12v DC automotive question. I replaced the engine-driven clutch fan with an electric radiator fan, controlled by thermostat and relay. I included an LED in the vehicle to show when the fan is ON. Everything works fine most of the time, but I find that when vehicle speed gets to around 55 MPH the LED is coming on even though the engine temp should not be triggering the thermostat. I'm convinced that the fan is being driven to turn (freewheeling) by the relative air at speed and the fan motor is generating current back into the system. Is there any ill effect of letting this condition persist, e.g., reduced fan life due to the heat by-product of the generated current, damage to other electrical components. If so I could add a switch to control it, or a diode but I understand a diode will limit the voltage to the fan when running in normal mode. Thanks.
If the relay disconnects power from the fan, then the fan isn't backfeeding anything except the LED that you placed across it.
You could also add a plunger that retracts when power is applied. that way your fan bearings would not be worn out by freewheeling. However you would also want to include a brake circuit to stop the fan prior to re-engaging the plunger. But as already mentioned if the relay is disconnecting the fan from the circuit you will not have any ill effects apart from shortening the life of the fan and possibly generating EMI.