Manual override for Fans.

[BlackBird_SR71]

Atom G4+

ave Coolant fans triggered by Auxiliary Output #2 ( Pin #20)  . Works fine controlled by ECU. 

Would like to add an Manual Override switch which would simply ground the #85 Terminal on the Fan Control Relay. As this is also connected to Pin #20 on ECU, are there any special precautions that have to be made. Such as adding a Diode to wiring to ECU to prevent any type of Feedback. 

[Adamw]

You cant have a relay that is live all the time connected to an aux output as it will cause a backfeed.  So if you are intending to use this override switch when the ignition is off the only way to do it would be to move the fan relay to an ignition drive. 

[BlackBird_SR71]

This would be only for turning on the Fan while the engine is running. Usefull in some Racing situations such as sitting waiting on starting line before a Hillclimb or Autocross.  Or just as a general Override . 

 But I was thinking ( in my Sleep ) . I should be able to do this with a Generic input. Use Grounding Toggle switch on spare Auxiliary Input to actuate Aux #2 output to fan relay.  Hmm I could use the AC quicker function as well. Is that Voltage triggered or sink to ground triggered.  I think I just answered my own question. Now back to bed...

[Adamw]

If that is the case then your original suggestion of your switch grounding relay pin 85 is fine.  The other option is connect the switch to a DI and use a virtual aux & GP output for the fan function.