Engine almost stalls when fan turns on

[Dabigguy]

Are you using dual fans, or a single fan? The power sag is almost certain when using dual fans, unless you stage them (five second delay works well).

If using dual fans, be sure to use two relays and stage one of them.

How do you "stage" one of the fans?

 

[GNVenom]

Use an in-line relay delay:

http://www.installationinstructions.com/109086.pdf

It goes on the relay coil, delays it for around 4-5 seconds.

 

[Craig Smith]

One of the options on the old Gen 7 systems is one that I still like (and is still used in OEM applications I believe) and that is an option to kick the IAC open a few ticks when the fan output is activated. It's called feed-forward (predictive, as opposed to feedback which is reactive) and it was a very helpful tool that made easier work of minimizing the effects of electrical system load. Properly calibrating the injector offset table will always be an important task but having the ability to kick the IAC open by xx counts (user-programmable value) at the same time the fan output activates really helped. It was just a nice simple touch that simplified an issue with a lot of dynamics.