Should the auxiliary cooling fan on an 86 300sdl be running with the a/c on at all times, or is it just at a certain temperature?

In most cases, the aux fan in a Benz is primarily controlled by a PRESSURE switch in the receiver dryer. The fan shouldn't be running all the time, nor is a specific temperature required. The fan turns on when the pressure exceeds about 300 psi, which is higher than it would run in normal conditions. So in traffic when airflow is poor, on an extremely hot and humid day, the fan will run on low speed.

If the engine is close to overheating (around 105C), then a switch or sensor in the coolant circuit will trigger the fan to come on at high speed. You should rarely hit these temperatures.

my cooling fan is not functioning any longer on my w123. it was working most of the time here in GA. It was redone internally and is verified working if i cross the wires where the relay on the front fender well is. I have a new relay there as well. What would cause the fan to no longer turn on via relay?

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1978 300D

Hmmmn on my 85 300D, I think the AC compressor is controlled by the pressure switch on the dryer but the aux fan is controlled by the temperature switch on the dryer. I'll have to double check to be sure.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Check the fuses. There are 2 and they have to be good for the aux fan to work. one fuse for the relay coil, one for aux fan power supply to the relay.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That is correct for my 85 300d as well, but I did a little digging on the w126 and from what I found it looks like there are two pressure switches in it's system, so comparing the two systems may not be very helpful. I am very curious how thr SD's system works though.

Edit: Just noticed madmax's question: you may have a bad temperature switch. On the receiver dryer there should be two sensors. One of them will have short wires connected to it. Try disconnecting those and touching the two wires from the car together while the ignition is set to run. That should bypass the switch and the fan should kick on. Obviously be really careful here and don't get caught up in the fan or get zapped. If the fan runs when his switch is bypassed, that means either your temp switch is bad, or your refrigerent is not getting hot enough to turn on the fan.

There are usually two pressure switches at the receiver. One controls the low speed fan. The other is a high/low switch which prevents the compressor from turning on if the pressure is too high or too low.

There are a total of 2 switches on the receiver/ dryer of my 85 300D, one is pressure the other is temperature. AFAIK, there is only one speed for the aux fan. W126 and other models are apparently different.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked