Hot, hot, hot...

I had my 1991 420SEL idling yesterday (high humidity, 90°F) in my driveway for about a half hour with the A/C on high. Low speed fans kicked on normally but the gauge temp reached 100°C and still no high speed fans. I checked the gauge temp sender mounting point on the T-stat housing with a laser thermometer and it too read about 100°C, so I see this is accurate. The engine didn't get past that point, but at what temperature does the sensor on the block kick the fans on to high speed? I have had the cooling system flushed, new T-stat (OEM) and fan resistor replaced about 11 months ago by my reputable MB mechanic. I also purchased the kit from ROLLGUY to turn on the high speed fans whenever the A/C compressor runs, but my mechanic refuses to install it; it seems this may help(?) in my situation. I am somewhat knowledgeable when it comes to this stuff, but I can't figure out the photo diagram the came with the kit. Any advice?