Aux Fan & Overheating Issues

Hey Guys - I need some help with my car overheating. Its a 86' 535i 5 speed and it only overheats after sitting in traffic at low speeds. I have been researching and reading as much as I can find but nothing about my problem. The Aux fan works when I turn the air conditioning on but I have never heard it work when the car get hot. What/where would this problem be and how can I fix it? I am assuming it should be kicking in when the car gets hot but it doesnt, only on AC -

If the overheating is not because of no aux fan what else could it be?
I have changed the thermostat -
I do not get any moving coolant in the aux coolant resivour either. Any ideas?

Thanks in advance

Overheating while stopped in traffic means that your fan clutch is gone so its not doing its part (actually the only part since it is only needed when stopped). Replace it with a new one and you'll be good to go. As for your auxiliary fan not working, there are two things that you can do. First remove the plugs from each switch and jump them. If the fan comes on the switch might not be working. There are two switches located on the top side of the radiator on the right hand side. They both work at different temps and fan speeds (higher temp, higher speed). The reason for you fan not working might be that there is air trapped in this corner of the radiator which in turn makes the water to NEVER contact the switches. Check to make sure that the coolant level is correct, bleed the cooling system through that small 8-mm bleed screw on your thermostat housing, check level, drive around and repeat. These cars tend to trap air and some people go as far as removing the top switch on the radiator to ensure proper air removal.
Remember, you have to do two things: replace the fan clutch and then bleed the cooling system (you could test the switches by removing them and placing in hot water with a thermometer and a continuity meter). Just as a precaution I would replace the thermostat if you don't know its age. Its cheap and easy to change in about 5 minutes!)

Good Luck!