It's a new starter from simons, but the old also had problems with the same symptoms....
So it was probably not the starter in the first place and you took a guess, which has cost you a new starter motor unnecessarily. If a professional technician does something like this, everyone shouts about rubbish garages and rip-offs. Yet I see people here doing it all the time themselves...
I can hear the starter clicking when I turn the key. And when i disconnect the small "wire" is does nothing. So believe that wire is okay..
I have disconneted the big wire (the one from the batteri). Tried with a testlamp, but there is no power going to the end... Conclusion there no power in that cabel...
Not necessarily true. When you say you tested the main cable with a test lamp, where exactly did you make your test lamp connections?
You should have the test lamp earth on the battery earth, and first test the test lamp by connecting the other end to the battery positive. If the lamp lights you have proved everything in your test circuit is fine. Now you should move the positive connection of the test lamp to the end of the main cable you have removed from the starter BUT DO NOT changed anything else. I know this is more awkward with the battery at the front and the starter at the rear, but you simply require long test leads - what is the problem. If the test lamp no longer lights or only very dimly then the main lead has a problem.
If you move both the positive AND the negative of your test lamp, then it proves nothing until you have verified you have a good earth too. On any vehicle but especially these cars with galvanised chassis, that should never be taken for granted.
There is also a "mediun" wire.. But can't measure anything on that... I think I took it off the same place as the "big" wire
If it came off the same connection as the main power lead then it is probably the alternator supply and won't have any power until connected with the main lead as that is where it gets its power!
I can almost not turn it, despite having jumpcables on the starter itself...
But where is the engine earth strap located..??
The power unit main earth is a braided cable that connects to the differential housing.
Thought the starter simply got the earth from the bolts...
The starter motor does get its earthing through contact with the engine block and mounting bolts, but if the engine itself is not earthed as the main earth lead has broken or is not making a clean contact then that supposition is false...
Everything else works fine besides the starter...
But nothing else requires the amount of current that the starter does!
Will the engine turn easily by hand? If the engine is seized, or hydraulically locked, no amount of power to the starter will have the result you require. :-)
First, you need to make sure you really have good connections and the main cable is fine. You could try the starter on the floor (removed from the engine but with the same main cable - pass the main lead down under the car and connect to the starter, then you will need a heavy duty earth lead to the starter casing and back to the chassis but make sure that really is a good earth to the battery. If the starter now works, you have eliminated two things - the cable and motor.
All problems like this are a case of doing tests, collecting information, and possibly doing more tests on the basis of the results until you can track down the root cause. It's detective work and you need to do things logically.
Roy