I received all parts mentioned above and installed them. While installing I removed the extra SATA port on the Newertech AdaptaDrive since I think it is useless for my purpose and so I connected the iMac SATA ports directly with the OWC Thermal Sensor to the SSD's SATA ports.
The iMac is working and I installed 'OS X El Capitan'. Sadly it seems that the 'OWC Thermal Sensor' is not doing its job correctly. At a room temperature of 25°C and 15 minutes after starting the computer, it is showing 32°C for the Hard Disk respectively the SSD temperature and so the fan is turning at about 4340 rpm. Both, the showed SSD temperature and the fan speed, are constantly rising and the fan noise is getting louder and louder.
Now about 30 minutes after I turned on the computer the SSD temperature is showing '''35°C''' and the fan is turning like mad at '''6030 rpm'''.
The CPU is at 40°C and the CPU Fan is turning at 1198 rpm.
The optical drive fan is turning at about 1000 rpm.
I opened the iMac again to check the sensor cables but everything is connected correctly. I already done a SMC/NVRAM/PRAM reset.
I thought that the 'OWC Thermal Sensor' will solve the 'fan noise problem after a SSD <-> HDD replacement ' without having to use a special software. But it seems that it only hands the SSD temperature to the OS and then the OS is handling it as a HDD and speeds the fan up as it would for a HDD. Maybe somebody knows more on that but shouldn't the fan turn slower because it's an SSD? What are the normal fan speeds for average temperatures?
Or maybe the sensor is simply faulty.
I did not contact OWC yet but for now I installed the free software 'SSD Fan Control' and I think I'm going to live with this software solution since it's really annoying to take apart the iMac a third or fourth time to get out the sensor and send it back to OWC and then wait for a new one and try it again and so on. Also I'm living in Austria and shipping costs are therefore another reason.
I read that a SSD is not generating that much heat as a HDD and that overheating should not be a common problem with SSDs. Is that true?
I will use the option 'SMART' in 'SSD Fan Control' and there I set the following configuration:
1100 rpm @ 45°C
6300 rpm @ 70°C
So now the software 'SSD Fan Control' is showing me 39°C for the SSD temperature and 1500 rpm fan speed. I barely hear the fan now and it is really great.
Is this a good solution or are there any long-term problems with that?