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Dan, Pontiac Technician
Category: Pontiac
Satisfied Customers: 3490
Experience:  GM Certified, GM Mark of Excellence, ASE Master Certified.
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I have a 99 pontiac Grand prix 3.1L I have replaced the

Customer Question

I have a 99 pontiac Grand prix 3.1L
I have replaced the following
Head Gaskets,ECT sensor (Autozone),ECM (Autozone),Radiator ,thermostat
My Problem (No Fans at 200 degrees and above)
The temp gauge (Dash) gets to 235 degrees F without turning the hi or lo Fans on
The temp reading on my scanner is at 210 degrees F a difference of about 25 degrees F
No coolant loss
When I turn my A/C on the low fans kick in when I turn a/c off they cut off
I can turn the hi and low fans on and off with my scanner (eliminates the hi and low fans circuits and ecm control of the fans)
When the temp sensor plug is unplugged the high fans come on
The only code present is an EVAP code no other dtc's before changing the temp sensor I had ECT intermittent high voltage I found that the plug was corroded and I repaired it

What I need to know
what are the conditions needed for the PCM to turn the fans on
is it possible that the temp sensor is bad why the difference btw the gauge and the scanner reading
Submitted: 7 years ago.
Category: Pontiac
Expert:  Dan replied 7 years ago.

There will be a difference between the gage and scan tool data because it's 2 different sensors...ECT and temp sender for gage. One is in the cylinder head and the other is in the intake manifold. Here are the fan parameters.


Engine Cooling Fan Description - Electric

The electric cooling fans are used to lower the temperature of the engine coolant flowing through the radiator. They are also used to cool the refrigerant (R-134a) flowing through the A/C condenser.

Object Number: 181868  Size: SH
Click here for detailed picture of above image.


The electric cooling fan operates when the engine cooling temperature exceeds a certain value. The cooling fan on this engine is controlled by the Powertrain Control Module (PCM). The PCM turns the cooling fans ON by grounding the coil of the cooling fan relay(s) when certain conditions are met. When the A/C is requested, the cooling fan will also be turned ON.

Power for the cooling fan motors are supplied through Maxifuses®. The cooling fan relays are energized when current flows from the fuses in the Underhood Accessory Wiring Junction Block , and through the relay coils to ground through the PCM. The Low Speed fans control circuit is grounded for low speed fans operation. During low speed fans operation, both fans run at a slow speed. The High Speed fans control circuit is grounded for high speed opeartion. During high speed fans operation, both fans run at high speed.

Important: : When certain Diagnostic Trouble Codes (DTCs) are present, the PCM may command the cooling fans to run all the time. It is important to perform the Powertrain On Board Diagnostic (OBD) System Check prior to diagnosing the engine cooling fans.

If a problem that involves the low speed cooling fan relay control circuit exists, DTC P0480 Cooling Fan Relay 1 Control Circuit should set. If the problem affects the high speed cooling fan relay control circuit, DTC P0481Cooling Fan Relay 2 Control Circuit should set. A problem with the ECT sensor should set DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage DTC P1114 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage . Any of these DTCs will affect cooling fan operation and should be diagnosed before using the Electric Cooling Fan Diagnosis .



Customer: replied 7 years ago.
I have not seen any sensors on the cylinder head it is my understanding that the ect sensor on this car has two circuits the ect circuit and the temp sending circuit thus when the plug is unplugged the temp gauge drops and the scanner reads -38degrees farenheit.

I read a statement in a forum that the fans dont kick on until 238 degrees F I havent seen the temp rise that high on the scanner reading yet
I plugged the extra sensor to the ect sensor harness and exposed it to an open flame after a few minutes the high fans came on.
I also read another statement said that a ect sensor from autozone gave similar readings to mine but when replaced with another sensor by a different brand the scanner and gauge temps were almost the same
What temp should the fans come on at? 210 ,238,240

Expert:  Dan replied 7 years ago.
The fans should turn on between 225 and 230 degrees and turn off below 210 degrees.
Customer: replied 7 years ago.

Customer: replied 7 years ago.
Relist: Inaccurate answer.
I need to know what conditions need to be present in order to turn the fans on