Working on a 99-02 Silverado. Did the efan swap. Went to the junk yard got a Radiator, Fan harness, alternator, and electric fans from a 2005 Silverado. Installed all of that using pin #33 and #42 to control the fan relays. Programmed the PCM to run both fans. After all of that the fans work perfect as far as high and low speeds go as the truck gets really hot. Well as most of you who have done this know, the AC command DOES NOT turn the fans on high due to a lack of a pressure sensor on the high side of the AC. With that there are a couple ways around it but none of them fix all the problems. Some use AC command which then never turns fans off at speed, some wire in a sensor then reflash OS but then gauges and some other things don’t work right. I think I have come up with a fix. Now if there is already a fix out there please let me know but I wanted to share this with some of you and get your thoughts. Drawing is not perfect.

 

You lost me...what your trying to do is turn the fans on high speed whenever the ac is on?

 

I plan on doing an efan set up soon, and I am going to bypass all this PCM crap and just put in a regular high/lo temp sensor into the coolant. Also run the 3rd relay off the AC switch in the cab.

 

Yes turning the fans on high with Ac but being able to shut them back down when your at highway speeds. The PCM for that year won't control fan high with Ac on. This is a way of getting around that.

 

Question: what amperage alternator would be acceptable with the efan swap? I have an 03 with a 105 amp alternator, and wasn't sure if a 130 would do the job.

I was going to purchase a Nelson harness (not sure if any nearby pick and pulls have all the components), and they want $35 to activate the fan circuits in the 03 PCM. Would anyone with EFI Live or HPTuner be able to perform such an procedure?

 

Plus not being able to control the fans while at highway speed is going to have the alternator working overtime to keep up with those fans constantly running at high speed all the time. You could install a manual switch between the AC circuit and fans to turn off the AC circuit once you reach a reasonable road speed.

 

130a should be enough, unless you have a decent sized stereo system. My truck holds voltage pretty well for the most part (only 400w system), but if my system were any larger (say 600w+) I would definitely need a 145a or bigger alternator. LS1 fans and blackbear harness is my setup.

 

im runnin a 145A and two batterys.
my PCM turns my 05 fans on low when the AC is on. Its plenty to keep it cool and the AC blowing cold.
Why do you want them to come on high?

 

If you swap the pins around you can let the PCM turn Fan 2 on high with the AC.

 

But that would only be one fan running...