fan relay

I wanted to run a second control switch to the fan relay, if there was a need, I could turn on the fan before the ecm. I don't see why I can't use the original relay and just tap into the control wire already there, have a toggle switch under the dash connected to switched ignition voltage. That way the ecm could do it's thing and I could do mine. Although, I do wonder if the ecm control wasn't active and I activated the circuit with the toggle, if the backflow might create a problem at the ecm? I guess as long as that circuit wasn't grounded, it wouldn't do anything, correct? Any thoughts. thanks

What fan are you talking about ? is it the radiator fan or a cooling fan for your amps?"I'm So Loud It Hertz"

It's the radiator fan. I know I can use a separate relay and splice into the power wire at the fan, but I thought it would be easier to just splice into the control circuit at the original relay. I did worry some about the back-flow through the ecm circuit, I guess I could use a diode to stop the back-flow. Sounds like it would work, and I wouldn't have to wire a second relay. thanks

yea just check if it goes positive or negative to turn the fan on and put a diode in, should work fine as far as i know...

If this particular cooling fan has one of those delay relays that will activate automatically when the key is off (temp sensor controlled), you can splice into the wire running to the temp sensor and run the other side of the switch to ground.  Most relays of that sort are on a time-control circuit, so you could probably get up to 10 minutes of fan run time before the circuit would reset itself.  The key off fan circuit will probably run at high speed if you have a two speed fan. 

Use the fan temperature override circuit and splice into the temp sensor wire, running the other side of the switch to ground.