AC compressor relay and Trinary switch wiring question

[Richard]

Originally Posted by Davidf

Hmm, I believe the condenser fan runs anytime the compressor is engaged on all of my BMW's. Albeit at a slow or high speed depending on need. But, it runs. Why would you NOT want the fan on when the compressor is engaged. The more the condenser is cooled, the more efficient the a/c system will be. The more efficient, the more fuel that is saved as the compressor will cycle off more often.

Newer cars are all about efficiency. There is no reason to run a fan when airflow is free at highway speeds. GM uses a pressure sensor to activate and deactivate fan for this reason. Not just to protect compressor. Have had two BMW's myself. Would bet they operate the same way. Unless you are using an 850+ watt fan in ideal conditions there is no way it will flow more cfm than running down the highway at speed with air being forced through the front of the car. So by running fan at speed you are most likely restricting airflow, not increasing it.
Older vehicles such as the 67-72 trucks use a clutch on the fan to disengage it when not needed. It is regulated by heat. The electric sensors are just taking this a step further. There was nothing to fully engage the fan when a/c was turned on and the ac worked fine.