Bad temperature sensor

Your engine should have two coolant temperature sensors. The single-wire sensor is for the dash gauge, and the two-wire sensor is for the engine computer. Both of those have a very low failure rate because there is just one component inside them. More problems occur from wiring and connector terminal problems.

If the old fan motor had tight bearings, that would have made it draw very high current, and typically blow a fuse. Where I would start is by locating the fan relay, removing it, then checking for twelve volts on one of the terminals in the socket. Some vehicles used a fuse link wire, and when that burns open, it leaves a carbon track behind. That can conduct enough current for a digital voltmeter to see a voltage and give a false reading. Testing for that twelve volts is best done with a test light. If a fuse link wire is burned open, the test light will not light up. That is the more accurate tool for this type of problem.

Yup. That proves the high-current circuit is working. The suspects now are a defective relay, or the Engine Computer is not turning that relay on. On most cars, if you turn on the ignition switch, then unplug the two-wire coolant temperature sensor, the fan relay will be turned on by default. The Engine Computer will see the sensor circuit has a defect, and will turn the fan on just in case there is an overheating problem.

If the fan runs when you unplug the sensor, you likely do not have a problem. A lot of GM engines have to get as high as 226 degrees before the fan turns on.

If you hear the fan relay click, but the fan doesn't run, suspect the relay. Swap it with one of the other ones like it as a test.

Dandy. Happy to hear you solved it.

Be aware temperature sensors have a very low failure rate. As they get hotter, the resistance of its internal temperature-dependent resistor goes down. The Engine Computer reads that resistance to calculate temperature. A much more common cause of this problem is corrosion on the connector terminals. That will add to the resistance and cause the computer to interpret a lower-than-actual temperature. The total resistance may never get low enough for the computer to turn on the fan.

If the problem occurs again, try cleaning the terminals in the plug.

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Cheers, Ken