Those are terminal numbers on the wiring diagram, not amps. A starter solenoid draws around 7 to 8 amps, not 15 not 50. If the ring terminal does not fit the bigger stud, make it fit! Crimp a new terminal or cut it into a spade terminal (that's what I'd do).

Using a relay to fix a weak igition switch is a valid/ cheap/ inexpensive way, especially if the ignition switch has a history of being weak, or hard to replace, or unavailable.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked