UPDATE: I still had fan issues after the "shorting trick" described above. However, running Apple Hardware test I found that I had a ambient CPU temperature sensor error. THIS DID NOT SHOW UP AS AN ERROR USING FAN CONTROL SOFTWARE, all sensors reported OK. I t turns out that sensor sockets can very easily be installed reversed when reassembling. It was very easy to fix when the sensor was identified. If you have trouble running the Apple Hardware test, have a look at the sensor connections: The solid plastic side should be down.
I now have no fan issues with an SSD installed as secondary, keeping the original HDD intact to make a great fusion drive. The "shorting trick" is probably only necessary if you replace the drive with the SSD.
The morale is: RUN THE APPLE HARDWARE TEST, do not trust the sensor values reported by the fan control programs.
Booting with D or OPTION-D should start the hardware test. This did not work for me, so I googled to find instructions on bootable USB stick installation.
EVERYONE WITH FAN ISSUES: After searching for days only finding conflicting explanations for my fan issues, this site explains it all:
I installed the SSD as described below the optical drive, but the fans still went full blast. I thought this was only supposed to be happening if you replaced the original drive?
Well, at least it is now properly explained, and a solution exists.