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Coolant Temperature Sensor

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Hi Guys,


Just need your technical advice. My ride is a Espero 1.5DOHC and I'm having a problem with my CT sensor. It's newly replaced but the reading is off the true temperature ( it shows higher temperature than actual). So to get rid of this offset, i thought of putting a potentiometer in series with the CTS then i adjust the total resistance based on what i think is the actual temperature.

Here's what i did:

1. i turn on the engine and wait until the thermostat opens fully (this should place the needle on the middle of the temp gauge right?)

2. while i'm doing this, i have a voltmeter connected to the ECU terminals of the CTS.

3. then i turn the potentiometer to adjust the voltage to the voltage set for normal operating temperature which is (from what i read on the manual) 1.5 to 2 volts.


another method that i do (which i'm not sure if correct) is

1. i set the the potentio to its lowest setting (0 resistance, which would also contribute 0 voltage drop)

2 then i wait again for temp gauge to be at middle (at this point, even when the temp gauge is on the first line or lower portion, the fans would turn on, and the compressor would not switch on, meaning the sensor is already registering an overheat)

3. when the temp gauge reaches middle, i then turn the potentio to a higher resistance (making the "measured" temp colder) until the A/C turns on and the idle sets at its lowest - 800rpm- which i think are the conditions for normal temperature operation)


the problem is when another day comes and i have to start the engine again, it would not have the same setting i made. the rpm would be higher when the temp gauge is at the middle.


so do you think there's something wrong with my procedure? can you give advice on an alternative solution?

for now, i refuse taking it to a shop with a scanner to see all data at once since they charge a lot per scan (O.T. is there a work around here, i mean can i have an improvised equipment and software to have the scanner functions )


need your badly as this has been my problem for months now. Many thanks!




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Well, according to the manual:


- the A/C fan should NOT start automatically (I'm talking about the smaller fan, located in front of the radiator)

- with the A/C off, the fan will have two speeds:

    * 1st speed (low), activated directly by the radiator thermocontact (a "termostat" as we call it), when the coolant temperature reaches 90 degrees Celsius
    * 2nd speed (high), activated by the ECM, based on info given by the CTS sensor, when the coolant reaches 105 degrees

- if you turn on the A/C, the cooling fan (not the A/C condenser fan) is automatically started on the 1st speed, through the A/C compresser relay

- while using the A/C, the cooling fan might be switched to high speed automatically by the ECM, based on: A/C system pressure, vehicle speed and coolant temperature (must be over 105 degrees)


Since you changed the CTS, my bet would be either on the thermostat (have you replaced it ?) or the worn-out indicator (from what I know, the dashboard needle is sustained by a little spring, which becomes weaker with time, therefore leading you to think that the indicator is showing incorrect indication).

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Hi Vaxxi,


Thank you very much for the prompt reply.


Regarding the thermoswitch on the radiator, i'm not sure if i have that one. what year model are you referring to? mine is 97-98. How can i know if i have that thermoswitch?


Moreover, the idle will only depend on the CTS right? how come everytime i restart the car, the idle would be higher than what i set previously



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I have a similar problem or have you found a solution?

Edited by VAXXi
no spam links

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