Gday everyone, fresh join from Melbourne Australia here

Ive got a 2009 Zd30 Crd and am having trouble starting it since having it open to fix a timing cover leak.

when i try to start it, The starter motor engages for a second, turns the engine over once, then the solenoid disengages and the starter is freely spinning without engaging the flywheel teeth.

I initially thought it was a problem with the starter and bought a brand new one to the same effect. I have bench tested the starter relay, which works when given power across the coil, i can even hear it engaging when i turn the key

The battery is full, 12.8 volts, and i have a second battery in the engine bay that ive joined with jumper cables

Ive tested with a multimeter while someone turned the key and the voltage drops to 11.2 volts a short time after the starter engages, so it seems like there is something earthing out, or the starter is getting disengaged too soon.

Has anyone had a similar issue to this before? any help would be greatly appreciated