It is very hard to tin the cable ends well and tinning is key to soldering. I have a proper soldering iron, but no special device to only tin cables (I’m blanking on the right word).
Hmm, you don't need a device though? Hold the wire somewhere, touch the iron onto the wire and then touch the tin onto the wire as well. Wait a bit, it'll tin thoroughly, (maybe change the position of the tin to tin all sides more easily).
Yes, this is how it works on thin cables, but the ones on the ECU are very thick and very hard to tin with soldering iron. I finally managed it, but it was a fight. I’m using the TS100 pen.
Oh hmm, I see. Did you try pressing the button for a boost? I just tinned 12 AWG wire very easily, were yours much thicker? Also keep in mind that if you don't run the TS100 at 24V it'll have more difficulty keeping the temperature.