Things to check, Thermostat, radiator cooling fan, radiator fins for road grime and dead insects, blow it out with shop air, weak water pump. Leaks! Pressure test it for leaks.
DESCRIPTION All FWD, and some RWD vehicles use an electric cooling fan. This fan is used for engine and A/C condenser cooling and operates when A/C is on and if engine overheats. OPERATION All electric cooling fans operate when engine coolant temperature exceeds a certain value. The ECM completes ground path for the cooling fan relay. Relay contacts then close and complete circuit between fusible link and fan motor. When the engine cools, the switch opens and fan stops. If coolant sensor fails, the ECM will command constant fan operation. A/C equipped vehicles have a separate signal line to the ECM for fan control. When A/C control switch is " ON", and A/C system low pressure switch is closed, the ECM receives a signal on this line and turns on the cooling fan. The compressor clutch does not have to engage for ECM to turn fan on. On some models, when engine is off, ECM may turn on the fan relay and run fan for up to 7 minutes. This occurs if hot conditions were present while engine was running. Hot conditions are based on Manifold Absolute Temperature (MAT) signal to ECM, coolant temperature and time from start. TROUBLE SHOOTING CHART C-12, ENGINE COOLING FAN 5.7L - VIN 8 The cooling fan is totally controlled by the ECM based on inputs from the coolant sensor and fan control switch. The fan should run if coolant temperature is more than 226 F (108 C). Battery voltage is supplied to the fan relay on terminal " E" and ignition voltage to terminal " D". Grounding circuit No. 335 (relay terminal " F" ) will energize the relay and supply battery voltage to the fan motor. Once the fan relay is energized by the ECM, it will remain on for a minimum of 30 seconds.
NOTE: This article contains test charts that are part of General Motors Computerized Engine Controls. Only those charts required to test electric cooling fans are included. Other diagnostic codes may appear while performing electric cooling fan diagnosis. For complete information on General Motors Computerized Engine Control systems see COMPUTERIZED ENGINE CONTROL section. Seconds. The ECM will remove the ground to circuit No. 335, if vehicle speed is over 40 MPH (unless engine is overheating). The fan control switch (mounted in the A/C high pressure line) will open when head pressure exceeds 233 psi (16 kg/cm 2 ). This input causes the ECM to ground circuit No. 335. If a Code 14 or 15 sets, or the ECM is operating in the fuel back-up mode, the ECM will turn on the cooling fan. Diagnostic Aids If an overheating condition is suspected, verify if this is due to actual boil over. If gauge or light indicates an overheat condition, and no boilover is in evidence, inspect the gauge/light circuit for malfunction. If vehicle is overheating and the gauge or light indicates the same, but the cooling fan is not coming on, check the coolant sensor temperature using a " SCAN" tester. Sensor may have shifted calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system. Fig. 1: Engine Cooling Fan 5.7L - VIN 8 (1 of 2) Fig. 2: Engine Cooling Fan 5.7L - VIN 8 (2 of 2) WIRING DIAGRAMS Fig. 3: " Y" Body Electric Cooling Fan