Look for a road with plenty of potholes, shound'nt be too hard too find!
Seriously it might be the resistor pack or the brushes in the motor getting worn and sticking , the bump jolting them back into contact with the armature. try banging the motor when stopped with it switched on. A faulty switch even , it'll be a case of trying them all.
Hi there, had a similar problem for a few weeks. Sometimes the fan would work and others it would make a noise like it wanted to. In the end decided to take it out and have a look, it’s easy to get at just remove the lower passenger facia. The fan motor is held in with three screws. Once you’ve unplugged it and removed it will strip down to just the motor / fan. Just 2 screws hold the motor / fan in the housing. You can then see the carbon brushes. I found the brushes OK plenty of wear left in them and free movement. Motor looked good and would run with a 12v feed to the red and black wires coming directly out of the motor. I then turned my attention to the circuit in the motor housing, it has a couple of transistors on it and a relay. Found the relay contacts were dirty also the solder joints on one of the transistors looked a bit cracked. Cleaned relay contacts and gave the circuit board a good clean and re-soldered the joints I could get at and rebuilt it all. Up to now it’s been working fine. Also while I had the motor out I applied a small amount of oil to the bearings. Hope this is of help, I have explained it the best I can. Dave
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Adding the oil to bearings like that is something I have done for years,one of the better tips.
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Cheers for the replys gents much appreciated
And spot on Trader97