Question: How does the 914's airbox temperature sensor work?
The Rotax 914 airbox temperature sensor is a JUMO manufactured Pt-100 RTD type (Resistance Temperature Detector) and requires 5 VDC provided by the TCU to measure temperature as a function of resistance. The resistance increases with temperature and the TCU converts that to temperature for display on the Rotax TLR 4.6 software "MONITOR" page on a MSDOS laptop computer. “Pt” is the chemical element symbol for the probe's internal platinum coil and “100” is the ohms resistance at 0ºC. When inserted into a cup of melting ice at 0ºC, the 914’s Pt-100 RTD should read 100 ohms. A specification sheet on the JUMO probe is attached with highlights on the 914's version.
Resistance increases with temperature and the TCU converts that resistance to temperature on the TLR 4.6 software for display on a laptop computer running MSDOS. If there is increased resistance due to corrosion or a poor connection on either end at the probe or TCU, it will see that as a rise in temperature. At 88ºC the TCU will command a reduction of turbo boost with no warning in either the red or yellow TCU lights.
As RTD temperature sensors have a “self-heating” error due to the resistance current, the Rotax sensor shows an 8.4ºC higher temperature of 34.6º after about 3 minutes of being powered as compared to all ten of the other temperature sensors on the engine display showing actual ambient 26ºC. Note the circled builder installed J-type thermocouple probe showing 26ºC with the engine is not running. J-Type CHT probes having 1/8 NPT male mounting threads fit into the the mount adjacent to the Rotax sensor and are supported by most engine monitors. This builder used a Westach 712-7DJK for $64.
The 5 VDC from the TCU can be measured at the airbox temperature sensor connection.
John Spratt
