[RichardJ]

That diode is there to protect any electronic circuitry in the evaporator suitcase. If you're using any kind of aftermarket AC system that uses semi-conductor circuitry to open/close the doors and regulate the temperature, that voltage spike Rusty mentioned, from the clutch coil can fry it.
keep the diode and remember to observe the polarity. The white band end, stays on the clutch wire.