My aux fan is coming on intermittently on the '85 300D.

ACC set on 'econo', engine temp 85C - verified with IR thermometer.

I have also experiences random battery draining events - just changed battery, may or may not be related.

What could cause this?

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

First thing that comes to mind is the Temp Sensor in the Water Jacket that
"Trips" the Auxiliary Fans.(It's Bad)

Or Short/Ground in wiring.
Or any other number of things.

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'84 300SD sold
124.128

Does someone have a wiring diagram for the '85 300D? I can't seem to find one.

I need to now what turns on the aux fan. I know there is a temp sender in the block, and an AC refrigerant temp sensor. Thats all I know about.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Disconnect the temp switch on the receiver/drier and see if that doesn't solve your problem.

The receiver dryer temperature switch (the one with the long pig-tail wires, not the one with the spade connectors directly attached - that's the pressure switch) closes when the R/D gets hot. That closing completes a circuit that when the ignition is ON, trips a relay. The relay connects an always hot circuit to the fan. I do not think the Aux Fan is in any way connected to the temp sensors for the engine, it's solely for the A/C and should act on the temp sensed at the R/D.

If your battery is also draining AND your fan is running randomly, I'd suspect a sticky or intermittent relay. If it closes while the car is turned off, it may cause the fan to run and drain the battery.

Or check whatever else is on fuse C and maybe pull the fuse?