Cooling Fans not Coming on

[Jake Camacho]

Fellas and Gals! 
 

I am having a problem with my cooling fans not coming on my 2006 GMC Sierra, 5.3. I can force them on by running the A/C constantly. But if the A/C isn’t on, the fans won’t come on till the truck reaches almost 230 degrees F, which I’m on the road to cracking a head at the temp. I’ve already replaced the fans, thermostat, engine temp sensor,  checked to make sure I’ve got good water flow through my radiator, I’ve replaced all three cooling fan relays and I’ve checked the wiring harness. The fans will come on when I reach in there and manually unplug the harness from the temp sensor while the truck is running and it sometimes will throw a code after that. The only thing I can imagine may be happening is the CPU is not sending the ground signal to command the fans to come on when the engine reaches the appropriate temp (≈ 202, without the A/C being on). Has anyone ever experienced the CPU messing up and not send a ground signal to the system, what am I missing? S.O.S.  PLEASE SEND HELP!! 

[sefiroxx]

Check the resistance of the temp sensor against specifications.

Measure resistance of the temp sensor at the sensor. Then pull wire harness from cpu, measure resistance in the wire harness, should be the same or close.

Now the tough part. Resistance in the sensor increases as the temp rises- reducing the voltage on the return leg. Unplugging the sensor creates high resistance, and 0 voltage, enough to cross the threshold set by the cpu. A short on the return leg could be sending incorrect voltage signal back. Unplug the sensor, check for shorts with either leg against all other wires in the harness.