Nothing seasonal is running in Kuala Lumpur & the west coast in September — which is its own answer. Pack for the temperature.
Going in March
A second wet peak around the inter-monsoon, with April the wettest of the three months.
What this means for your bag
A packable rain shell
Not a winter coat — it stays warm through the rain. A light hooded shell that folds into its own pocket ends up in the day bag instead of the hotel room.
See the Weather Protection packQuick-dry fabrics
Cotton takes two days to dry in this air and starts to smell before it does. Synthetics and merino wash in a sink and are wearable by morning.
See the Summer Sun Kit packRoom for humidity
Everything stays slightly damp. Pack fewer clothes and plan to wash them, and keep anything that must stay dry in a sealed bag.
See the Summer Sun Kit pack
Build a packing plan around the season you are actually travelling in.