If you have a thermostatic fan, which is a fan that’s not electric but driven by a drive belt on the front of the engine, you handle things a little differently. This is a thermocouple device that is supposed to lock up when it gets hot enough and run the fan at full power.
These types of fans came about to help improve fuel economy. By design, they are only supposed to work when they heat up, and when not needed they kind of free wheel, which allows the engine to work less, thus saving fuel. The symptoms of a failed thermostatic fan are the same as an electric fan failure: You’ll start to overheat while you’re sitting in traffic, but when you start moving, things will start to cool down again.
An easy test is to spin it. Obviously don’t do this while the engine is running. With the engine off, just grab one of the blades and give it a spin. You should feel some resistance and it should only spin about a half a turn or so. If it spins freely, or keeps spinning after you spin it, then replace it. Also, if you see fluid on the outside of it, this could indicate that the fluid inside the assembly has leaked out; if that’s the case, replace it.
Another old-school test is to hold a piece of paper in front of the radiator when the engine is running and up to temperature. The paper should be pulled toward the radiator if the fan is working properly. If not, it might not be moving enough air through the radiator to cool the system. This one is a bit more difficult to nail down because it’s a mechanical device, but if you use the above information, it should guide you through it.
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