[camcojb]

usually the ecu controls the fan by grounding the fan relay as you said. The a/c has a binary or trinary pressure switch inline and on mine it has a ground output which I tied into the fan relay ground, along with the ecu ground wire. That way the ecu will command the fan on at a certain temp and regardless of temp whenever the a/c is switched on the trinary switch will ground the fan relay to run the fan, which should always run if the a/c is on.

I turn on both fans at the same time, but you could run one off the ecu, and run the other one or both off the a/c pressure switch. I am not familiar with the XFI but if it has two fan controllers then set one ground wire to each fan relay for the ecu control, and then tie in the a/c pressure switch to the relay ground of one or both fans, depending on your preference. If there is an a/c input wire for the XFI then you'd run the ground or 12v off the binary/trinary pressure switch there, instead of directly to the relay. I do not know if it does though.

EDIT: I downloaded the XFI manual. The a/c control they give you is to override the a/c relay and let the ecu shut off the a/c by temp or throttle percentage. You can set max temp to operate, so it will not come on if the coolant temp is beyond where you set it. It also lets you set a tps setting so once beyond a set % of throttle it shuts off the a/c (like full throttle) to reduce load on the engine. There is no input, you just hook the output wire (B11) to the a/c relay ground terminal and let the ecu control it. It doesn't need an input as it only shuts the system off by temp or tps % through ungrounding the a/c relay. It will not affect or control the fans off this circuit.

To get the fan control do what I outlined above. The XFI system can only control one fan, so you'll need the a/c pressure switch ground output to ground the second fan relay, or use a fan controller like the preset ground switches that screw into the intake or block and ground the circuit at a certain temp, like say 200 degrees, and run the second fan relay ground to that switch.

Jody