Okay, so here is a little bit of history on the subject...I just had my engine replaced a few weeks ago and after about a day of having it the radiator cap erupted and my car over heated a little...I replaced the radiator cap and noticed that the fans were not working anymore. I decided to replace the radiator, hoses and fans. I bought the fan shroud similar to the mishimoto setup which has two fans on a slim shroud with two wires from each. I did some research and found the wiring...I wired the fan on the drivers side, the 2 greyish black wires to the black and the two greens to the blue. The passenger side fan, two yellow to the black wire and the two blue wires to the blue on the fan. Soldered and heat shrunk the whole set up. Now when I turn on the car it runs fine, but the main fan doesn't turn on, even with the a/c turned on. After the car is on for about 20 min or so the passenger side fan will turn on for a bit then shut off. I'm assuming that the cooling fan 1 & 3 relays are bad. Is this a correct assumption or could it possibly be some sort of blockage in my cooling system causing this? I am at a complete stump and I would like some input before I go and buy the relays(Nissan wont allow returns on electrical parts). Any ideas would be very helpful!

Side note, there is a new thermostat and waterpump.

 

Im going to follow this thread closely and try to help you out.
I have similar issues.
My car will overheat while my ac is ON and cool to normal shortly after turning OFF my ac.

 

update*

So I purchased some relays on ebay that should be very similar to the ones we have in our cars. They are Jideco 170 9441-A 12v relays. I am waiting until Monday to give it a try. I don't overheat at all but I would like both of the fans working. I also purchased a new water temp sensor and I will be replacing that if the relays don't work. Can you give a little better description of your issue kwise? Like, is your radiator hose that connects to the bottom of your radiator hot after driving for a while? I might be able to help you a bit.

 

installed the new relay's and nothing changed...can someone who has done this before just tell me if the fan is supposed to come on when the a/c is on with the way I have it wired...or is it the blower fan only that is supposed to turn on when doing so?