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Hi everyone,

I seem to have a random issue with the waste gate servo on my 914 UL (TSN 200hrs).

At power on TCU lamps self test for 1-2 sec, than switch off.

Wastegate servo performs self test from fully open to fully closed position, but then seems to overturn into the closed position and gets stuck there. ( image 1 in the attachment shows servo in the stuck position).

Yellow TCU warning light starts flashing, as it should.

If I power off and power on again the servo stays stuck and yellow TCU light starts flashing right away.

To free the servo up again I opened it up very carefully and reassembled it.

After that it worked for some time until randomly it turned too far and got stuck again.

I think the servo turns way too far into the closed position sometimes, but I am not sure about the cause:

Is my wastegate cable too loose?

Or is this an issue with the wipers inside the servo?

I also checked the nominal resistances at the servo connectors according to Rotax Maintenance Manual Heavy  912/914, 78-20-00 ( see image 2 in the attachment):

Connectors: E-C: 5,00 kΩ > ok

Connectors: E-D: 5,02 kΩ > way too high, should be 0 to 70 Ω ! 

Connectors: C-D: 5,6 kΩ > ok

Connectors A-B: 10 Ω > ok

 

Did anybody experience this issue before maybe and know a solution?

Many thanks for your help everyone!

 

 

 

10420_1_IMG_4227.JPG (You do not have access to download this file.)
10420_1_IMG_1012.jpg (You do not have access to download this file.)
  • Re: 914 wastegate servo issue

    by » 5 months ago


    Hello Tom,

    I really don’t know however this might be a mechanical issue rather than electrical. I wonder if the high resistance reading you found on pins E-D is because the servo had “over travelled”? Was it in the 0% position?

    The Line Maintenace manual 12-20-00 Page 65 describes the inspection and lubrication requirements for the waste gate. It makes reference to Heavy Maintenance Manual 78-00-00 for adjusting the waste gate but I have found that information to be in the Heavy Maintenance Manual 78-20-00 Pages 22-25. (It describes the installation and adjustment of the servo motor and waste gate control components.)

    It might be worth cleaning, lubricating and adjusting the mechanism before looking for electrical problems.

    Rotax Owner video here 

    https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/280-wastegate171

    Cheers, Des.

    39562_2_IMG_2400.png (You do not have access to download this file.)
    39562_2_IMG_2401.png (You do not have access to download this file.)

  • Re: 914 wastegate servo issue

    by » 5 weeks ago


    Hi Des,

     

    many thanks for your input and very sorry for my late reply.

    I had a problem with my wastegate servo randomly turning too far, blocking the wastegate in the fully closed position. Tension of the cable and lubrification of the wastegate spindle have been according to the books. (LMM Manual and HMM Manual, as you quoted.)

    The most likely reason for the overturning is the servo´s wipers, their lubrification and contact to the printed poti counterpart.

    As I have very painfully found out in the meanwhile, the stuck servo has already caused an overboost of my engine before I bought it.

    In the meanwhile I have replaced the entire servo and checked the nominal resistance values of my new servo before putting it into the aircraft:

    Strangely the resistance values E-D where at 5kOhm on the brand new servo, too! ( it was measured, I hear you, in the correct position).

    The mechanic that helped me had another new servo with him coincidentally. Out of curiosity we measured the resistance value E-D of this other brand new servo. It was also at 5 kOhm and would therefore have to be considered not servicable. There is something wrong here.

    We suspect an error in the published resistance values for E-D, in  Rotax Manual Heavy Maintenance 78-20-00.

    According to my mechanic, who is a seasoned Rotax specialist, working for the company, this would not be the first time.

    Anyhow, I found my crankshaft to be twisted by 6° by the overboost and have to deal with an expensive overhaul now.

    I can only stress out to take any trouble with the Rotax 914 TCU servo very serious and rather replace the damn thing. 

    The former owner of my aircraft tried to save € 480.- by tinkering for too long with an unreliable servo and caused a € 10000.- repair. 

     

     

     


    Thank you said by: RotaxOwner Admin, Des Howson

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