One last post before I leave on my trip. John - just to make sure you know - If the engine temp. is not at 80C or above on my 560sel - I can remove the connector to the temp sensor (two wire) on the end of the upper rad hose and the aux fan will NOT default on. It will only default to on when the connector is removed if the engine temp is over 80C - must be another sensor somewhere that has to kick in before the ECU will detect an open fault when the connector is removed. Before you start condemning your aux fan circuit as faulty - try the test with the engine temp up all the way. If your fan still does not kick in when connector is removed - maybe Arthur can suggest what other sensor may be going flaky that prevents the ECU from reading the default signal when the connector is removed. I will check for your response early next week when I return.
Morris