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I’ve been searching these forums trying to find an answer to my AC add to my LS swap-lots of info but also a lot of conflicting info. To add to the confusion theres a lot of info that’s 10 years old.
First an overview of my setup. 2004 LQ9 out of a Cadillac Escalade. Using factory PCM and main wire harness, DBW. I have HP Tuners. Engine and trans swapped over into a 67 C10, using aftermarket radiator with 2 electric fans with 3 relay harness -using green and blue wire to
drive fans via the ECU similar to the schematic below.
I’m adding a Classic Air system, and from what I’ve read using the trinary switch is the simplistic way to go.
My confusion comes from some guys saying you need a diode wired inline to the relay to keep from throwing the check engine light, to current videos showing a simple setup like here:
Others show using a small separate relay. Problem there is I couldn’t find any info on relay size, how to connect it, etc.
What’s the easiest way to accomplish turning on the fans when using the AC with my setup?
Should I be exploring anything outside of the trinary switch?
I have dual fans set up for both to operate high/low.
High/low from two temp ground signal from PCM and both on high when ground signal from PCM.
I added the 3 diodes. Two to block trinary signals from reaching PCM and the third keeps the trinary “high” signal from going to the PCM “low” wire when the AC is on and the engines temps are lower.
In my case, the the temp signals from the PCM latch low then high (both remain latched).
I wired the diodes with the flow direction like you would normally think but it didn’t operate properly. After buying different diodes and hours of “cyphering”, I just flipped the diodes “backwards” and everything works great! Tested both sides of each diode, and have been running it like this for a couple of months. I’m no EE so maybe someone can explain the diode direction deal but I figured maybe because it’s a ground signal it worked backwards?
Anyway, not sure if you are doing a high/low or all on high all the time (which is much easier) but this is how I have mine wired. I found this basic schematic online and added the trinary & diodes to it.
This is the same setup I am running on my 72 Ventura swap using the underhood relay center from a 95 Trans Am donor (T56 donor too). I am using a forth relay with the diodes to do what a trinary switch does, as I wanted to keep my Classic Air pressure switch on the receiver/drier, wiring cleaner and already had the relay and space for the diodes in the relay center I was using. I am using @jozw30 excellent diagram with my relay in place. I will print this out and add it to my build folder!
In the above diagram I don’t see a trinary switch. With this diagram, the fan will run all the time when the compressor is on. The purpose of a trinary switch is to command the fan on only when the pressure is high enough to need it.
Many people will run it this way with the fan on with the compressor. While this will work, it puts more strain on the electrical system and will not increase the performance of the AC system. While it’s possible to integrate aftermarket AC with the computer, using a trinary switch works well and is much easier to set up. A simple diode in the trinary ground signal will probably be necessary to prevent a check engine light.
I run a 3 relay strategy like GM (2 speed fans) with a trinary switch tied in the low speed circuit. Works great, and I find the when the trinary switch is triggered, the fans are usually running from the engine temp.
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Another Trinary switch and PCM question. 
