Hi, everyone! New poster here. Apologies if this post is in the wrong place, or this question was asked and answered in another thread yesterday and I missed it.
tldr version:
09 Vue with the 3.6 V6 motor. Radiator fans come on at lower temperatures but not at high temperatures, unless the AC is turned on.
Long version:
Daughter's car. It warned it was overheating as she pulled into work. At the end of the day she checked and found it was low on coolant, which she topped up. On the way home the Check Engine light came on. I looked at it the next day; the air temperature was around 66°F. The coolant level was midway between min and max. The code was P0128, which indicates the engine is not reaching normal operating temperature like it should. I cleared the code and cranked the car and let it idle while I monitored the engine temperature with the scan tool. I cross-checked the temperature readings with a "point and shoot" infra-red thermometer aimed at the metal pipe that goes to the top hose, and got readings that were consistent with the numbers shown on the scan tool (about 3°-4° lower). The temperature climbed slowly, but at about 162°F the fans came on, or more specifically, the left (driver's side) fan came on; the other does not seem to work at all (multi-meter probes stuck in both sides of the power side of the connector showed 14.2v, and it didn't run with jumper wires from the battery to the fan side of the connector). As the temperature climbed the fan would periodically shut off for a minute or two, then come back on again. After about 45 minutes the temperature reached ~194°F and stayed there for a while. After another 15 mins or so the temperature started climbing again, the fan was off and did not come on again. When it reached 230°F, with the fan still off, I turned the car off and let it sit for a while. When I went back to it the scan tool showed 148°F, and when I cranked it the fan came on at once. It cycled a few times as the temperature climbed, but at some point stopped coming on and the temperature climbed to 228°F before I shut it off again. I let it cool for a few minutes and tried again. The temperature reading was 210°F, the fan did not come on, and the temperature climbed. At around 225°F I tried turning on the AC. The fan came on, and the temperature started to drop. I turned off the AC, the fan stayed on, and the temperature continued to drop to about 208°F. At this point the fan turned off and stayed off as the temperature climbed again. I repeated this a couple of times, with the same result.
There are no trouble codes showing, active or pending.
It's my understanding (I could be wrong) that there is a single temperature sensor that supplies temperature information to the ECM, which then controls the fans accordingly. The ECM is apparently getting good information from the sensor, because the scan tool is showing appropriate values that are close to the readings from the IR thermometer. Nevertheless, the fan comes on when it's not needed, but not when it's required. Any ideas, anyone?

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Update: I had time to look at it some more. I cranked it and let it warm up from cold. This time the fan did not come on of its own accord at all. When the temperature reached 230°F I cycled the AC, and the fan came on, but this time it turned off at 215° instead of 208° like it did last time. I let it climb to 230° twice more, cycling the AC to turn the fan on, with the same results.
I found three fan relays, "fan main", "fan cntrl", and "fan aux", and tested them (correct operation, resistance between 85 and 86, and voltage drop between 30 and 87/87a). All three worked fine. I also tested two other relays and swapped them for "fan main" and "fan cntrl", but this didn't change the behavior of the fan. There isn't another relay to match "fan aux" in the box, so I couldn't swap it out.

 

Where Is the ECTS located physically?
Where are you measuring with your IR gun?
Is the resistance change and the voltage drop across the ECTS consistent in the sense that it either slowly rises or slowly drops except at the high temperature end where it will drop quickly and the low temperature end where it will rise quickly.

 

Thank you for replying, Derf.
The ECTS is immediately to the left of the dipstick as you look at the motor from the front of the car.
The top radiator hose attaches to a metal pipe that is shaped like a twisted-up T, with the hose going to one arm. I was measuring on the "upright" of the T.
I measured the resistance of the ECTS before cranking the car, then monitored it as the car warmed up. The initial reading was 3270 Ω at an indicated temperature of 55°F (car) and 54°F (IR thermometer).The resistance dropped rapidly at first, then more slowly. With the ECTS unplugged, the fan came on at once and stayed on. Eventually the resistance stabilized at ~400 Ω, and the temperature appeared to be ~195°F. Unfortunately, my temperature readings proved to be unreliable, so I can't give accurate numbers for resistance versus temperature. With the ECTS unplugged, I couldn't get temperature readings from it. Instead, I used the IR thermometer, but aimed it at the sensor where it screws into the block, instead of at the pipe to the top radiator hose. My readings turned out to be too high. It has a laser to tell you where it's pointing, but I didn't consider that the measurement area is rather bigger than just a dot, so it was probably catching some of the exhaust manifold. Whatever the reason, when I plugged the ECTS back in, the temperature shown by the car was ~165°F. Checking on the pipe to the top hose as before, the equivalent pipe to the bottom hose, and a couple of spots on the block, all gave readings more consistent with 165° than 195°.
With the ECTS plugged in, the fan didn't come on. The temperature climbed to an indicated 230°F, and I shut the car off. I checked resistance (which involved putting small alligator clips on the pins in the sensor so I'd have something I could get to with the multi-meter probes), and got a reading of 165 Ω. I removed the alligator clips (which took much less time than putting them on) and plugged the sensor back in. The indicated temperature was 220°F. At this point I ran out of time, so I don't have information on how fast the resistance changes with temperature at the hot end of the scale.

 

How's your coolant level?
If it is too low, the cooling system cannot build up enough pressure to keep the mixture from getting hot enough to boil, expand in volume, and overheat due to insufficient cooling.

The average thermostat opens between 185 and 195. The fact that you're reading approx 230 near the radiator but lower through the temp sensor is consistent with the coolant not properly circulating in the cooling system because there is a significant volume missing. Therefore, the temperature measured by the coolant temperature sensor may not be the actual temperature of the coolant as there may not be sufficient coolant to cover the sensor to obtain an accurate reading.

The only other explanation left is that it is a PCM problem because the relays all seem to function as designed, both fans work, and the failsafe mode operation is correct because both come on when you disconnect the sensor. There is no evidence that the sensor itself is bad as it gives reasonable readings throughout the temperature range.

You may wish to inspect the pigtail connector for corrosion in the connector slots themselves. Same thing on the sensor pins it connects to. Corrosion there would add resistance to the overall circuit which would make the PCM think the engine was colder than it is. And that extra resistance would be a larger percentage of the total resistance of the circuit as temperature rises.

Alternatives include wiring the fans directly to a constant 12 volt source that is key on engine on only. Doing so will have the cooling fans running all the time which I suppose is better than not at all when they should.

Depending on where you live, I suppose you could drive with the AC on all the time as AC engagement runs the fans via the AC relay.

Or you could wire up a manual switch.

The temperature sensor response you saw was as expected. My statement about the high temperature resistance behavior was incorrect for which I apologize. I was looking for crazy values or discontinuities in the values.

 

First of all water pumps only suggest circulation in a cooling system on a good day. Deadhead the system anywhere and no coolant moves. These are not positive displacement pumps. My 55 T-bird overheated during the summer due to cavitation due to a design flaw on the cooling system. Plug a heater core and you get no heater. Plug a radiator and you get no cooling even “IF” the thermostat opens. I have seen one car that the fins on the coolant pump corroded away. I have seen a couple International Diesel engines in Fords have the plastic impeller break in half.