I am using Serial Cycling with a pressure sensor from an 05 1500 that had electric fans. I wired it into the PCM just like the 05 trucks and my old Pressure Switch is just zip tied off to the side. With Analog, the compressor is either on or off, there is no distinction when it comes to the pressures. With serial cycling you have to use the pressure sensor rather then a switch as it sends different signals based on the pressure that will turn the fans on/off or from low to high. Its the same configuration as the f-bodys. With the pressure switch you will also only have fans on low, which for the majority of people is more than fine unless you buy the kit from PCM of NC or do the relay method, but i wanted my fans to not constantly be running with the compressor and to turn on high should i ever need it so i went with the pressure sensor instead! But with the sensor i can also do analog because it kicks the compressor on and off with the compressor, and just ignores the pressure in the line.

Step one is just remove the wire that currently resides at C2-17. Cut the pin off of it and strip back the insulation so that you can put a butt connector on that wire.
Step three is connecting that wire to the black wire coming off of the low pressure switch over on the accumulator. I went to the junkyard and pulled a harness apart and got as much of the low pressure switch harness as I could. Took that home, depinned it and used the black wire from that to connect it to the wire that used to be at C2-17.

I understand all of that. Where I am lost is how you are using Serial Cycling on a 99-02 truck when it doesnt have an HVAC module that communicates over the bus like 03-06 trucks do. The two systems are activated very differently and are even wired different.

That I am not sure, I guess at that point it it may relying directly on the OS/tune file recognizing that change on the 3rd wire coming from the pressure sensor/transducer when it sends a signal to the PCM, which if that is the case it would not matter if it the HVAC module does not communicate over the bus. I have my vcm scanner setup to monitor: AC requested, AC active, AC pressure Sensor, Fan1 and Fan2 and have watched the AC requested signal but the fans not turn on until the pressure sensor reaches the correct voltage to trigger Fan 1. Once it drops below the voltage, fan 1 turns off and my A/C stays active, so i know its working as intended/expected. I know a lot of things utilize the bus, but without sitting down and really digging into i cannot fully speak to it though!

And the A/C compressor cycles when the low side pressure gets too low?

I have an 01 Silverado that has the 05 electric fans and I too was tired of the old harness that has the one fan run on high any time the ac is on.
So after seeing this post and reading through it I purchased the BP Automotive fan harness and ac update pressure transducer.
Got everything installed and the settings in the tune for the ac set to two fans and serial cycling.
Started the truck and as some people have experienced the fans came on immediately and would not shut off.
The OS in my truck was 8322 so I upgraded to the 02 OS 2156 and now fans work correctly.
After I loaded the 2156 OS I got a P1336 code and had to do the case relearn this was a little frustrating as I tried the procedure many times and it did not work.
I have HP Tuners so that is what I was using for the case relearn.
I started comparing the old 01 OS and the new 02 OS before I had made the updates and I saw where the P/N rpm cutoff is different.
On the 01 it is set at 6,000 rpm and the 02 is set at 3500 rpm so I changed it to 3500 rpm re-ran the case learn and bang 1st time it worked.
Just wanted to mention this as maybe it could help some others out.
Also want to thank the OP for starting this whole thing and the other contributors to this post and for me a big shout out to Cfdurant for talking me through the OS swap.

This thread is pretty thorough but just looking for confirmation of steps before I tear into the ECU.

Am I missing any steps in the ECU config that would prevent this from working?

Thanks for the assistance!

2002 5.3 Truck: (Assuming my ECU OS is 12212156)
Currently have installed a 3 Relay Fan Kit to OE 2005+ Fans (Low / High speeds via blue/green grounds)
- Blue Wire (ground) to Blue PCM C1 - Connector 42 (Previously Empty)
- Green Wire (ground) to Red PCM C2 - Connector 33 (Used for mechanical fan recirculation door, de-pin and replace with new wire)

3 Wire AC Pressure Transducer (DIY or BP Automotive harness)
- Transducer Black wire to Blue PCM C1 - Connector 49
- Transducer Gray wire to Blue PCM C1 - Connector 8
- Transducer Red/Black Stripe Wire to Red PCM C2 - Connector 14

HP Tuner Changes:
AC Type Fitted: Change from 'Analog Cycling' to 'Serial Cycling'
- This presumably activates wires 8, 14 and 49 to the new transducer and becomes the new AC compressor reading/controls

Fan Settings:
Fan Type: 2 Fans
Fan 1 (First Stage):
On ECT: 205F
Off ECT: 195F
Enable Speed: 30MPH
Disable Speed: 35MPH
On Temp AC On: 284F
Enable AC Press 215PSI
Disable AC Press: 190PSI
Enable Delay: .3s
Disable Delay: .2s
Min Time: 90.0s
Fan 2 (Second Stage):
On ECT: 215F
Off ECT: 206F
Enable AC Press: 240psi
Disable AC Press: 212psi
Enable Delay: .3s
Disable Delay: .2s
Min Time: 120.0s
Running Conditions:
A/C Fans Disable Speed: 45MPH
Ignition Off:
Enable Temp: 230F
Run Time: 2.000MIN

A/C System:
Hardware:
Recirculation Fitted (or Recirc Option): Fan #2 (Changed from 'A/C Recirc')
Type Fitted: Serial Cycling
Disable Conditions:
Disable RPM: 5,250RPM
Re-Enable RPM: 5,000RPM
Disable TPS: 100%
Re-Enable TPS: 0%

Engine > IDLE > Airflow:
Cooling Fan Airflow:
Fan 1: 2.0lb/h
Fan 1 2: 4.4lb/h

That looks correct to me! I will also mention that it was worth trying on your 2002, even if the OS is NOT 12212156, as there have been people who have had luck with other OS's, and it just seems to be hit or miss on others. Even with my 12208322 OS that I had, there were several people who upgraded to that OS from their previous OS and had luck, and then others like myself who had to upgrade to a different OS. But if it saves you $100 without having to buy extra credits, its worth it. But everything else looks correct!

I will also mention that i ended up having a similar issue as @iamlemarksman with my front A/C being warm and the rear A/C being ice cold. After taking a look i realized that my high side schrader valve for my A/C was leaking and replaced the original stock style ball schrader with the Dorman 800-955 that uses a regular style valve and so far everything is good and no longer dealing with a slow leak.

Thanks for the confirm, I'll be buttoning this up this weekend and I'll report back with any issues.

Just as an update, everything worked flawlessly. ECU was already 2156 so no updating required. All settings in HP tuners are available in an advanced view.

Uploaded all the changes and drove around for about an hour in mixed city traffic and highway and never saw temps above 208 with a super humid/95 degree FL day.

Fan 1/2 cycled properly with the AC and coolant temp commands. The truck is so much more quiet than with the mechanical fan which is a huge plus. Butt dyno confirms the truck feels a bit more peppy.

All and all this was a success thanks to everyone in this thread!