My 2002 dodge ram 1500 is overheating. Changed thermostat and burped the cooling system. The truck can sit idle and not overheat but when you drive it, it doesnt take long to overheat. No radiator or water pump leaks. It sucking the coolant in just like it should. Changed out the temperature sensor and when I did this I notice the ac/ cooling fan started working but only when I have the heater or ac on and the truck stays cool. I dont think the fan was working before changing the temp sensor. Once I turn the heater or ac off the fan stops and it overheats.

  • Welcome to Motor Vehicle Maintenance & Repair! Which engine do you have? Does your truck have one or two fans? I'm not seeing an option for anything other than a single mechanical fan for the 2002. May 6, 2021 at 16:40
  • It's a 4.7 engine. It has 2 fans but one is for the ac. When I turn the heater or ac on the fan starts up and keeps the engine cool but once I turn it off the truck overheats.
    – user63505
    May 6, 2021 at 16:53
  • Since only one fan is turning on, this leads me to believe there is one of three issues going on with your truck. The fan itself might be dead (the one to cool the engine regularly). The relay to the main fan might be out. The third is a little harder to explain. Depending on how the fan is triggered (through the ECM or via direct temperature sender), there might be an issue there. I don't know which it might be, because I'm not sure how your truck is setup. May 6, 2021 at 16:58

1 Answer 1


The engine computer looks at coolant temperature and based on what it sees sends a signal to the fan controller to spin the fan. That controller is a Pulse Width Modulation (PWM) controller, and controls the speed of the engine cooling fan.

Additionally, there is an override for Air Conditioning (A/C). Whenever the A/C is engaged the cooling fan(s) runs at high speed all the time, everytime. Not sure of the logic of two fans on your vehicle, but I doubt the one fan = engine, one fan = A/C thought. 2002 Chrysler, its possible, but I doubt it. High demand should be both fans running full speed. You should be able to tell by looking at the cables running into and out of the fan controller. Modern controllers run BOTH fans.

I'm assuming that you have used the onboard diagnostics (OBD2) system and verified that the temperature sensor is working correctly. If you haven't done that, test this first. Free at most autoparts stores. Or you can purchase a bluetooth module that mates to your mobile phone typically for less than $20. Its possible that you replace the temperature sensor, but the computer is still not getting that signal because of a cut in the wire.

One question. When the engine is off (and cold!) can you spin the blades of the fan by hand easily? Be safe, do NOT DO THAT TEST ON HOT Vehicle. Hint, use a screwdriver to push the fan blade, not your finger.

If that is whole, my guess is either dead fan or dead fan controller. Historically the fan controllers do go out. Replacement fan controllers run around $120 or so. And they are a pretty easy part to replace.

Funny I just worked on a vehicle with troublesome overheating... Spent a lot of time on it. Turned out the radiator needed a flush, bad. I really hate when folks use tap water in the radiator. That's just bad. Distilled water/approved coolant only.

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