You need some form of sofware (such as HPTuners or EFI Live) to set the on/off temps in the pcm

Yes, the harness works with single speed fans and actually provides high and low speeds itself by using relay coil resistance for low speed. As nonnie mentioned, if you have a newer truck/pcm, there are more options that can be set in the pcm for high and low speed. My truck is a 99 so I'm limited in that respect, but the Nelson harness is more than adequate for my needs

 

will the truck is an 02 2500hd w/a 6.0l, even thou the truck uses a mechanical it still has settings in the pcm for fan on and off temps?

 

The settings are input via the tuning software, the pcm sees engine temp from it's input, then grounds the appropriate pin (pin 41 or 42 I believe, can't remember at the moment), according to the temp that it has been set to by the person using the software. When the engine temp drops below the "off" setting the PCM then removes the ground, turning the efans off.

Just an example, my truck is a 99 that originally had a clutch fan, and I now run efans that are controlled by the PCM...as well as turning on when the ac compressor engages, thanks to a wire going to the compressor clutch B+ that goes to a relay in my Nelson harness.

 

do you have to tune for e-fans or just hook them up and they work fine?

 

They don't work on their own, you either have to tune for them, run a manual switch, or use a temp probe that goes into the radiator fins. The temps probes are prone to failure though, so I recommend against that method. PCM controlled efans is by far the best method, and you can still have a manual switch if you want.

 

I had to have mine tuned for the e-fans dealerships tried but couldn't do it.....never overheats and a/c works twice as good.