Three diodes are used for circuit protection.  A 1N5408 3A coil suppression (flyback) diode with quick-disconnect terminals came as part of the motor/controller/contactor package.  The terminals make it easy to connect to the contactor coil.

1N5408Contactor Flyback Diode

A MR752 reverse protection diode is wired upstream of the contactor coil, per the controller wiring diagram.

MR752MR752 Diode

A 1N4148 flyback diode is wired across the coil of the smaller relay.  The bands of the flyback diodes go on the high voltage side.

1N41481N4148 Flyback Diode

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