goes to say that the newer the system these days the more test equipment is needed to fault find and calibrate the system, ie it would be nice to have a Garmin G1000 but who can repair it when it goes wrong, Garmin!

The fuel gauging systems in GA aircraft is rubbish, however the old float type could be made to work quite well, then Cessna went on to this superduper capacitance system and well you have the end result.

I have said that this system CANNOT be troubleshooted with a screwdriver and a DVM, you need a test box with the adapter cables.

Good luck and let me know.

Regards
Pete

Ernie and Pete:

Thanks to both. Rest assured I am not frustrated at the people. Everyone is trying to help. I am very frustrated at the situation. After all man has had fuel gauges on every engine driven vehicle since the beginning of time. It seems to me that this should not be so hard. They are just fuel gauges. What was Cessna thinking?

Ernie and Ed

Give somebody like Yingling a call and see if they would hire out a box, all of the Cessna twin engine airplane have this system so they must be around so that you can calibrate the system correctly. Surely you maintenance guys will have some contacts. Give Cessna a call and see if the box they made is still on the go.
I am very surprised that these airplanes are certified without fuel gauges and maintenance shops that have not got the proper equipment to fix a problem?

Regards
Pete

Pete and Ernie,

I just returned from my shop with your email (Pete's) and amazingly my shop has your magic box. My mechanic says he has rarely had to use it as he was always able to solve these problems by repairing connections. He did say the test box is required equipment to have at a "Cessna Certified Repair Station" which my shop is. As I headed over to the shop, I was prepared to "pull the plug" on this enjoyable little project and live with intermittent fuel gauges but since they have the test box, I decided to let the saga continue. They should have the plane back in their shop by this afternoon (Chicago time) so stay tuned!

Ed

Well, the problem is still not solved but I guess it is as good as it is gonna be. The good news is that the fuel gauges work about 98% of the time and are stable while they are working. We just cannot seem to completely eliminate the problem. My mechanic saw nothing unusual while utilizing the test box other than he could impact voltage readings simply by by puting his hands around wires. I have no explanation for that. This was not an attempt to wiggle wires to search for a loose connection (although we have done that plenty) but an attempt to simply see what happens if you put your hands around wires.

My original signal conditioner has been sent back to Aero-Mach for good because my shop was able to get that one to consistently fail with the associated behavior of the gauges. They were unable to get the overhauled conditioner to fail but I knew I did when I intitially installed it myself. During the install testing with the test box the overhauled box behaved perfectly but shortly into my first flight (before I left the ground) the problem reappeared. We may be chasing multiple problems here. Who knows......

So my plan is to just fly until the annual in August and see if I can make any correlations with the supposedly random brief failures. My shop is going to work more on the connections at the gauges but as I said, not until August. The airplane is back together now and is going to stay that way and is working well otherwise. If any major developments should occur, I will certainly pass it on. If anyone else out there solves this problem, please share!

Ed
This sounds a good one! I wonder if you have a bonding problem with the tank units. The wires from the tank units to the conditioner are screened and that should not happen. If you squeezed the wire you will alter the capacitance. I think now you need to check the whole bonding of the tank units and the wing joints.
However that is as you say a job at the annual when the panels are off, have the pleasure of flying it for the summer, enjoy!

Pete

Hmmmm....thanks Pete, I will share that thought with my mechanic!

Ed

Just got an interesting phone call from Aero-Mach Labs. They told me after receiving my original signal conditioner back at their shop for the third time (after I decided to keep the "overhauled" unit), they bench tested it again and found 5 more failed transistors. This really makes me wonder if the "overhauled" unit I now have is any better than the original one. They did say that under warranty for one year they would be happy to give my overhauled unit that I now have a free "check-up" to see how it is doing. I think I will do that in August/September 2009 when the airplane goes in for annual.

Ed
That is rather annoying, as these people have had the box twice or is it three times and found faults that during the first test should have been cleared. You know it is much like the UK now to get a unit repaired like yours is almost impossible, to get a radio repaired in the UK is the same. The skilled people are just not here any more.
The Skymaster is a easy airplane to both service and troubleshoot all you have to do is go back a few years and think logically.

Regards
Pete