Why my nissan sentra b14's fan won't stop working

Hi, I’m having an issue with my 1999 b14 sentra’s cooling fan lately. When I turn on the ignition the fans would start working inmediately and same thing happens when I turn it off, they keep working until I completely shut it off. Now, what worries me most is that the revolutions have raised up to 1400 rpm with the fans running all the time. A mechanic advised me to buy a new fan relay and replace it, which I did today, but to my surprise, the problem persists and frankly I lost my faith to that mechanic.
I’ve done a few tests, for example: removing the terminal from the coolant thermo switch, but still they running and it turns the check engine light on. With the terminal off the thermo switch I placed a new one to check if the fans would shut down, nothing happened.
I read in another forum that in the case of a 1994 Honda, it would be a fan module failure, however, I don’t know if that would be the case for 1999 nissan sentra.
I would highly appreciate an accurate advice on this issue. Thanks in advance.

Can’t speak to your 99 Nissan specifically, but on my 90’s Corolla if the coolant thermo temp switch becomes disconnected, or fails in the “off” state, the radiator fan will run 100% of the time. Are you able to post a schematic of that circuit? On my Corolla the temp switch connects to the fan relay. When the coolant is cold the temp switch is closed, and when the coolant reaches above 200 degrees it closes, which turns on the fan relay, which in turn powers up the radiator fan.

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Thanks George, I can hardly draw a circuit since I’m not an electrician. However, I have replaced the radiator’s fan relay and the coolant sensor and the problem persists, the fans won’t shut down even when the coolant’s temperature is cool. What I can tell from this is that somewhere in the relay - coolant sensor circuit there’s a failure to deliver the correct signal.

Check with a Missan dealership to find out if this car has a fan controller or fan control module.

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From what I see, on the 99 Sentra 2L SR20DE engine, the ECM (engine computer) controls the fan operation via an output signal to the fan relay. The ECM decides whether to turn the fan on or off by juggling 4 inputs: vehicle speed sensor; engine coolant temperature sensor; AC switch; ignition switch “start” signal. So there must be something wrong with one or more of those inputs, or the ECM output that controls the fan relay. The automatic transmission version of this car has two cooling fan speeds, low and high. The manual transmission, only high is supported. The start signal involvement is probably to prevent the fan from turning on during cranking, when all the battery energy is needed for the starter motor. The input which tends to put the fan “on” irrespective of the others is the AC. Suggest to focus on that. Perhaps the ECM thinks the AC is turned on, when it isn’t. At over 50 mph (and coolant temp less than 203 deg F) the fan should turn off even if the AC is turned on. If you can figure a way, see if the fan kicks off at over 50 mph. Might provide a clue what is happening. Beyond that you’ll need an auto-electric shop to probe the circuit’s test points to see at what point they are failing.

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Hello George, thanks for your follow up response. Based on your input, I made a test on the fuse box and took off the AC fuse off wich solved the idle acceleration issue. With a tester took off the coolant sensor harness to check for power and guess what, there was no power which leads us to the relay problem of not being able to cut down the fan activity. What I need to do now is find out where the power shortage in the coolant sensor is to fix it. Thanks once again for your useful help in trying to solve this problem, indeed it’s been a great relief.

Maybe double check that removing the fan relay turns off the fan.