Hi, I have an E39 530D with 135,000 miles on the clock and have had some problems recently that may or may not be electronic.
Firstly the car experienced a significant drop in power 3/4 way up a mountain just when I turned on the fog lights but full or nearly full power returned shortly after that which was strange, however a noise like a squeaking fanbelt was apparent when putting the engine under a bit of pressure. I brought it to the garage and they told me the noise was coming from the turbo. Turbo was removed and sent off to be checked and it came back with a clean bill of health with advice to change something (some gasket?) on the outlet side of the turbo, just in case this was faulty, this was done and turbo reinstalled and when the car started up again the noise was there for about 3 minutes on idle and then it went away. It failed to come back upon testing. When I got the car back again it was going fine for a day and then under pressure going up a mountain again I turned on the fog lights and again the engine power dropped. It turned out as well that the dash lights had gone off when in the garage but returned again and the window and mirror controls in the door have completely stopped working some time recently as well. ALSO the aircon and fan stopped working but then returned again at a later stage the same day. It all seems connected to the electronics in some way. My mechanic admits that the turbo sound could feasibly be caused by the an electronic control incorrectly influencing it. Does anyone have any experience of a similar problem? thanks in advance.