No fan operation

Take a step back for a moment. You already identified the radiator fan motor is not turning on, so there is no point in replacing all those unrelated parts you mentioned. The fan relay was a good suspect, but not the most logical one. That would be the motor itself.

Modifying the circuit to use a toggle switch was suggested by someone who does not understand how these simple systems work. Doing it that way can lead to a rather unusual problem where the engine will not stop running right away when the ignition switch is turned off.

The coolant temperature sensor is not a good suspect because temperature sensors have just one component inside them, so they have an extremely low failure rate. If there is a wiring problem related to that sensor, the fan will run all the time. In fact, that is a quick test we use. This system can be divided into two parts, the high-current fan motor circuit, and the sensing and control circuit. Half of the relay is in each part. Almost all electric radiator fan systems will default to turn the fan on when there is any problem with the temperature sensor. That is because the engine computer will not know coolant temperature and the engine might be overheating. The fan gets turned on to prevent that.

The test is simply to unplug the coolant temperature sensor. Be aware that most older engines had two coolant temperature sensors. One had only one wire plugged onto it. That is for the dash gauge and has nothing to do with the radiator fan. You want to find the coolant temperature sensor for the engine computer. That one will always have two wires in its plug. That is the one to unplug while the ignition switch is on. It might be smart to have the engine running in case that is a requirement on your car.

When you unplug the two-wire sensor, the fan relay should click on and the fan motor should run. If it does, the system is working properly. If it does not run, feel the fan relay while you unplug the sensor. If you feel it click, we will need to check in the motor circuit, but a bad fan motor is the likely suspect.

The resistor you are referring to is for the different speeds of the heater fan in the dash. That is not related to the radiator fan.

The sensor on the coolant reservoir is to turn on the "low coolant" warning light. That also is not related.

Your most important comment is, "and the fan does turn on, but plug it back and nothing." That proves the fan circuit is working properly. There are two things to look at now. The first is GM cars are designed to have the fans turn on at uncommonly high temperatures, often around 226 degrees. The best approach is to connect a scanner to view live data and see exactly what the engine computer is seeing for coolant temperature. In the rare event it is seeing well over 226 degrees but is not turning on the fan relay, the computer would be suspect.

The better suspect, since this is obviously something new that is going on, is a cylinder head gasket is leaking. That can cause combustion gases to sneak into the cooling system and pool under the thermostat. That will prevent the thermostat from opening. Thermostats must be hit with hot liquid to open. Hot air will not do it. The problem with this story is this combination of symptoms usually occurs on engines that use two different coolant temperature sensors; one for the dash gauge and one for the engine computer. Yours uses only one sensor for both functions, so if the dash gauge is reading too high, the computer is seeing the same thing.

What may have changed is how well the radiator is giving up its heat to the air flowing through it. Radiators are very efficient in that respect as long as nothing interferes with their operation. In the past, the two temperature sensors were in different places, so the sensor for the dash gauge could see the engine getting too hot, (and that is what you would see), but the sensor for the fan was on the other side of the closed thermostat where the hot coolant had not gotten to yet, so the computer would see the lower temperature and not turn the fan on. Now, with a single sensor you and the computer are seeing the same thing, but while you see it as higher than normal, the computer might be seeing it as not yet high enough to turn the fan on. This is where you would need to connect the scanner to see exactly what the actual coolant temperature is. If it is not up to at least 226 degrees, it is acceptable that the fan has not been turned on yet.

If you do find less than 226 degrees but it makes you uncomfortable, look for anything that affects the radiator that might have changed. There should be a rubber seal that runs along the bottom of the front edge of the hood. That is to prevent air from bypassing the radiator by sneaking over it. Look for anything bolted to the front sheet metal surrounding the radiator that was removed, or any holes that were cut in it that would allow air to bypass the radiator. There is usually a shroud around the fan blade, or, with electric fans, it is built as part of the housing. That is needed to direct air out under the engine. When it is missing, air can go down underneath, then forward and back through the radiator again where it is too hot already to absorb much more heat.

Feel the radiator to see if there is one area that is considerably cooler than the rest. That would indicate some of the cooling tubes are plugged. This is especially common with the miserable red Dex-Cool coolant GM uses. GM advertised it as "lifetime" coolant to make their cost of maintenance appear to be lower than that of their competitors, then they put a sticker under the hood that says to replace it every three years. Even the Dex-Cool company does not recommend that. They say to replace it every two years just like every other brand of antifreeze because the important additives in it wear out in about two years. Those include water pump lubricant, corrosion inhibitors, and seal conditioners. Dex-Cool turns into mud when mixed with other types of antifreeze. That will make it harder to be pumped around the engine.

Be aware too the fan is not needed at higher speeds when natural air flow is sufficient. If there is a problem with the radiator fan, overheating will only occur at lower speeds or when standing still. If the temperature gets too high at highway speeds, look for the things that affect how effectively the radiator does its job.

You do not have an electrical problem with the fan. It runs when you unplug the coolant temperature sensor. It can only do that when everything is working. This is the equivalent of saying, "my car runs great up to ninety mph, but I am still going to check to see if the tires are flat". We need to be looking at things other than the fan circuit.

