You know, the one with that distinctive load roar to it? Well, I don't think they're working properly in both the 300SD and 300D. The other day, I was in stop-n-go traffic with the a/c on full blast in the 300D, and I noticed the temp gauge was up near 100c. I did not hear the electric fan kick in, although it cooled down as I drove at a steady higher speed. I also read in some old threads that the electric fan is supposed to come on even when the A/C is on? Or something like that. The fan never comes on in either car with the A/C on, although a few months ago in the SD, it worked once, and never worked since then. What do I need to look at to get these fans properly functioning?
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

My A/C system doesn't even work And my fan still comes on loud and strong when its real hot out and I'm in traffic just sitting. It comes on when the temp gets up around 95-100C.....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

try jumping the plug at the ac dryer.
There are two sets of wires going into the dryer, one is for the compressor and tells it NOT to turn on if the coolant pressure is too low indicating that it will not get lubrication and the other set is for the fan.
If the fan does not come on when you jumper these wires then press on the center of the fan with a screwdriver or something or tap it with the butt of your hand a little. If it kind of starts and then stops or even if it begins to run like normal then the brushes are probably worn out and are not long enough to engage the fan anymore. Pretty simple to pull it apart and replace them. I had to get a couple of brushes that were close and then file them down to fit in the holders.
You can also jump the fan at the connector which is usually mounted in a little bracket at the bottom of the radiator. At least this way you can test to see if it works at all.

On my 81 it sometimes needs a little assistance to get working i.e. worn brushes that I have yet to replace. I can sometimes just spin the fan once by hand and it takes off.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique