Originally Posted by dieseldiehard

Another thing you may want to do is replace the fan thermo switch, the three terminal one. As busybenz raised earlier there is a lower temp one available from the later series diesels. gsxr recommends this "up-grade" (see below).
The reason I suggest replacing this switch, even if with the original type, is because they tend to fail after a few years because they carry the entire fan current (about 12A at least and more when they start). There was an idea which proposed adding a small relay to take the high current load off this switch, that is something I'd like to do when I get time.

quoting gsxr: 3) The electric fan high speed is only triggered by the 3-prong sender up at the water outlet on the cylinder head, near the upper radiator hose. The 603.960 switch is blue and rated 128/105, and costs about twice as much as the 603.961 switch (which is gray and rated 120/105). If you replace it, get the cheaper switch, which will also engage the AC safety shutoff at 120C instead of 128C, which IMO is ridiculously high.