the stock one locks at 107C - but that's actual temps around the viscous fan clutch, which equates to around 120C coolant temps. (this came from an article in MBCCA's "The STAR" ). in other words, too late. the tropical duty clutch is discussed in this thread:
i got a quote for $160 using the part numbers in that thread. i just did a search with the various parts suppliers and gave them the part number. i'm sure it can be had for less since the price in the Philippines was $105.
BTW i think the article in "The Star" was the basis for the VFC modification described in JimF's pages. the tropical duty clutch has the advantage of working just like the regular version up until lock up. the VFC mod with the screw is manually locked and unlocked (say for winter use).