914 wastegate servo issue

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.

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.