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Debugging a TCU Problem

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paul.mcallister

Joined: 09 Jan 2006
Posts: 177
Location: Waukesha, WI USA

Posted: Sat Aug 26, 2006 7:33 am Post subject: Debugging a TCU Problem

Hi  all,

  

 I am looking for  some ideas on debugging a TCU - Turbo - waste gate issue. The background is as  follows:

  

 About a year ago,  every now and then the orange TCU light would flash.  I'd power cycle and  it would go away.  Over time it is almost at the point that this  happens every time.  At first I thought it was just the waste gate  sticking, but after further investigation I realized that if the waste was  stuck, the servo would move and the cable would bow in and out. (Lockwood  confirmed this).  

 In my case what  happens is the the servo just doesn't move.  I can hear is making the  normal "growling" noise, but it doesn't do its normal cycle on power up.   If I start the engine and power cycle the TCU then all is  well.

  

 I asked the Lockwood  guys and they think I have a faulty servo.  I have reseated all of the  cables but it didn't make any difference.  I really don't want to buy a  $370 servo just to find out it that its not the problem.

  

 The next step is to go buy a servo to prove this theory, but  before I do I'd be interested in an other ideas.

  

 Thanks,   Paul



    [quote][b]

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Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Aug 26, 2006 7:49 am Post subject: Debugging a TCU Problem

Hi Paul,



 	  Quote: 	
		    

 About a year ago, every now and then the orange TCU light would flash.


	


Do you mean, while engine was running ? During cruise, for instance ?



 	  Quote: 	
		   I'd power cycle and it would go away.  Over time it is almost at the 

 point that this happens every time.  At first I thought it was just 

 the waste gate sticking, but after further investigation I realized 

 that if the waste was stuck, the servo would move and the cable would 

 bow in and out. (Lockwood confirmed this).


	


Are you sure the servo is so powerful as to overcome any cable binding 

or wastegate sticking ?

Have you tried disconnecting  the cable at the servo end to observe what 

is going on at power on ?



 	  Quote: 	
		    

 In my case what happens is the the servo just doesn't move.  I can 

 hear is making the normal "growling" noise, but it doesn't do its 

 normal cycle on power up.  If I start the engine and power cycle the 

 TCU then all is well.


	


If it growls, then it is trying to do its duty. What prevents it ? Cable 

? Wastegate ? Stuck bearings inside ?

By the way, did you install it in the safe environment of the cabin, or 

in the engine compartment inferno ? (We chose the cabin for the TCU and 

servo).



 	  Quote: 	
		    

 I asked the Lockwood guys and they think I have a faulty servo.  I 

 have reseated all of the cables but it didn't make any difference.  I 

 really don't want to buy a $370 servo just to find out it that its not 

 the problem.

  

 The next step is to go buy a servo to prove this theory, but before I 

 do I'd be interested in an other ideas.

  


	


I would first remove it from the airplane, and conduct a close 

inspection and some bench tests.

But first, does it work with no cable ?

And what about the TCU connectors cleanliness, condensation, etc...?

BTW, the yellow light (if correctlly wired) might indicate a TCU 

malfunction ;-((



Keep us posted,

Best regards,

Gilles Thesee

Grenoble, France

http://contrails.free.fr

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Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Aug 26, 2006 8:10 am Post subject: Debugging a TCU Problem

Paul McAllister a écrit :

 	  Quote: 	
		   Hi all,

  

 I am looking for some ideas on debugging a TCU - Turbo - waste gate issue.


	


Paul,



By the way, connecting a laptop to your TCU during tests might provide 

interesting data.



FWIW,

Best regards,

Gilles

http://contrails.free.fr

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paul.mcallister

Joined: 09 Jan 2006
Posts: 177
Location: Waukesha, WI USA

Posted: Sat Aug 26, 2006 8:42 am Post subject: Debugging a TCU Problem

Hi Gilles, 



Thanks for your ideas, see my answers to your questions in the txt

____________________________________________



 	  Quote: 	
		   Do you mean, while engine was running ? During cruise, for instance ?


	


No, this is when I first turn on the master prior to engine start. I

have never had a problem in flight or after engine start up

 	  Quote: 	
		  Are you sure the servo is so powerful as to overcome any cable binding 

or wastegate sticking ?


	


Lockwood Aviation tell me yes.  John Hurst (Ex US Manager of Europa)

confirmed this.



 	  Quote: 	
		  Have you tried disconnecting  the cable at the servo end to observe

 what is going on at power on ?


	


Well, no, but given two independent confirmations I didn't think it was

worth the effort.  



Also, this isn't a repeatable problem.  It happens with the first 

power up, but once it works it says working. So, I could go to all the

trouble to try what you suggest, but it I couldn't conclude any thing

for sure.



 	  Quote: 	
		  If it growls, then it is trying to do its duty. What prevents it ? 

Cable ? Wastegate ? Stuck bearings inside ?


