Despite what it may seem, you DO want the A/C system managed by the factory LS PCM. I keep seeing a lot of mis-information on how to wire these things, and sure you can get by, but you lose a lot of functionality and frankly efficiency/safeties when you hardwire the AC system. Namely managing AC pressures by switching between low/high fans, shutting the fan off after a set MPH, shutting the compressor off after a certain RPM/Throttle threshold, shutting the AC off if the truck overheats etc.

So I wanted to provide some alternative thoughts and ways to properly wire in and integrate the AC System from the truck with the LS PCM.

To start off, whether you're DBC or DBW does not necessarily matter. What does matter is which PCM you're using. You need an Analog input capable PCM for your swap. This essentially means the AC request input at the AC can accept a 12v on/off as it's signal.

0411/P01s are analog right out of the gate. Very specific P59s have it as well. However, a great deal of P59's do not and use a serial connection for this communication. The fortunate side is you can solder in the resistors to a non-analog P59 to make it be analog if you so wish.

Once you have an analog input capable ECU, you need to wire the AC on request wire to your PCM AC request input. Now when you hit the button, you tell the PCM you want AC. It then uses its own output to the clutch relay to activate the Clutch as long as it sees it is within the TPS, MPH, Pressures, and CLT thresholds.

If you're on an 0411, it will likely run out the PCM and thru the factory pressure switch on the accumulator, which has a low and high PSI threshold. With a P59, you will only need to use the factory 3 wire pressure switch from the donor that feeds the low and high pressure values back to the PCM. With these values fed into it, the PCM can manage the pressures via fan control low/high switching, ensuring your AC quality is the best it can be. You can get this sensor off any P59 equipped truck/SUV.

Keep in mind you can actually tune these pressure values in HPTuners to dial in to your specific truck where you want the AC to shut off at for a low/high. It provides another layer of customization that doesn't exist in a full on analog system.

Now to wrap this up, the typical alternative I see presented around the internet is AC switch 12v out to a relay, from the relay you run to the compressor thru the low/high switch, which then activates the clutch. That same signal feeds another relay for the low side fan. And sure, that works, but again you're running your low side fan at 70mph, the fan/clutch won't disengage at WOT, and pressures aren't regulated, only used as a basic threshold to shut the system down. Most of our trucks are cammed, headers, and generally "hot" under the engine bay. Being able to monitor and regulate the AC pressures is critically important if you want it to perform well.