What temperature should this fan turn on? With the a/c on and engine temp around 90c the fan isn't running. If I bypass the temp switch (jumper) the fan will operate.I Put a new switch in but still the same. Also would this condition explain the fact the engine temp will rise up to and slightly over 100c at highway speed of 70 and up?

222500 miles

The 300SD eventually had the fan only turned on by the refrigerant temperature in the drier. It explains why they often run cooler with the A/C on. The earlier diesels also used engine temp. If it has a eng temp switch it would be at 100deg C.

Steve,
I have a 1980 300D also. I've almost got the wiring diagram in my head from memory after troubleshooting so many things on it. The aux fan has one temp switch on the a/c receiver/dryer that activates it. My experience is that without running the a/c, it is VERY hard to get a properly tuned 617.912 non-turbo into the 100+ C range on the dash gauge.

You can verify fan operations by shorting the two leads on the recvr/dryer temp switch together. The fan should come on if the ignition switch is on. If not, go to the relay box on the left fender and check your "ice-cube relay" by switching it with another. You can hear it pick up if you short out the temp switch leads with the engine off and the key on. If the relay is picking up, you may have a melted brush assembly, the leading cause of death on aux fans of that vintage.

I'm trying to piece together a working fan from parts of several for just this reason. If anybody has a spare aux fan that runs, let me know if you'd be willing to deal on it.

Mike

80 300D a/c elec fan
 

Thanks for the info. What else would cause engine to run hot if not the electric fan?

The first thing I'd suspect is the t-stat. Then the radiator, followed by the water pump. Most pumps crap out by blowing the bearing and leaking before the lose pumping capacity, but it could happen. Other than that, there isn't much load on the engine.

You can't really tell how hard the a/c is working because the condenser is ahead of the radiator. That's why the watem temp goes up when you run the a/c, you have a freon to air to water heat exchanger.

Thanks Mike, I'll check the radiator and pump. T-stat changed recently.