High Rotax CHT

[dforster]

Dear Sirs,

I have an EMS-D10 which is approx. 2 years old, running software version 5.0.0. It is connected to a new Rotax 914.

Although the CHT's read close to correctly at room temperature, during the first engine run yesterday, the temperatures climbed abnormally quickly until the engine was shut down at the max CHT operating temperature of 275 F.

Today, the senders were checked in cooking oil using multiple cooking thermometers. The EMS readings appear to be approx. 35 degrees too high, showing 313 F when the cooking oil is at 275F. The EMS CHT's gradually converged with the cooking thermometers as the oil cooled, with everything close to agreeing at approx. 150 F.

I confirmed the 1.21 Kohm resistors are installed as per the diagram on page 3-15 of the Installation Guide and confirmed they show 1.21 Kohm on an electrical meter when disconnected.

Could you please advise as to what I need to do in order to get accurate CHT readings when close to max operating temperature?

Thank you,

Dave Forster

 

[dynonsupport]

Dave,
The Rotax CHT's are resistive devices. They have about 1K or resistance at 76F/25C. But up at 275F, they are only about 25 ohms. The difference between 275 and 313F is only about 6 ohms.

So if you have 6 ohms of resistance between the probe and the EMS, you'll have a big temp error when hot. But when cold, 6 ohms only makes about 0.1 degrees of difference, which you will never notice. At 150F, 6 ohms is less than 1 degree.

We've got thousands of Rotax engines using our EMS, and this is not a typical problem. The tables in the EMS are right. Given this, we have to guess that something about your install is adding some resistance. Do you have a lot of connectors in line with the sender wires? Something unique like a firewall connector? Are you using wires that are at least 22 gauge? Is the engine well grounded to the airframe and battery? Is the EMS well grounded to the battery? Are you using more than just the electrical ground on the EMS to ground the EMS?

 

[nacepro]

I am having the same problem in a Flight Design CT, but I have two d120, and when one is on the high temp is only off by 10 degres, but when the other unit comes on the temp goes up another 20 degres