Working backwards on the wiring diagram I viewed, the DIC receives data from the Data Bus. The Temp sensor sends its signal to the ECM. From there it is how I described before. The actually factory wiring diagram shows, AT the fan, Pin1 12V, Pin 2 Ground, and Pin 4 feeds the ECM signal to the COOLING FAN SPEED CONTROL module. So, a correction is in order; IF the ground is GOOD, then one has to diagnos BACK from the fan starting with the Fan Speed Control Module. These modules often fail on all manner of cars, not just GM.
As for the
from the dealership, they can (would) use their Tech 2 scan tool to command the ECM to activate the fan.
[One can do that at home. Start the car and warm it up, open the hood, turn on the A/C, and listen for the fans to come on. You may need somebody to activate the A/C with you head sort of buried in the front of the engine compartment to listen for the fan. If it doesn't come on, then that nearly eliminates the Temp Sensor as those commands are independent.] If the fan will not come on, they don't NEED a hot engine, as THE FAN WON'T COME ON. If they insist on duplicating the problem, the fans are programmed to come on at (I believe) at about 230deg F. Have them start the engine with the A/C on. The engine will get hot enough. Politely ask the Service Writer or SVC. Mgr. the following question; "Would you like to correct the problem in cool weather, or have me bring in a car with all the coolant blown out of the cooling system because it all boiled out in traffic? After all, we may be talking about an ENGINE, not a RELAY." This should work. If not. customer service at GM will. Dealerships HATE those calls from GM. Regardless of how good or bad a store is, they live/die by their CSI ratings, and GM is using CSI scores as one of their criteria for closing stores.
BTW, since you are attempting to measure voltage at ground, it WILL be zero. What you should do is measure continuity between Pin 2 on the wiring harness plug and a good known ground point. Supposedly, that point will be a tab bolted to some sheet metal at the right front corner of the engine compartment. It should be ZERO on the ohm setting of your meter.
