Do any of you have an S-TEC pressure transducer you are not using (you switched to Tru-Trak) that you'd be willing to part with

Above from Ed

>>>>>>>

 

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If you can part with the pressure transducer for a few days I'd be happy to
check it out for you - we are the manufacturer.  You won't learn anything by
probing with an ohm-meter as the sensors are active devices.  Monitoring the
output while flying is a good way to check them out, though the extra wire
connected could possibly cause an EMI issue that will confuse things.
Interestingly, my Cessna with an S-TEC 30 installed, is having the same
sorts of problems although I have an experimental sensor installed that
shouldn't suffer from this sort of thing.  I haven't had a chance to check
it out.  Also, the symptoms you describe, while not impossible, are an
unusual failure mode for a sensor.  A flaky static system can create 50 feet
of uncertainty as well - is the sensor connected to the static system or is
it measuring pressure inside the cabin?

Above from Gary Casey

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just a data point.   I had an stec-55x in my c172rg.
  It took over a year to get the installation to work properly.
  All sorts of failure modes: uncommanded turns, going offline,
  failure to lock on etc.   We replaced lots of stuff.
 
  In the end it ALL came down to the silly connector they put on the back of
  the tray.
  Replacing the connector and all the pins and the connector housing solved
  the problem, but that might have been overkill.
  It turns out the connector housing is plastic and is held on by two bolts
  that go into the back of the tray.  Every time you take out the computer and
  put it back in again, you are wearing out those through holes in the plastic

housing.

Above from Colyn

>>>>>>

Gentlemen,

I have an S-Tec 50 (circa 1992) that has been updated over time and utilizes GPSS via a G430.  I have been having altitude hold problems all summer and so far this fall.

 

But first, the main connector to the computer is locked together with a spring-loaded latching mechanism.  I have had connectors apart many time and have installed my breakout box between the cable and the computer and I have not seen a problem on the ground.  Colyn and Gary, I hope that the connectors are not the problem as other A/P functions are fine.

 

Here's where I am at:

 

1.  The first thing I did was to clean the brushes and armature on the pitch servo motor.  It ran fine on the ground and in place although it sounded different when run in opposite directions.  It could be heard running from about 3 to 36 volts.  Higher voltage (up to 40 volts) is used by the local avionics shop (JA) to clean the buildup of carbon and I have been told this is good for about a year.  Physically cleaning is a longer term solution although I was recently told by S-Tec that the FAA does not allow S-Tec to "overhaul" or service these motors - they must replace them with new because they have been known to jam from a brush blocking the armature or a short across armature segments.  The motor should turn slowly at very low voltages.  The tech also told me that the brushes can be "hardened" against chipping by heating them with a hair dryer until they smoke a bit, thus getting all the moisture out. (I have not done this because ????).  Anyway, the next flight test provided no control of altitude - that is, hand trimming the elevator was the only way to obtain relatively level flight.

 

2.  There is no way to fully check out the system on the ground because the accelerometer affects elevator management also.  Thus, I suspected the pressure sensor.  Previously I had removed the pressure sensor from the static system thinking that might be a problem.  Using cabin pressure made no difference.  Note that I have carried a spare altimeter aloft using the cabin air pressure which is generally .2  less (200 feet higher than the external static source.  OK, I bought the new, improved sensor for about $800 and temporarily mounted it in the horizontal orientation in a relatively vibration damped area using cabin pressure.  Drat, no difference!

 

3.  S-Tec tech suggested there might be a problem with the computer - so I sent it in.  Returned with a clean bill of health for a mere $215!

 

4.  I cleaned and tested the motor again - this time after completely removing the servo located in the battery compartment behind the co-pilot seat and requiring that the battery be removed also.  BTW, I am now utilizing more nutplates on the battery hold-down and certain A/P servo mounting brackets.  After another failed test flight, I found that low voltage would not turn the motor.  So.... I ordered a new one (note that S-Tec will only sell thru a distributor) for about a mere $600!  The new motor will turn even though I used a single AA battery.

 

5. Let's see, I have now invested about $1600.  Why don't I just switch to a Tru-Trak?  I said to myself, "Self, you have a beautiful fiberglass panel with the three large pilot side instrument holes (TC, DG and AP) that would be replaced by little airplane and knob.  What would I do with those other big holes?"

 

6.  I reconnected the original pressure sensor and went for a flight.  The first engagement of the ALt Hold function caused a rather large climb and dive.  The second engagement got me in a climb angle I didn't like.  OK, OK I did change something else too. I noted that at elevator neutral, the connecting rod end was positioned at about 10 o'clock - towards the nose.  Thus, "large" movements of the servo would result in small fwd-aft movement of the elevator pushrod.  Hmmmm..... pretty much de-sensitized.  Thus, I repositioned the pushrod so that it was at the 12 o'clock at elevator neutral (having forgotten why I had set it up the way it was).

 

7.  My next two flights will be such that first the new expensive (Gary?) pressure sensor will be checked and second (if necessary) the pushrod will be returned to its' original postion.  I will report the results and any further conversations with S-Tec.

 

 

And A Steady Stick Hand.

 

PS: Ed, I may have an extra pressure sensor, but it may not work right???