It is rainy season in Kuala Lumpur & the west coast right now, and it runs October to December. The rain tends to come hard and short rather than all day, and it stays warm through it — so the thing to bring is a shell you can fold into a day bag, not a coat.
Going in September
Nothing seasonal is running in September.
Packing for Kuala Lumpur & the west coast?
Build a packing plan around the season you are actually travelling in.
Seasonal ranges are typical, not guarantees — a monsoon can arrive early and a dry season can break. Check the forecast before you seal the bag.