Verifying 914 TCU Sensor Data Input

Question: How can the 914 TCU sensor data input be verified?

Data sent to the TCU by Rotax’s 914 engine sensors can be checked for accuracy by comparing with other builder installed engine sensors. TCU inputs of RPM, ambient pressure, airbox pressure and airbox temperature can be displayed on a laptop computer running MSDOS using the Rotax TLR 4.6 software. Details are in the 914’s Heavy Maintenance Manual (MMH-914 Rev. 6, Section 76).

The Rotax “MONITOR” display from the Rotax TLR 4.6 software on the left below shows the sensor inputs that can be compared to the engine display on the right to validate correct data. The units of measurement were changed to Celsius and millibars to match the Rotax TLR’s “MONITOR” page.

The manifold pressure of 849 millibars compared to the Rotax pressures of 824 and 828 millibars. The pressure altitude from the altimeter set to 29.92inHg/1013mb was 842 millibars from a conversion chart.

The temperatures compared well with three Rotax supplied sensors circled in magenta, oil temperature and the left and right coolant (CHT) temperatures. The other circled sensors were J-type CHT thermocouples (red & white wires) supported by most engine monior systems. The green circled AIRBOX is a probe style J-type thermocouple in the 1/8 NPT mount adjacent to the Rotax airbox temperature sensor. For more detail see separtate posting labeled “914 Airbox Pt-100 RTD Sensor”.

The engine monitor was turned on with the TCU CB/fuse pulled with the laptop running and the TLR program window opened. After the engine display came on the TCU CB/fuse was closed and immediately the “MONITOR” page was selected and the 14.2º C was seen. Over the next 3 minutes it heated up 9ºC to 23.2ºC.

The Rotax airbox temperature sensor, p/n 966435 is a Pt-100 RTD (Resistance Temperature Detector) that receives 5 VDC power from the TCU and heats up once powered. It is only comparable the instant power is supplied to the TCU.

 The “MONITOR” page with the engine not running shows zero for rpm. With the engine running it should be the same as the engine rpm display. The “LOAD” is the throttle position which was at full throttle and correctly at 115%.