Fuel "Control monitor" or "Signal conditioner"

[edasmus]

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?

[Pete Somers]

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

[edasmus]

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

[edasmus]

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

[Ernie Martin]

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

[Pete Somers]

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

[edasmus]

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?

[Pete Somers]

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

[edasmus]

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