MGL Avionics User Forum

We have a newly installed Rotax 914 and we're having trouble trying to get the Xtreme to produce sensible readings. The oil pressure shows 1.9 bar, when a mechanical gauge (correctly) shows 4.0 bar. The fuel pressure is showing 1.2 bar, when we're expecting around 0.5.

The fuel pressure sensed is a UMA 0-7 psi DIFFERENTIAL unit (for a 914) and the model number (N1EU07D) is not available in the drop down menu.

So, first things first, is the differential model available on later firmware? Much more important, where can I find the latest firmware for the Xtreme? I've looked in all the normal places you'd expect but no luck.

The aircraft was previously running an Edge converted Rotax 912 (fuel injected), so I can understand the fuel pressure side of things being different but why the silly oil pressure readings when they're the same probes to the RDAC?

Probe and RDAC grounded to the engine, other indications seem to work okay.

Further investigation talks of a 100 Ohm resistor between the Oil Pressure input on the RDAC and Reference Ground for the Stratomaster. Just wondering if the Xtreme requires the same for the Rotax oil pressure sender.

Re OIL.P- the RDAC needs the 100ohm resistor input to ground to convert the current output from the sensor to a voltage for the RDAC to read.
Re FUEL.P- set the sensor type as 0-5V T1EU07.

Thank you, Stuart. We’ll do what you say and see how we get on.

We did as suggested and had success with the oil pressure (with the resistor) but the fuel pressure is still foxing us.

The UMA pressure sender is definitely sending a voltage. With the pumps turned off we'll get anywhere from 0.2v - 0.99. When the pumps are turned on we'll get 2.55v. SO we know the sender is good.

The RDAC is an XF. We've tried with the dip switch on and off, although it should be on and that is the position it has been left in.

With UMA TIEU07 selected we get nothing at all with the pumps on or off. The fuel pressure remains at zero.

With UMA TIEU35 selected we get zero with pumps off and 1.2 bar with them on. The reading should be around 0.6/0.7 bar, so at least we're getting something, albeit double the expected value.

Noticeable in all these scenarios is the ADC value does not change, which I think is odd.

I tried to use the USER set up. Pumps off the ADC value sat at 1096 (1137 with the dip switch off). Pumps on.....nothing changed. I expected to see the ADC value change, so I could then 'assign' that value to 0.6 bar on the user set up.

I am disappointed that simply selecting the correct UMA pressure sender i.e. TIEU07 doesn't work and just shows zero fuel pressure. That would have just been all too easy.

I'm now wondering if the RDAC is knackered, hence the lack of ADC change or whether the Xtreme is knackered because it's not processing the RDAC signal (IF it's working correctly).

I'm also wondering if we use a couple of 10K Ohm resistors (in a voltage divider circuit) to half the fuel pressure signal voltage to the RDAC and use the TIEU35 setting. In theory I think we'd go from 1.2 bar to 0.6 bar.....which is what we expect. But what a phaff, just because something is not supposed to do what it says on the tin.

Of course, all of this assumes the TIEU07 settings DO work for the NIEU07, something the cynic in me questions but the logic is sound, since it is just a voltage from the sender that the RDAC is looking at.

Any other wise words out there. I'm especially curious about the lack of ADC value change.

Thank you.

I would first check the RDAC channel is working.
Disconnect the sensor and switch the pull-up resistor for that channel ON.
Check with a voltmeter for approx 5V on the input (pulled up by the resistor).
Check the ADC reading is around 4000.
Repeat with the input grounded- input voltmeter reads 0V? ADC reads around 0?
If so, the channel (from input to Xtreme) is working fine, and the issue is either with your sensor or your FUEL.P setup.
If not- almost certainly the RDAC input channel is dead, as the rest of the RDAC appears to be working fine.

The UMA sensors are active devices (voltage output), so the pull-up resistor for that channel should be OFF.
From what you say it appears your sensor is working (2.55V is about half scale - for the EU07 that is 3.5psi or 0.17B).
You are expecting 0.6-0.7B?? That is 8.7-10psi... overrange for a 0-7psi sensor.

Thanks again Stuart. We'll try out your suggestion for testing and hope to prove what is or is not working properly.

I do agree with your logic for the sensor press versus what we'd expect but we were told very specifically to get the N1EU07D because of the installation on a Rotax 914. I'll look at the availability of a greater scale but still with the D for differential pressure.

Here we go again.......

The NIEU07D is what I use on 914s.
The differential pressure (fuel to airbox) is typically around 3.5psi, so I suspect your sensor is doing exactly what it should.
I suspect the problem boils down to 3 possibilities- bad RDAC channel, wrong Xtreme fuel.p setup, misunderstanding/expectation of what is being measured.