Vehicle Is Overheating

The Problem noticed on 1999 Pontiac Sunbird.The vehicle overheats and Cooling fan not turning ON.
But when the Obd code scanner is connected to DTC port the fan comes ON and when OBD code scanner is disconnected the Fan Turns off and will not turn ON.

WHY FAN TURNS ON WHEN ERROR CODE SCANNER IS CONNECTED?


The cooling  fan operating system is pretty simple. You scanning it, and the fan coming on tells you the ECM is turning on the relay when requested. So we know that the Relay, and ECM command CKT to the relay {Dk Green/White wire} is engaging the relay.That is why Fan is accepting the voltage from relay and Turning ON.

SEE THE FAN WIRING DIAGRAM AND OTHER PARTS WIRING THAT MAKE THE FAN OPERATE


SO WHAT DO YOU THINK THE PROBLEM IS?

Coolant temperature sensor and its signal to ECM needs to be inspected.
It seems that coolant temperature signal voltage is high.

You have to verify the coolant Temp Sensor is sending proper info to the ECM. If the DATA stream on the scanner reads 226 to 230 degrees, and the fan does not come on, then you know the sensor is operational, and the ECM is not processing the info correctly.
This is common issue noticed many a times at our garage.The Fan gets inoperative due to faulty ECM.

SO WHAT IS FAULTY COOLANT TEMPERATURE SENSOR OR ECM?

To confirm that testing is required.I have already provided you the wiring diagram.That will help you the most to confirm what is faulty as per voltage test.


In the the diagram I attached above, you see the yellow and black wires to the Temp Sender. You need to verify that you have a good ground on the Black wire, and 5 Volts on the yellow. If you do, jumper the pins. If the fan comes on, replace the sensor. If not, chances are the ECM is bad.

WHAT IS MOST POSSIBLE OUT OF ECM OR SENSOR?

As per you description i assume its faulty ECM.

The fact that the Fan came on with the code checker installed, this puts the system in "Field Service Mode" and will have the ECM turn on relays, so we can assume that the ECM has an internal issue if the Sensor and it's CKT's are good.

This will help.

Thanks.

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