5.3 Thermostat Housing with Temp Ports

[ls1nova71]

Originally Posted by mongocanfly

Ls1nova might have a better answer than me..but if the ecm makes the ground for the relay (and now I'm just guessing ) if you remove the wire from the ecm to the relay and make a jumper wire from the ecm side of the relay and take it to ground the relay should engage and fan should run when you turn the switch on..again I'm just guessing but if so that would prove you have ecm issues,,,, I think

This is exactly what I was getting ready to suggest! It is the quickest way to verify the wiring without a scanner that you can command them on with. like mongo said, unhook the PCM connectors from the PCM, turn the ignition to "on", then find the fan1 and fan2 wires in the PCM connectors, and try grounding them one at a time. If the fans don't come on, you have a wiring issue, if they do come on, either you have a programming issue, or the truck hasn't reached the temperature needed to turn them on.