Originally Posted by
JFB Tech
The fans are putting quite a load on the alternator and this drops the engine revs as it takes more power to run the alternator. You'll notice it on quite a few motors when you load up the electrics by switching on high load items such as fans, lights and heated screens.
You will notice it on all cars, OP the fan switch does not switch the live, it is also very dangerous just to spur off the battery that feed has no protection!
Get the fans working SAFELY and properly, I would guess they aren't automated at the moment as you have modified the existing wiring?
Martin