I installed 34" rad and efans with a nelson harness. The truck is overheating when i stop moving. Im guessing its a relay not turning on the high speed fan? I know the fan comes on when i turn the ac on and when it gets to 210 ish, but im not sure how to test for highspeed? Anybody know what i should do?

 

The high speed rairly turns on in my truck, you should see if your fans work on low speed.

 

Last Nelson harness I messed with they would only come on high with the ac on. I am not sure if this has been changed as that was awhile ago. How did it hook up? How many wires are running to the PCM? As for whether or not they should need to come on high, that depends what thermostat you are running and what fan temps you are commanding on and off. For guys running stock thermostats and higher fan temps then you will rarely see high used as low usually is all thats needed. However if you are running a 160 stat and have fan temps in the 180's then you will see high come on. I have a 160 stat and my high speed will come on if its hot out and I am in traffic, mine is set at 190 on and 183 off for high.