Wouldn't it be simpler to arrange a switch or relay that cut off the ECU feed as it switched in the direct 12 V feed.
EDIT sorry i hadn't seen that Cobber had got the first. However I don't think i needs to be as complicated as he describes
I am unaware of them coming on otherwise but then the car's temp gauge has never gone over the half way mark during my ownership; there has been no reason for the fans to come on.
...….I am unaware of them coming on otherwise but then the car's temp gauge has never gone over the half way mark during my ownership; there has been no reason for the fans to come on. ………..
...….It could be that the radiator fans will not come one with the ignition turned off (had a VW that worked this way), unlike the engine fan which has come one many times just after the car has been parked. ………..