[MDPotter]

Posting this for anyone that has a Gen 3 or 4 LS engine, Vintage Air, and dual electric fans.

When it came time to wire up my dual fans, I did a lot of research. By recommendation of a few members, I decided to run 2-speed fans. When I say 2-speed, that means both fans running half speed (in series, 6V) or full speed (in parallel, 12V). This way, the fans are quieter and they pull air evenly across the radiator like most modern day dual fan setups in factory vehicles. This isn't too hard, GM has a schematic for it that involves three relays. This gets a little more complicated is when you need to integrate a trinary switch from Vintage Air. Gen 3 and 4 PCM's supply a ground signal to the fan relays in order to turn them on and off. Conveniently, a trinary switch closes when line pressure reaches 254 psi and also provides a ground signal.

Going back to the GM PCM function (specifically the 3-relay schematic) - typically, when coolant temp reaches 195, a ground is sent via the "low speed" green wire and both fans run in series (half speed). When coolant reaches 205, a ground is sent via the "high speed" blue wire and both fans run in parallel (full speed). So when your A/C line pressure reaches 254 psi, you want both fans to be running full speed. It's not as simple as tapping into the correct wire of the GM diagram to turn on both fans full speed though. This is because the PCM runs diagnostics and it will see a faulty ground in that blue wire when the trinary switch closes and will then turn on the check engine light. To avoid this, you have to install a resistor and a diode on the blue wire that connects to the trinary switch. Credit to user prodjay1 for this. Using a resistor and a diode will only allow current to flow one way and will keep the PCM happy (or unaware of that other wire).

This all sounds great and in theory would work, but not all fans are two-speed. Sure, you can run a fan at 6V or 12V and it will work, but the fan may not be designed for that. In my instance, Spal fans are not designed to be ran half speed and will "prematurely wear out" according to Spal. So I found a wiring diagram for a 2-relay setup from Speartech which is based on Gen 3 and 4 engines. I then added the trinary switch, resistor, and diode to the diagram. I wired this up last weekend and here is the sequence of operation: Coolant temp reaches 195, fan #1 comes on full speed. Coolant temp reaches 205, fan #2 comes on full speed (fan #1 remains running). Trinary switch closes, fan #2 comes on full speed. So the PCM and Vintage Air are happy and both have control over fan #2. For the ignition source, I had not used the electric fan circuit from my American Autowire chassis harness and it is a "key on" ignition source so it worked perfect to utilize this formerly unused circuit.

Now, the trinary switch has no control over fan #1. If you want to take this a step further, you can tie it into fan #1 as well. But the likelihood of the A/C line pressure reaching 254 psi prior to the coolant temp reaching 195 is pretty unlikely.