(1996 Impala SS) Now if the A/C is OFF the primary fan cycle's on and off... If I turn on the A/C the Primary fan comes on and stays on. When I pull of the sensor plug both fans will run.

. Now I have a few questions... When does the secondary fan come on? I thought the secondary fan suppose to come on when you turn on the A/C but the primary fan comes on. (is this correct) What suppose to activate the secondary fan? coolant temp? pressure in the high side of the AC? What sends the signal to the PCM that turns it on? Since the car won't run over 175-185 degrees with our current weather.

So is everything working correct and it'sjust hasn't gotten hot enough to turn on the secondary fan or is there a problem still in the PCM?


Thank's
Bill

B/D-car LT1 (Caprice/Impala/Roadmaster/Fleetwood) Cooling Systems:

Standard equipment for all LT1 equipped B/D-cars is a dual electric fan setup with a 150-watt primary (RH) fan and a 100-watt secondary (LH) fan. The electric engine coolant fans are independently operated by the PCM (Powertrain Control Module) based on the inputs from the Engine Coolant Temperature (ECT) sensor, A/C Pressure Sensor, Vehicle Speed Sensor (VSS), and various other inputs.

The B/D-car coolant fans operate under PCM control at the following engine temperatures and A/C system pressures:

Fan Mode ... Temp ... Pressure

Primary (RH) Fan ON ... 109°C / 229°F ... 189 psi
Primary (RH) Fan OFF ... 105°C / 221°F ... 150 psi
Secondary (LH) Fan ON ... 112°C / 234°F ... 240 psi
Secondary (LH) Fan OFF ... 108°C / 227°F ... 210 psi

Additionally, the PCM will turn off the fans at higher vehicle speeds (above 48 MPH I believe) since running fans can actually impede airflow through the radiator at high speed. Each fan also has a minimum running time. Once activated, the primary fan will run for a minimum of 50 seconds, and the secondary fan for a minimum of 26 seconds. Finally, certain Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one or both fans.If either temps, or pressures, get past those points, a fan turns on.
When the secondary is on, the primary should also be on.
Obviously, if the AC is off, AC pressures should never reach those numbers, so Coolant Temp Sensor readings alone would activate the fans.
When the AC is on, the primary fan will almost always be on, while the secondary will also come on from time to time.

Most PCM reflashers lower the fan thresholds towards the 'fully open' temp of your thermostat. I know of none that change the AC pressure thresholds (of course, I could be wrong ...)

thanks..... that's the information I've been looking for....


everything is working perfect, just hasn't gotton hot enough to turn on the secondary fan. I may send the PCM for a reflash to turn on the fan earlier.

Sounds like your car is fine, I never see my secondary fan come on. If everything is in good working order than I doubt anyones sec. fan come's on.

There are posts at www.thirddgen.org that indicate the GM engineers intentionally run the engine hot as an emissions issue. The articles also indicate better performance is seen by lowering the fan on/off temps approximately 10 degrees from factory settings.