Idle unstable with fan on

[LZeppelin513]

I am running a 3500 with 7730 on nast1 code. Idle is pretty solid normally, but when HVAC fan is turned on it starts hunting. My ECM volt reading drops from 14.6 to 14.3 when switched on and wideband jumps up about .7. Also, BPW increases from about .65 to .75.

Should the BPW be increasing when ECM voltage drops a bit? Or is the .3 voltage drop tricking the ECM to think there is a lean condition and adds fuel? Any thoughts? Thanks.

[robertisaar]

BPW will increase with lower voltage, normal
BPW will also increase with higher load (electrical, in this case).

when it does start hunting, are the IAC counts moving in the opposite direction of RPM(ie, when RPM swings high, IAC drops like a rock, then as it comes down, IAC starts coming back up and repeating the cycle)?