Yes you were right, could messaure there are no connection when bending the cables... soldered it together but still no luck ... also one of the fans having a broken cable ... ordered new fans, then the problem should be solved ... thanks!
Be careful when you solder together. You want the short loop wire to connect the black of one fan to the red of the other. And the remaining red and black cables to go to the correct wiring. If you do it backwards I don't know what will happen. Either nothing or it will burn out the fans.
This is a common problem on the UM2. It's the connectors/wiring. In the video the fans are on for part of the circle and not the other part. That is because of the flexing/bending of the electrical cables that are next to the bowden.
You might not even have to take apart the head. So there are 3 fan connectors but the side 2 fans are special because they are in series so you will see a jumper between two of the connectors coming from the main printer. It's hard to describe in words so google something like "ultimaker 2 fan connections" or "UM2 fan cabling" or something for some photos.
Anyway, slide up that plastic weave that hides the connectors and you should see them above the print head. Set the fans to 100% and then just squeeze and gently tug to try to figure out where the problem is. You might even see a wire come loose and then "oops you broke it" but the good news is you know exactly what the problem is at that point.
The connectors aren't really needed. I NEVER disconnect those cables and I've taken apart the print head on all 4 of my UM2s multiple times. So the connectors aren't useful. So if you find a bad one just cut the connectors off and solder things together (or find a friend who is good at soldering).
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