After a scorching hot day yesterday that proved to be too much for the a/c and the coolant (with the a/c on, the coolant temp got up to 100°C before I shut the a/c down to prevent overheating; the coolant dropped back to normal after I killed the a/c, although I also need to do a cooling system flush). Granted, my electric fan doesn't work and is probably contributing to this, but I realized that I don't think I've ever heard the engine-driven fan kick in. I have an idea of what it should sound like (you should hear my truck when the fan kicks in; it sounds like a jet engine taking off!), and don't believe I've ever heard it. Spinning the fan by hand, there's some resistance, but I guess it isn't enough.

So, I think my fan clutch is dead...any reliable way to confirm this beyond a shadow of a doubt? Also, there isn't by chance an aftermarket supplier that makes electromagnetic fan clutches (such as this), is there? I like the idea of the engine fan sensing coolant temp and going from not-moving-at-all to full-speed-ahead without dragging any...

Thanks in advance!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

The ETM for my 1983 300SD says the aux fan switch closes at 100C (212F). Probably similar on your 300D as we use much the same engine.

If you unplug it and short the two feed wires, the fan should come on. The switch is the red thing with two wires on the receiver-drier - near the driver's side headlamp(s). (Don't confuse it with the (larger) pressure switch, also with two wires). That will verify whether the fan works. It unscrews. On the early r-d (like my 380SLC) you don't lose freon; on the later ones, you do, contrary to what has sometimes been written on this forum. Pix on my site in the a/c piece.

Test it in boiling water with an ohmmeter to see if the switch works - I would guess you could raise the boil temp a few degrees by adding common salt. Resistance should fall from open circuit to next to nothing at 100C.