Im not sure how the one pin harness works but im going to guess it is always on low and kicks on high when the AC is turned on.

The 2 pin harness works like this: when the computer says turn the fans on low, pin 42 is grounded and the fans are powered in series. When it wants high it keeps 42 grounded and grounds 33, and doing this now puts the fans in parallel. Long story short, things in series have the same current and split the voltage (so each fan gets 6v, hence low speed), while in parallel each device gets the same voltage but splits the current (12v = high speed). That is essentially how this harness works, and its pretty clever if you ask me.

The real question is what makes the computer decide to turn on the fans. In HPT this is controlled only by the coolant temp (for 02-04 and maybe others), while I am unsure if the AC settings work through EFILIVE.

If you want the fans to always kick on low when the AC comes on, splice a wire between the AC sense and one of 2 things: 1) green wire only if want them to come on low, or 2) both green and blue wires if you want them to come on high with the AC. You need the wire that is grounded when the AC is on for the AC sense wire.

I personally wired a switch that grounds the wires manually so I can turn them on high whenever I want, even if the engine is off. It really comes in handy for cooling down the engine bay before working in there if I need to do something.

I hope this clears up some of the confusion.

 

The single pin harness works exactly like a double pin harness. the only difference is instead of the one relay triggering off of the PCM ground it triggers off of the positive signal from the A/C clutch.
That is why it is easy to change between the two. You only have to change the one relay from a positive trigger to a ground trigger.

The regular one pin harness works like this.
When the A/C is off and the engine is cold both fans are off. Whenever the A/C compressor is cycled one fan comes on high. If the coolant reaches a certain temp. and the A/C is off then both fans come on low. And if both the coolant temp and A/C is triggered then both fans come on high.

 

So the function of "pin33" is the done by the AC sense wire, nice.