Replaced the fan clutch of the 1984 W126 300SD. The fan clutch should turn freely and not engage until temperature reaches a certain level. If yours is difficult to turn with a cold engine, it is time to replace.

My original Behr was constantly engaged. I decided on the URO replacement (see photo). Looks identical to the BEHR is was replacing. Took time to clean the components too. (BTW - don't replace your fan and clutch with an electrical fan modification. That just sucks power from the alternator, which has it own energy conversion losses. The clutch directly takes mechanical energy from the engine, which is a 100% efficient conversion vs. the alternator mod.)

Install was a straightforward, no surprises and same hardware used. Turns much smoother than the broken OEM Behr fan clutch. Once the temperature rises, it engages the clutch as it should.

Attached Thumbnails

   

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Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

Cool!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Interesting. I always heard others say that a "good fan clutch" should be a bit tight even when cold. I've never felt one that felt free-spinning when cold, indeed that sounds like it might have lost its silicone fluid. But, since your fan blows hard when hot, sounds like what I thought is wrong.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

^ The silicone fluid is what transfers the rotational force to the fan (a bit like a torque converter in an auto trans), but a temperature-sensitive bi-metal spring is what determines the valve aperture (that controls the fluid flow). It's pretty ingenious. Found this explanation online:

"Temperature sensitive bimetal turns slide valve to open and close flow control aperture. Opening and closing action connects or disconnects passage of silicone oil in reservoir to function chamber (labyrinth). Raised temperature opens flow control aperture by turning slide valve and centrifugal force pushes silicone oil flowing into function chamber. Viscous silicone oil mass transmits pulley shaft revolution through rotor and case to fan.

Slide valve closes flow control aperture at lower temperature. Silicone oil in function chamber discharges through exit aperture by centrifugal force coming back to reservoir. Reduced mass of silicone oil loses torque and rotor idles. Then fan reduces revolution."


Historically the bimetal strip/valve combo on our clutches have been "aggressively" tuned, meaning they engage the fan more strongly than OEM along the temp/rpm curve. A few years ago our engineers worked to relax the tuning a bit on some part numbers, after a few "loud fan" complaints, some of which could simply have been due to the owner being accustomed to the silence of a dead (empty) OE clutch.

Better to be a bit loud and cool well, vs quiet and not cooling.

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