GGR: Thanks for the link. Behr added a nice point of confusion for us, this is like Chrysler registering Hemi as though they invented the hemispherical head with the implication that only chrysler makes motors with hemi-heads.
Its a good explanation of how the clutch works, but it doesn't address how the oil gets back into the 'store' and what is happening when you first start the car cold. It sounds like at cold starting, there is no oil in the clutch, so no faning. After the bimetal heats up, the pin opens the oil hole and lets oil flows, as oil flows and the fan fans (clutched on) and more heat, more oil, more fanning. I can say that I could hear my fan fanning well when my 250 was at 200F and the ambient was 100F and my heater was on too.
GGR: An important thing they didn't say: when you first start these cars cold, at about 20-25 degrees C, the fan clutch IS engaged and stays engaged to high rpms until the car warms a little. That is a fact. As a result, you get very knoticable loud fanning on cold start. (It has always been counter intuitive to me why this happens.) The fan keeps fanning as you go through at least 2nd. Once the bi-metal heats up from the radiator heating up (and maybe some internal friction), you don't hear the fan at the higher rpms any more. Sounds like your visco clutch thermo band isn't moving the pin and it is staying in this cold mode. It may be that the oil is still in the clutch and it needs to spin it back into the 'store'??
I've just recored my 250 radiator. I am waiting for a motor clutch to run it up to temp, so I don't know if the radiator solves my problem. BUT earlier, I pulled the themo (has anyone registered Thermo for Thermostat?) and watched it open in a pan of water on the stove with a thermometer. It was opening at 70 and only half way. The new one worked fine. Its an easy test if you aren't on the clock and if you have your new thermo, put it in the water too, to ensure it is working properly before you put it in. Watch it open then poor in cool water and see when it closes.
And my Fan clutch was 20 years old so I bought a new one on EBay for $60 It looks like the stock MB part. (not URO, which is a little cheaper.) Both clutches were fine but the new one is a bit louder when cold, which tells me the friction is a little higher & fan is fannnig harder with the new one. After replacing both parts and renewing the coolant, no change. I still had to run the heater to keep the motor below 100C (Euro guage cluster) ambient 100F (hot, hot) and it was worse at idle or traffic. But neither change, the fan or thermo made noticeable difference. 20-30 years ago the car never went more than a hair above 80C
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Dr Benz, I like the test strips method. Is there a brand name? to make sure ican find them. These cars often sit long periods and the coolant can corrode some parts like inside the head and clog others like the block and radiator when the protection deteriates. I hate to guess, so being able to measure the effectiveness is great.
BTW, you can look into the radiator just enough at the two ports and with a light and small mirror see the chemical deposits accumalated on the core, as just an indicator of radiator health and how good your coolant has been. My suspected bad radiator had 3 to 4 mm stalagmites visible.
