Bence Bozsik

I have changed the oil in my newly bought '98 A4 with the AFN 1.9 TDI, and couldnt fill the new fuel filter to the top so I thought im just gonna clean the system of air by cranking. I had been cranking the engine 4-5x for about 6-8 seconds each time with breaks between them. The last round could have been a couple seconds more as i was hearing that it is finally starting to get some fuel, then it finally started and then never again.

Symptoms: It works perfectly when you switch to ignition on, dashboard lights normal, everything works. When you try to crank it, it gives exactly one very audible click, sometimes turns a whole rotation on the crankshaft but then it stops.

I have tried:
New battery: Same
New starter motor: Same
New ignition switch: Same

The thing now starts by having a cable tied to some ground cable inside the engine compartment (presumably starter motor ground) and when you turn igniton on, you just contact the other end to the battery positive terminal and boom. Starts on first crank.

What could still be bad? Some relays maybe?