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Discussion Starter #1 (Edited)
Read a bunch of posts and can't seem to find the problem
2002 wagon 230k miles.
Relays good, fans good, resister new, cht new.
Temp gauge works as it should. Tried three temp sensors

If I disconnect the head sensor fans will go on.
If I plug in a new sensor , do not install it and heat it with a heat gun, fans go on.
Put the sensor in the head and fans will not go on, temp just keeps climbing.

What am I missing?????

It appears when you unplug the connector to the head sensor both fans turn on what appears to be the high setting.

What triggers the low speed fans to come on? Does the ECU interpret the signal from the sensor and trip the relay? I am starting to think it may be something outside of the items we are currently tinkering with.
 

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Discussion Starter #3 (Edited)
I only have the factory gauge, it will go all the way into the red. Then I usually shut it down. When I test with a heat gun the fans will go on well before the red.
 

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Thats What She Said
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10,666 Posts
Got a bad connection in the connector? Dosent the head sensor go in coolant, you just plugging in another one leaving the old connector in the head?

Flush the coolant system? all the hoses look good? Thermostat working?
 

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His 02 wagon would have to be Zetec ( Maybe SPI?) and Temp sensor doesnt go into water on the Zetec , I dont know how he would heat it with a heat gun tho

Tom
 

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Discussion Starter #6
What triggers the low speed fans to come on? Does the ECU interpret the signal from the sensor and trip the relay? I am starting to think it may be something outside of the items we are currently tinkering with.
 

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Thats What She Said
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Correct, the ECU receives an analog voltage signal from a temperature sensor which then is received into a Analog to Digital Converter (ADC) and is then referenced to a "table" by the processor to know what value it is in degrees F. (Really the ECU uses some value that is likely coded as a 2^x value (not sure how many bit the ADC is) so the trip point is not a degree and is only translated into degrees for debug) The ECU then has threshold to engage the solenoid in the relay to close the low speed fan circuit. when temps get hotter or the AC in in use it send the signal to engage the relay which bypasses the resistor on the fans. The resistor just acts as a voltage divider and runs the fans at a lower power while dumping the other power out as heat.
 
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