Hello Paul
Will first make a few comments on your problem. After my name will comment
on:
"I often think that most of the TCU's functionality could be replaced by a
7 lb spring on the waste gate !!"
Page 10-24 of the owners manual
10.4.6) Orange caution lamp of TCU blinking
Indicates a failure of a sensor, sensor wiring, TCU, or leakage in the
airbox.
Did you hook up a computer to see if all parameters are within reason? It
sounds like if something is out it will not cycle servo.
Is sounds like the growl you are talking about is the slight fighting the
motor/pot/computer does to take control of the servo. I have no first hand
but have read that noise is pretty common out of servo. If the wastegate
is stuck, since the cable is pushing to open, like you said it will just
bow, the servo is more than strong enough to do that.
My experience on model aeroplane servos with dirty or failed pots, is
often they will jitter, or will not park or stop in the same location. On
some servos if there is no pot feedback, it will allow the servo to drive
beyond the stop and break things.
Hook up a laptop first and make sure all is happy. Make sure that the
switch you are using to power the TCU is not making a high resistance
contact, where it is actual dropping battery voltage to TCU (measure
voltage on turn on at the TCU if you can) and when you pump a little more
voltage when alternator does its magic, the TCU is happy. Long wires
feeding TCU or poor contacts/ground can drop voltage as well. Change
switch or take apart and clean contacts if you can. Red #2 pencil erasers
sliced to fit usual do a great job! Be sure to clean out debris after.
If you think the servo is suspect, and you are not quite comfortable
ripping it apart to try and fix it, I will be glad to do the honors for
you. My experience with model servos is precise cleaning with a little bit
of contact cleaner (with lube) usual resolves. It also allows a very close
inspection of the pot, after a while the pot itself wears out and becomes
unserviceable. Sometimes the shotgun spray without disassembly will work,
not reliable enough for me. Sometimes the pot does in fact develop a dead
spot. I have had fair success adjusting the wiper track to a non worn out
part of resistive disk or slider.
Ron Parigoris
"I often think that most of the TCU's functionality could be replaced by a
7 lb spring on the waste gate !!"
I don't agree a spring on wastegate will give anywhere near as much
control that is needed.
Function of TCU only does 2 tasks. Opens or closes Wastegate, and opens or
closes enrichment solenoid. Bad if one tries and make things simplier than
necessary. There is a reasonable amount of things that need to be in order
for the engine to boost over 100%. When it does the enrichment solenoid
will enrich. TCU processes to allow normal boost, but if some input
happens that is undesirable will open wastegate.
Airbox pressure is measured in absolute terms. To achieve 100% power, more
turbo boost is needed at altitude compared to sea level. If you were to
use a 914 on a airboat, and were willing to accept only 100% power, I
could see using a pneumatic solenoid to actuate the wastegate, where
airbox pressure would feed the solenoid to open the wastegate. If you were
to make such a device with an aneroid you could probably use it in your
plane. It however would allow boost if intake air was too hot, would not
know if turbine had potential of overspeeding (that is what I believe
function is of static air sensor, algorythm measures difference of static
and airbox (and other inputs) and knows when overspeed is likely). TCU may
open wastegate if it senses engine overspeed?
If you wanted to put more burden on pilot, you could probably introduce a
aneroid style blow off valve on the airbox (only Cont. and Lyc. I know of
that make such a thing, and probably too big, and allows too much leak)
that will not allow overboost, and have a pilot controlled wastegate.
Instead of a aneroid style mechanical blow off valve, you could take input
---From absolute airbox pressure sender with a little circuit and open a
solenoid to leak out airbox pressure. I was actual toying with the idea of
putting a blow off solenoid in the airbox so in the event the wastegate
stuck closed, you would at least not blow up engine. Solenoid that would
suit bill is not easy to find. I found one that would probably work fine
and filled my requirements, but it is used on the Space Shuttle, lead time
was in years and would cost half as much as a 914. The operator manual
speaks DO178B (10-18) take offs where you turn off TCU with wastegate
closed, and the pilot adjusts throttle for max boost and RPM. Once
critical altitude to clear obstacle, you then turn on TCU. The reason for
this operation is in event of wastegate opening by the TCU at a critical
moment, it may ruin your day. Anyway the reason you don't just leave the
wastegate closed all the time, or for that matter not even have a
wastegate and just let the turbo run full all the time, is when you adjust
manifold pressure with the throttle, yes you may be saving the engine, you
have a severe high airbox pressure that will do good for turbo long life.
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