	


Well, it has been my observation that is always makes barely audible

"growling" noise when it is its "steady state" condition.  It's not 

sounding like it is straining.



 	  Quote: 	
		  By the way, did you install it in the safe environment of the cabin,

or in the engine compartment inferno ? (We chose the cabin for the TCU

and servo).


	


Mine is in the cabin

 	  Quote: 	
		  I would first remove it from the airplane, and conduct a close 

inspection and some bench tests.


	


I agree, a close inspection of the unit and the integrity of the crimp

connectors is in order.  One theory I have is the feedback pot

on the servo might be faulty, may be the wiper is intermittent and

all it takes is an initial movement and its okay.  Perhaps the engine

vibration is enough to do this. 



I have inspected all of the cables in the engine compartment.



 	  Quote: 	
		  BTW, the yellow light (if correctlly wired) might indicate a TCU 

malfunction ;-((


	


Yes, the cost of a new TCU would make the price of a servo look like 

small change.  I often think that most of the TCU's functionality 

could be replaced by a 7 lb spring on the waste gate !!



Thanks for your input.



Paul

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Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Aug 26, 2006 9:27 am Post subject: Debugging a TCU Problem

 	  Quote: 	
		   No, this is when I first turn on the master prior to engine start. I

 have never had a problem in flight or after engine start up

   

Do you confirm the MP always stays within specified limits ? At 
	


take-off, for instance ?

If so, have you considered the "yellow indicator failure" hypothesis ?

 	  Quote: 	
		  

   

> Are you sure the servo is so powerful as to overcome any cable binding 

> or wastegate sticking ?

>     

 Lockwood Aviation tell me yes.  John Hurst (Ex US Manager of Europa)

 confirmed this.

   


	


If they did not examine your setup, I would not trust them too much on 

that...

What is sure, is that if the wastegate is stuck, the servo won't be 

powerful enough. It happened to some pilots I know, and sure enough, the 

servo could not overcome the friction.



 	  Quote: 	
		     

> Have you tried disconnecting  the cable at the servo end to observe

> what is going on at power on ?

>     



 Well, no, but given two independent confirmations I didn't think it was

 worth the effort.  



 Also, this isn't a repeatable problem.  It happens with the first 

 power up, but once it works it says working. So, I could go to all the

 trouble to try what you suggest, but it I couldn't conclude any thing

 for sure.

   

If it were my airplane, I would first make sure it is not the cable or 
	


wasegate.

Disconnecting the cable is no big deal.



 	  Quote: 	
		   .....

 I agree, a close inspection of the unit and the integrity of the crimp

 connectors is in order.  One theory I have is the feedback pot

 on the servo might be faulty, may be the wiper is intermittent and

 all it takes is an initial movement and its okay.  Perhaps the engine

 vibration is enough to do this.

Could be, that is if it is the servo...
	


Best of luck in your troubleshooting,

Best regards,

Gilles

http://contrails.free.fr

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rparigor(at)suffolk.lib.n
Guest

Posted: Sat Aug 26, 2006 9:25 pm Post subject: Debugging a TCU Problem

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  (10-1 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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acrojim(at)cfl.rr.com
Guest

Posted: Sun Aug 27, 2006 5:27 am Post subject: Debugging a TCU Problem

Paul

  

 Try disconnecting the big cable connector and  reinserting several times on the TCU. Sometime's a pin will get corroded or  dirty.

  

 Jim

 [quote]   ---

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Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Aug 27, 2006 2:49 pm Post subject: Debugging a TCU Problem

Jim Brown a écrit :

 	  Quote: 	
		  

     About a year ago, every now and then the orange TCU light would

     flash.  I'd power cycle and it would go away.  Over time it

     is almost at the point that this happens every time.  At first I

     thought it was just the waste gate sticking, but after further

     investigation I realized that if the waste was stuck, the servo

     would move and the cable would bow in and out. (Lockwood confirmed

     this).




	


Paul,



Went flying today, and paid special attention to the operation of the 

TCU and lights. The following behavior may serve as a comparison with 

your engine.



On power up, the red and yellow lights illuminate as advertised for 

about one second, the yellow light going off last by about 1/10 second. 

Could not hear the servo behind the panel, due to the gyros spinning up.

At holding point, it takes 25" MP to achieve the 4000 RPM for magneto 

and CS governor checks.

At take-off, I advanced the throttle steadily to the 115 % stop in about 

1.5 second. The MP needle went up direct to the 39.9 "(40) red mark and 

stayed there. The aircraft being lightly loaded I was airborne in a very 

short time.

During climb out I reduced to 5000 RPM/31" i.e. 75 % power.



If your MP remains just at the red line during take-off, you can be 

assured that your TCU and servo are working as expected. I seem to 

remember you told us that you had no problem in flight. So your problem 

may just be that the yellow light doesn't illuminate at power up ?

Have you considered that the problem may lie with your light ?



FWIW,

Best regards,

Gilles

http://contrails.free.fr

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