AC fan speeds up with brakes

I have a 2001 550, everything works as it should, but one thing that I've noticed is that the AC/heater fan speeds up when I press the brake pedal, then slows down when I release the brakes. It gets a bit annoying after a while, but I'm more concerned that it may be an indication of a problem that will become worse. I'm assuming this is not typical, but figured I'd check to see if anyone else has experienced this.

 

Only happens with the brakes, doesn't happen with the turn signals, wipers, headlights or any other electrical item. One thing I forgot to mention is the AC/heater controls and the headlights have been wired together somehow so the headlights come on and off with the ignition as required for Canada. If I turn off my lights, I have no Fan, but I can turn off the fan and still have lights. At first I thought this must have something to do with the fan speed issue, but the brake lights are the only lights that effect the fan, not the other lights.

I guess I have some exploring to do, I want to reverse the lights fan wiring anyways.

One other thing I notice is the headlights dim a bit when the radiator fans turn on, brighten when they turn off. Other electrical items don't seem to effect the headlight brightness, even turning on and off the AC has no effect, only when I hear the cooling fan relay click, I see the headlights dim. I assume this too is not normal.

 

Stefano,

Thank-you for you idea, I am going to check this. I just have to find the non-stock wiring. So far everything I've checked looks untouched. The bad wiring is in there somewhere, I just have to find it!

 

Well, after much staring at electrical schematics I'm happy to report I've solved the problem. I assumed that my issue had to do with somebody wiring up the headlights to stay on for Canada, because if I would turn off the light switch, I would loose the climate control system, brake lights, turn signals, wipers, cooling fans and probably some other things I didn't notice. However it tuns out that the daytime running lights were not the issue at all, but it did lead me in the right direction. I took off the steering column covers and found somebody had spliced an extra wire between two of the connectors to the turn signal stalk switch (see green wire in photo). So I removed the wire, but after I did that, all the functions I mentioned above that worked only with the light switch in the on position stopped working regardless of the light switch position. So some more thinking and staring at wiring schematics, and I found that one of the wires that had been spliced into went from the turn signal switch to relay N. Relay N is labelled UNDER KEY, so I swapped this relay with a headlight relay and bingo, all the functions I mentioned came to life and worked with the lights either on or off!! I got a new relay, but everything back together (without the extra wire or course) and all is now working as it should. The climate control fan no longer speeds up when I hit the brakes! Always feels good to solve a problem and even better to know that the electrics are now back to the way they should be.
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And that's the biggest reason I'm happy now, to be able to return everything back to the way it was intended to be is a relief. On me and the battery.