HOBBS counting without oil pressure.

[jefvervoort]

To study the possibilities of my SV system ( 10"&7",ems, autopilot) in the hangar, I power it up with a ground power supply to the main bus.
Last weekend while studying the use of a flight plan, I noticed that the ENGINE RUN TIME had accumulated 20 minutes, and the HOBBS had also gone up.
I suppose this is not normal, as there was no oil pressure.
Has anybody experienced this?
What should I do?

Thanks
Jef, in Belgium, RV9A, OO-156

 

[KRviator]

The installation manual suggests it requires Oil Pressure >15PSI, or Engine RPM >0RPM to count up.

Have you configured your cruise RPM in the Engine Setup pages? IF you have your cruise RPM still set at 0RPM, that would probably explain why your Tach and Hobbs time increased by the same amount you had the system powered on.

If you have an O-320 in your -9, you can reset your timers manually, and then program your cruise RPM to 2400 or thereabouts. That's what I use in my -9 with an O-340.

 

[dynonsupport]

This could also be caused by an improperly set up oil pressure sensor. To figure this out, you need to actually look at the displayed RPM and oil pressure displays on the engine screen and make sure OIL PRESS is zero. If it isn't displayed, shows an error, or shows pressure, then this will cause issues with the HOBBS timer.

 

[Raymo]

I initially set the wrong sensor for oil pressure in my system and it showed high oil pressure.

 

[jefvervoort]

Now I understand that I should have mentioned: I used the Dynon preconfigured sensor mapping files for the new Lycoming XIO-320 that I have. Did that in 2012, and had no problems.
This afternoon in the hangar, I powered the Efis, and the RUN time went up from 00.0 to 00.2; the Hobbs increased also with 00.2 . The oil pressure was 1, not 0 . RPM was 0.
Also a ( standard) warning came up: oil and fuel pressure low, as usual if the engine is not started.
I noted the oil temperature was 30 C°; in the hangar it was probably even a bit more. We had indeed an extremely hot day.

I waited some ten minutes, but nothing else happened; it went not up further. I ended the exercise.

I still think the increase of the hobbs and run time while the engine is not running, is not normal.

KRviator: how would you change the hobbs of a new engine; I suppose it is not done.
Ray: how did you set up the sensor wrong?

Thanks, Jef.

 

[Raymo]

I had selected the wrong sensor during setup of the EFIS. Sounds like you had already done this correctly but now see an issue. Mine was fine after setting it to the correct Kavlico sensor.


BTW: 30 C is not hot (in Savannah, GA)

 

[jefvervoort]

See attached screen shots.
The hobbs and run timers count while there is no oil pressure?
What could be wrong?

 

[jefvervoort]

Sorry, the previous message was wrong.
You should now read :
How can the system have oil and fuel pressure without the engine running.
In the preceding weeks, the hobbs and run timers would run without oil and fuel pressure.

Disregard the third screen shot.

An suggestion welcome. Thanks.
Jef. RV9A.

 

[n456ts]

Your oil pressure is 24 PSI with engine RPM at 0. You need to correct this.

 

[dynonsupport]

Yep, the issue is your oil pressure, which triggers the HOBBS. You might have the wrong sensor type selected in setup.

 

[Raymo]

Same for fuel pressure, assuming the pump is not running.

 

[jefvervoort]

Had some help from an experienced professional.
The two sensors, VDO type, part of the emskit, bought in 2010, seem to stay partly 'stuck' after engine shut down.
We measured 28,8 and 23,7  Ohm for the oil and fuel respectively.
After some gentle tapping on the casing, the resistance dropped, but not completely. On the casing itself we measured 0.4 Ohm.
We disconnected the wiring and loosened the oil & fuel hoses, and let the pressure come to zero.
And made a short flight.
Half an hour after landing and shut down, oil was 0 psi and fuel was 2 psi.
Anybody with a similar experience with the VDO sensors?

I see that Dynon has moved to Kavlico sensors in 2012.
I loaded a preconfigured sensor & mapping file in early 2012.
Thanks for your attention;
Jef.

 

[Raymo]

I suspect the sensors are not well grounded. See page 7-43 in the install manual regarding legacy sensors.

Mount the oil pressure sensor to a fixed location using an Adel clamp (see Figure 56) or other secure method. The oil pressure sensor must not be installed directly to the engine due to potential vibration problems. Dynon Avionics’ sensor is supplied with a 1/8” NPT pipe thread fitting. An adapter might be necessary for some engines. Please see the manual supplied by the engine’s manufacturer. You must use appropriate pipe fitting adapters and ensure that the case of the sensor has a connection to ground. This is critical for functionality.

 

[jefvervoort]

Ray,
The sensors were mounted strictly following the instructions in the manual ( revision K, March 2012) in all aspects.
I think the ground is ok: 0,4 Ohm.
According VDO technical documents they have a resistance between 10 and 240 Ohms. See my last post: what we measured was too much what means the sensors are not working properly.
Best wishes.