Dragging this thread up from the dead (sorry, everyone). Back when this thread was active, I only owned one M119 engine and didn't have any fan clutch issues, so I probably didn't even read it back then.
Now I own a half-dozen M119's and have spent a lot of time fiddling with them. I have found that only a couple of my M119 OE fan clutches engage as they should, i.e. by 95-100°C coolant temp, in warmer climates. A couple were completely dead and never engaged at any temp. Please keep in mind that the M119 fan clutch engages based on AIR TEMPERATURE, not coolant temperature. This means that in cold climates, the engine temp could be 100-110° and not engage the fan clutch, even with a perfect cooling system and perfect fan clutch. This is noted in the M117 FSM, see attached PDF file, read the "Caution" note on the last page... also note that the clutch is designed to be decoupled below approx 95°C coolant temp.
Anyway, I started doing some more scientific testing which was not mentioned in this thread, i.e. what the actual fan RPM's are relative to the drive pulley. I quickly found that this proved that some of the FSM data is wrong... the FSM indicates the engine RPM is higher than fan RPM, but this is backwards. The crank pulley is larger than the fan pulley, so the fan RPM is always higher than engine RPM.
Click here to view the FSM detalis on the M119 clutches, please look at all five pages in the PDF file and see my embedded notes.
Bottom line, my "good" fan clutches operate like LeaUK's. They are engaged by approx 90-95°C coolant temp on warm days, decouple by approx 3800 engine RPM, and re-couple when engine speed is reduced to approx 3000rpm. Contrary to previous opinion stated in this thread, with the AC operating, the additional heat given off by the condenser may cause the clutch to engage even at a relatively low coolant temp (80-85°C)... yes, a LOWER coolant temp, not a HIGHER coolant temp. That's because the AC condenser is giving off heat which increase the air temp at the clutch face (BMS), even though the electric fans are usually on. I've seen this repeatedly on my cars, with the AC on the mechanical clutch may be engaged at 80°C coolant temp on a warm day (with AC), but
not engaged at 90-95°C coolant temp on a cool day (no AC).
Side note: Fitchell-Sachs was bought out by Horton, and new OE "Genuine Mercedes" M119 fan clutches will arrive with the Horton markings as shown in the photos above. They are still using the Sachs design and probably built in the same manufacturing facilities, it's just a different name on the unit & box. They are identical physically and visually.
Now, the aftermarket clutches are a completely different animal... the ACM clutch seems to work fairly decent, although it decouples at a higher RPM than stock (possibly because ACM made the mistake of accepting the FSM docs as accurate, which they are not, as noted above). Avoid the no-name dirt-cheap junk clutches on eBay & elsewhere, there was a report (with photos) of those splitting in half in 200 miles. (!) The ACM, and most other aftermarket clutches, require a shorter bolt to install - see pics at link below. The ACM is $125-$150, OE is $550-$750, so if you choose to install a new $50 clutch - don't be surprised if it blows up and causes more damage than the $100 you thought you saved.
Detailed photos of the M119.974/.975 (400E/500E) fan clutches:
http://www.w124performance.com/images/M119/fan_clutch_119-200-01-22/
Link to forum thread with additional information & discussion on these clutches:
http://www.*********.com/forums/showwiki.php?title=Viscous_Fan_Clutch
