I have a motorcycle that is running a 12V system. My question is, What type of diode would I use for my cooling fans for my radiators. The power comes from the ECU on my bike. I want to install another positive wire and switch to the end of the power wire on the ECU power wire and a switch so I can keep the fans running even when the ECU turns off the voltage to that system if need be. The Power wire is connected to a relay and then on to the fans. I would like to Have the switch power in the same line just before the relay. I hope someone can understand my crappy explanation.
Taking a guess at what your asking. At the fan end there is power supplied to turn it (regardless if it comes from the ecu or relay). You'll need two diodes of sufficient current and voltage ratings (depends on how much current the fan takes and what sort of back emf might be generated when turning the fan off) connected as shown below. This stops either feed from supplying the other.The types shown are schottky types (low forward voltage drops). I'd also recommend some form of indicator to show you are overriding the ecu.
If your switching the relay it depends on how the relay is powered.