MGL Avionics User Forum

Hi,

I have an Extreme EMS wired via CAN Bus to the RDAC XP. It has been working fine. I have now added an MX1 and hooked to the CAN Bus. Everything works fine EXCEPT for the EGT's which seem to sit pretty much correct at ambient (about +26deg C) but as soon as I start the engine, goes into negative and sits around the -185deg C on both the EMS and the MX1. I have only 2 EGT imputs from the Rotax 912ULS - both acting the same!
Any clues/suggestions

Make that an RDAC XF

Even with the MX1 turned off, the EMS still reads the same.

Have you made any modifications to the RDAC XF wiring in any way? This is typically indicative of a backwards connection from the EGT thermocouples. In thermocouple land, Yellow is + and Red is -.

None at all Adam - that is what has me confused. Just tapped the MX1 to the CAN Bus after the Xtreme EMS and tapped an SP-6 to the CAN Bus before the RDAC XF. Never touched the EGT imputs at all.

I may try swapping the EGT imputs tomorrow but even that doesn't make any sense as the ambient is showing positive!! Doesn't go negative till after starting.

Thermocouples are polarized devices - if you swap the wires around the temperature goes lower when it should go higher.

If both ends of the thermocouple are at the same temperature (cold engine) then you will see the temperature measured at the terminals of the RDAC. The thermocouple in this case produces no voltage. A thermocouple measures the difference in temperature between the hot and cold end of the wire. To get the final temperature the RDAC adds the measured temperature at the cold side. This is called cold junction compensation.

Thanks Rainer - I swapped the thermocouple inputs at the RDAC and at least it now works in the right direction but still doesn't seem 100%.
The readings aren't linear with throttle movement and one is showing 200degC hotter than the other which it never did before. I find it very strange that I would have to reverse them to fix the problem when they weren't touched at all during the MX1 install. In fact they have never been touched since engine install.
Is it possible for other systems to be working correctly if I have somehow reversed a CAN Bus connection during the install? Probably just clutching at straws but something weird is going on and that is more feasible than for the EGT inputs to have gotten reversed.