You hit on a problem many people do not understand, but for a different reason. Testing must be done when the problem is occurring, as you said, but too many people think that applies to diagnostic fault codes and the check engine light. In fact, once that light turns on, the fault codes are in memory, even if the light goes off later. These codes tell you what problem was detected in the past, and that problem could be occurring right now too.

The issue here is the need for the scanner. Diagnostic fault codes can be read by simple, inexpensive code readers, and those only read codes in the engine computer. Some of the more expensive ones will read codes in a few other computers, but never all of them. Some cars as early as the late 1990's had up to forty seven computers. Only the most advanced scanners will access all of them and read fault codes stored in them.

Reading fault codes is only one or two percent of what full scanners do. If you have fault codes read for free at an auto parts store, they're using the more expensive code readers that cost as much as perhaps $100.00. Scanners used in the repair shops cost in the range of $5,000.00 to $8,000.00. Even the popular Snapon scanners only cost $4,000.00, but they rip the shops off by forcing them to buy annual updates for $1,000.00, and no years can be skipped if you want to buy the latest update.

Once you read the fault code(s), you only know which circuit needs further diagnosis, or the unacceptable operating condition. (Be aware fault codes never ever say to replace a part or that one is bad). If there was a problem with your coolant temperature sensor circuit, the typical fault codes could be, "coolant temperature sensor voltage too high" or "coolant temperature sensor voltage too low". On some older, less-advanced cars, you might only get, "coolant temperature sensor voltage out of range", then you have to figure out why. In either case, to do that, you would switch to the live data screen to see the sensor readings that computer is seeing. I have a Chrysler DRB3 scanner for all of my vehicles. That one lists fifty four things for my old 1994 Grand Voyager, just for the engine computer. You would scroll down to find the coolant temperature sensor voltage, then right after that it would list the temperature the computer has interpreted that to be. This is where you would see what the actual temperature is, and if it's anywhere around 195 to 226 degrees, the engine is fine and it is the gauge you have to look at.

If the temperature really is too high, there is another part of the scanner that shows "inputs/outputs" related to that computer. An example of an input is the neutral safety switch, air conditioner power switch, etc. In this case you would look for the "radiator fan relay" which is an output that is run by that computer. If it is listed as "off", the computer does not want to have it running right now. If it is listed as "on", but the fan is not running, that is when there is a problem in that system that we need to look at. Remember though, we are past that point. You made the fan run by unplugging the coolant temperature sensor.

Another part of the scanner I use quite often is the "actuator test mode", (ATM). This allows you to command the computer to do stuff. In this case, I'd select, "engine computer", "ATM", relays and solenoids", then "radiator fan relay". The engine computer responds by cycling that relay on and off about once per second. If I hear or feel that relay clicking on and off, I know that half of the system is working. Obviously, if I see the fan pulsing on and off, I know the entire circuit is okay. This way you can take voltage readings in the circuit without having to wait until the engine gets hot enough to require the fan to run. The engine does not even have to be running. On my van, the engine computer will even detect when the fan relay is missing from its socket or if there is a break in the wiring, so it tests that half of the circuit all the time. I do not have to even think about whether there is a problem there if are no fault code related to that.

If you look on eBay, there are very reasonably-priced "scanners" that will provide the information you need, but the low cost comes with a few drawbacks. I used one of these that cost less than $40.00. It did show the sensor data, but the screen only shows two or three lines of data at once. The professional scanners show up to twelve lines at once. The bigger issue is the rate at which the numbers update. That little cheapy updated the screen once about every three seconds. That is fine when you just want to know a value or see what a reading goes to when you do something, like unplug that sensor. When you are looking for the cause of an engine stumble or hesitation, for example, you might watch the throttle position sensor's readings while you slowly push the accelerator pedal to the floor. The signal might drop out for a tiny fraction of a second, then come right back, just as that stumble occurs. You would miss that with the inexpensive unit. Professional scanners update their readings multiple times per second so you can see those little glitches and dropouts.

The last important function of scanners is a "record" feature that lets you record a few seconds worth of sensor readings. A lot of common sensors on most engines are fed with 5.0 volts, then the acceptable range of signal voltage is from 0.5 to 4.5 volts. The voltage has to go outside that range for a fault to be detected and a fault code to set. But it is when the voltage remains within that acceptable range, but it is wrong, that elusive problems occur. You can have that stumble or hesitation because the sensor reading is 3.2 volts but it should have been 3.5 volts. Regardless, 3.2 volts is an acceptable value, so no defect would be detected, there would be no diagnostic fault code, and there would not be anything to tell you where to start looking. This is where the record feature is used on a test-drive. When the stumble occurs, you push the "record" button on the scanner. A few seconds of sensor data is recorded so it can be played back slowly, later, to see what changed. Because that data passes through the scanner's memory, the recording actually starts a couple of seconds before the button was pressed. It usually takes an experienced engine performance specialist to notice little things that happened during the "event". Many scanners can put this data on graphs too where it is much easier to see that something changed when the problem occurred. The last step then is to determine if a reading changed and the problem occurred because the computer responded to that incorrect data, or that reading changed as a result of the problem occurring. To say that a different way, most sensors feed information to the computer so that computer can decide what to do, but a few other sensors tell the computer what the result was of what it decided to do. Oxygen sensors in the exhaust system are perfect examples of that. The computer looks at other sensor readings to calculate spark timing and fuel metering, then it looks at the oxygen sensor readings to see what the result of those calculations were.