I made this oil cooler setup for a few reasons, but here is the main reason. When making near double the power and substantially more torque than what the M3 differential was designed for with the S65, I concluded that keeping the oil in the differential cool would be of a benefit.
I've rigged it up with two temp switches that switches on at two different temps. the lowest temp 88DegC (190degF) switches on the pump to start circulation of the oil. If the oil temp continues to go up and reaches 93degC (200degF) then the fan kicks on. I've added LED lights on the dash that indicate when each switch is triggered, that way I'll know if this was overkill or if it was worth it.
