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Someone check me on this if you will. I’m in the startup and get stuff working right phase of my marathon swap of a 03’ Yukon 5.3/4L60 into a 72’ C10. My biggest issue right now is that my fans will not turn on. I’m using a PSI harness and they programmed my PCM to turn fans on at 195 and 205. My temp passes through 210 with no fans.
As I study my diagram (penciled notes are my actual wiring) I’m thinking Derale led me down the path to failure telling me to connect the black wires (pin 86) to ground. I think the reason my fans aren’t working is because as they are wired per the Derale diagram, there is no switched 12V power available to close the relay. I’m thinking if the black wires from pin 86 of the relays are connected to a 12V switched power source the relays and fans should work when they get applicable grounds from the PCM or trinary switch.
My PSI harness is for a different engine/trans combo but the fan output is a ground signal. Not sure if that is something they keep across all harness combos or not.
Turns out that the relays operate better when they have a ground AND 12v switched power. The Derale diagram was for a switched circuit and threw me off. I took my black wires from pins 86 off of ground and to 12V switched power. Fans, or at least the No1 fan came on as advertised.
I haven’t considered that. I have the thermostat called out for my donor 03’ Yukon, which had a mechanical fan. Should I be looking for a 200 degree thermostat?
My crate LS3 came with an OEM 187F t'stat on the inlet which results in 195-197F temps at the outlet when you're cruising down the road with enough air flowing to keep the t'stat in charge. When you slow down, the fan controllers take over. You can see the problem -- if yours had the same t'stat and your first fan comes on at 195F - it's never going to turn off, because the off set point is lower than the on set point for obvious reasons. That 187F t'stat won't let the temps go below 195F or so. The t'stat sets the lowest temp the engine should see. The fan operating parameters set the highest temp the engine should see.
Your journey starts with knowing what temp t'stat is in your car. Sometimes the temp is printed on the body somewhere -- I prefer to put them in a pot on the stove with a thermometer and actually see what the opening range of temps is. My 187F stat began opening at 186F and was fully open by 188-189F. If you want to keep your fan settings where they are you'll need a LOWER temp t'stat, not a higher one. But I wouldn't want to go any cooler than the 187F unit. If you have boost plans you may want to run cooler to help control detonation. You're gonna want the OFF temp for the first fan to be at least 5-7F ABOVE the temps you're seeing at your outlet. That will allow the fan to cycle properly.
Just by way of example - with a 187 stat giving your 195F operating temps - you'd want your first fan to turn off at say 200F - so turning on at 210F. Perhaps the second on/off would be 220F/210F.
Good luck sorting it out.
Last edited by Michael Yount; Jul 27, 2021 at 11:14 AM.
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