Hi guys, I set my minimum airflow the other day with my not-stock single 18" electric fan running. I realized that when the fan kicked off, my minimum idle speed exceeded the commanded idle speed, which was causing the IAC to have some issues. My commanded idle speed is 750. If I set the minimum airflow such that I have a 700 rpm idle with the fan off, IAC disconnected, when I reconnect the IAC I idle at the correct 750 rpms. When the fan kicks on, the IAC compensates for the rpm drop and brings the revs back up to 750 ish (although as the IAC counts go up it starts hunting). Does this sound normal for me to have enough load on the alt. from the fan that I see the rpms responding to it?

 

Different engine, different car...... When I turn on the headlights on my 78 Malibu my idle RPMs drop by 50. And that's a little tiny 62 amp alternator on my buggy that isn't even very efficient at lower RPMs. I'm going to say that a higher-draw accessory like a cooling fan and a strong later-model 100+ amp alternator could probably drag down idle RPMs like you're seeing. If you have an inductive ammeter you might want to take some readings at the fan and the alternator power wire just to do a quick sanity check.

Cooling fans draw a LOT of amps when they first kick on, and then less as the fan comes up to speed.

 

My '86 z-28 used to do that, when the cooling fan kicked on the rpm would stumble for a second and then go way up.
Turned out to be an improper ground on the cooling fan.