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?
I just wired up a relay to come on with ignition power and draw power straight from the battery for my high speed fan. That will cause your fan to run constantly though. Been thinking about changing my relay source to the ac.
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