The fan speeds are determined by how much of the resistor pack the current has to flow through before it gets to the motor. Speed 1 goes through two resistors, speed 2 goes through one resistor, speed 3 goes direct to the motor.
The AC requests speed 1 by default, so puts current to the start of the resistor pack. The rad switch can also request speed 1, but is usually over-ridden by the AC.
The rad switch then requests speed 2 which puts current to the middle of the resistor pack, effectively ignoring speed 1. The start of the resistor pack is still powered, but as it has 12V on both sides of the resistor, no current flows and it is ignored.
In my car, speed 2 is disconnected from the resistor pack and goes straight to the new fan. When the rad switch requests speed 2 the new fan comes on (full) and the original fan keeps running at speed 1. Its like this simply to avoid having to run new wires to the engine bay - I have subverted the wire for speed 2 to drive the new fan, but if you wanted to run new wires you could have the new fan on, plus the original at speed 2. It doesn't need it though.
When the cluster requests speed 3 it puts current directly to the fan motor, bypassing the resistor pack completely and running the fan at full speed. In this situation, the rad switch would still be hot enough to be requesting speed 2 so in my car both of the fans would be at full speed.
Theoretically you could add a second resistor pack and a bunch more relays to have speed control of the new fan as well. That would be nice because it would run both fans slowly for AC when stationary, but its more hassle than its worth IMO, since I rarely sit in the car stationary wanting AC. I guess in London traffic that may be more relevant?
You could of course just put a bigger fuse in and run both fans in parallel, assuming the wiring would take it. You'd need to check the cross section and current capacity to be sure.
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21st May 2019, 10:13 AM
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