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First of all, the electric fans aren't "A/C fans". If they are coming on whenever the A/C is on, the odds are your system is overcharged. They are intended to come on at low speed only when the A/C pressure is too high, or on high speed when the engine is close to overheating. They are very powerful cooling fans...the pair in my 190D pull 25 amps, and probably put out close to 3000cfm. More than enough to do the full job without the pump fan.
On the left side of your cylinder head, there is a row of four or five 22mm bungs in the water jacket. One of these is used for the temp gauge sender, one is used to tap water for the heater. There are at least two spares. Take out one of the unused plugs and replace it with a Porsche or VW fan switch...available in temperature ranges from 175F to 205. I use a 190F switch to control the electric fans. Although the pump fan is still in place, the electrics deploy first and do most of the work. My pump fan almost never engages. Temperature is consistently "on the thermostat", even in stop and go.
One point...on many cars, there is no separate fuse for the high speed fan circuit. Instead, there is a fuse internal to the relay. If you car has a black relay for this purpose, you should replace it with the superseding orange relay, which has a fuse in the top. If the fuse blows in the black relay, not only can't it be replaced, but it can start a fire.
The pump mounted fan consumes huge amounts of horsepower. This is probably why it's designed to come on so late, and why your temperature fluctuates in traffic. The later fans with thermostatic clutches are less wasteful than the ones with electric clutches, but either way it's a waste.
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