CHT on D-180 connected to Rotax912 sensor

[sky]

I have installed the D-180 on an Aircraft equipped with Rotax 912 engine. The EMS indicators (displayed on the D-180 screen) read the correct values, except the CHT-L indicator. I have connected the CHT-L sensor (installed on the Cyl n° 2 by Rotax) to the D-180 as indicated in the D-180 installation Manual; I have not connected the CHT-R sensor (installed on the Cyl n° 3 by Rotax) to D-180.
The CHT-L indicator reads about 30°C more than the actual value, and this wrong value increase and decrease with throttle setting. I have the same problem with a second Aircraft ( identical to the first) on which I have carried out the same connections with a second D-180; therefore I have excluded that the problem is in the sensor.
Is there someone who can help me to solve this problem?

 

[dynonsupport]

This sounds like a grounding issue.

The ground path on the Rotax engines is through the engine case. If there are issues with the ground path between the engine block and the electrical system, you can see exactly these sort of problems. One - a voltage differential between the engine ground and the rest of the electrical system will cause the reading to be off. And two - the fact that you see things change with throttle (which changes the alternator or generator output) also indicates that there's something up there.

The solution, generically, is better grounding between the engine and the rest of the electrical system.

 

[sky]

I have connected the sensor ground directly to the ground point (in the instrument panel) to which all the ground points of D-180 are connected, but the same problems remain.

I have used a shielded wire for the connection between the CHT-L sensor and the pin 4 of D-180, but the same problems remain.

I have put a second resistor 1,21 KOhm in parallel to the one connected between the pin 18 (+5V) and the wire connecting the CHT-L sensor to pin 4 of D-180, and the CHT-L indicator reads correctly and isn't influenced by throttle setting.
Is this solution acceptable to you?

 

[dynonsupport]

This should not work at all actually, and if you really did have two working resistors hooked up in parallel you'd expect incorrect temperature readings. However, if there were something going on with that first resistor, you might experience what you're describing.

Try measuring the actual resistance across the first, suspicious resistor (with power off of course). Maybe it's the wrong value, or perhaps just faulty.

 

[sky]

Improving the grounding the problem reduced considerably.
Thanks