Clarification: With either the blue or gray switch shown in my photo above, the electric fan turns on high speed at 105°C (the two poles of the switch are closed, which triggers the relay, which turns the fan on). You can test this by shorting the two wires at the switch with the engine running... the fan should turn on high.

The third, single pole on the switch is for the AC safety... at either 115, 120, or 128°C (depending on the switch part number / color); this second stage will shut off the AC compressor to reduce thermal load on the engine until it cools sufficiently. It would be rather unusual to ever reach this safety net, though, most 603 owners get nervous far before those temps and switch the heater on full!


Paul, if your temp problems are not cured with your recent fixes... my next bet is the radiator. I'll save you the long story, but the PO had changed it on my car (new dealer radiator) not long before I bought it, and only 5 years later it was getting up to 110-115°C in the summer. I changed EVERYTHING else, assuming it couldn't possibly be the almost-new radiator. In desperation, I finally swapped the radiator, and voila... instant cure. The car refused to go *over* 100° after that, even in 110°F ambients (Sacramento summer). On the flip side, my sister's '87 300D has a similar issue, but a new radiator didn't fix hers - that one is still a mystery that we're working on!