Robert Coats

Why does the 964’s A/C system use a two-speed condenser fan?

If the "donut" round resistor used to control the low-speed circuit were removed and replaced with straight wire, would the fan run on high-speed all the time (compressor engaged)?

Just don’t understand why the fan has to run on low-speed…it seems like this would hamper cooling performance, but maybe some of you A/C folks can set me right.

DaveK

Actually - can I add a similar question (which I guess might be answered in the archives if I searched.....).

What controls when the fan is low / high speed? I know my fan is on all the time I have a/c selected - but I've never figured out when it is high speed or when it is low.

springer3

The 2 speed fan control extends the life of the fan motor, the alternator, and the wiring (connectors eventually fail, and fail much faster at high current draw. Two-speed control also saves fuel and improves performance when you don't need the extra cooling (the fan pulls a lot of power).

Randall G.

Well, this sucks. The reply I made to this thread yesterday has been washed away.

Anyway, as springer3 suggests, electrical loading and fan longevity are probably the main reasons Porsche chose to have a slow and fast-speed. In slow-speed, the fan probably draws about 9A, in fast-speed, around 15A. A second consideration would be noise--the fan is much louder when running in fast-speed. Similarly, we could ask why Porsche designed the oil cooler fan with a slow and fast-speed?

As you guys know, the condenser fan runs continuously in slow-speed with the A/C. When system pressure (proportional to load) reaches 17.5-19 bar, the "mean effective pressure" switch closes, sending a signal to the CCU to run the condenser fan in fast-speed.

I know of at least one person who has replaced the ballast resistor with a wire. Apparently, no adverse effects. As Robert has suggested, I would think it would only increase system efficiency. But, I'm not an A/C tech, so I need to tread lightly here.