I changed my radiator to double cell, after the single cell got leaking. I now noticed that the temperature gauge never goes up to normal gauge whenever I turn on the AC, and the Fan never idles.

When the AC is off, the temperature gauge would normalize and the thermostat and fan would work normal;( you will hear the fan speed increase to cool it down after which it turns off again)

But when the AC is on, the fan speed is at its optimum thereby cooling the engine way too much and temperature down.

Could it be the double cell radiator or any other suggestion?

  • Welcome to Motor Vehicle Maintenance & Repair! Mar 3, 2018 at 13:25
  • What kind of car? Check the temp sensor? Replace it just cause along with thermostat? Or it could be ac problem
    – Dan Z
    Jan 18, 2020 at 23:02

2 Answers 2


Not knowing what kind of vehicle you have, I'd suggest your thermostat is at fault. If you put in a lower temp thermostat than stock or the thermostat is stuck open (even slightly), your radiator is over cooling the system. When the A/C is on (compressor actively working), the fans will stay on to cool the condenser so the A/C will work properly. If the thermostat is stuck open, there will be a continuous flow of coolant through the radiator, which will cool the engine more than it should. Since there is a larger reservoir of cooled coolant in the radiator, this is a lot easier for it to happen than it did before the new radiator.

  • Thanks, Its a toyota camry 2007, ,,but when the AC is not turned on , the fan doesn t turn on until the temp guage stays at its normal point. and how am i going to test the thermostat while in the car
    – Kodi
    Mar 3, 2018 at 22:50

You definitely have a faulty thermostat that opens much earlier than it should. With the AC off, you should notice the same at highway speeds. The temperature, with a good thermostat, a good radiator, a good fan and a good coolant temperature sensor, should, after the engine had enough time to warm up, stay steady at the thermostat's opening temperature or slightly below it, but still steady, no matter what the vehicle's speed.

To test if the thermostat is faulty: look at the cooling system plumbing of your car and touch a pipe that looks like it directly links the thermostat housing with the radiator. Look for your engine's data and identify at which temperature the thermostat is designed to open flow to the radiator. Provided that you have a temperature gauge in your instruments cluster, if that pipe becomes warm/hot when the temperature is lower than the thermostat opening threshold, your thermostat is opening at a too low coolant temperature and it needs to be changed.

If the condenser uses the same radiator fan to cool down, the system is wired so that, whenever the AC system is on, the radiator fan switches on whenever an intermediate pressure is detected by a pressure switch or sensor on the refrigerant liquid line, so that the compressor's discharge pressure can be lowered. If coolant is flowing through the radiator at the same time, that coolant gets cooled too, even if the car is stationary.

Along with the thermostat, i recommend you to also replace the coolant temperature sensor. They're cheap and easy to change, and tend to become inaccurate when old. In addition, in modern cars, the ECU uses the coolant temperature value to adjust the fuel air mixture and other important engine running parameters, so it's an important part.

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