Diagnostic Aids
Cooling fans should be off if coolant temperature is less than 208°F (98°C) and high side temperature is less than 122°F (50°C). On-board diagnostics may assist in diagnosing cooling fan problem. Monitor parameters ED04 and BD21 for coolant temperature and parameter BD27 for A/C high side temperature sensor. See appropriate TESTS W/CODES article in this section for instructions on using on-board diagnostics. Disconnecting A/C high side temperature sensor should result in BD27 reading of less than -30°F (-35°C). If BD27 reading is not -30°F (-35°C), see Code B111 chart in appropriate TESTS W/CODES article in this section.
Disconnecting coolant temperature sensor should cause BD21 to read less than -30°F (-35°C). If BD21 reading is not less than -30°F (-35°C), see CODE 14 or CODE 15 chart in appropriate TESTS W/CODES article in this section. BD27 should read about 122°F (50°C) when A/C high side pressure is about 160 psi (11.2 kg/cm2 ). Comparing these might isolate a bad sensor.
Normal cooling fan operation requires low speed if engine is running, vehicle speed is less than 45 MPH and coolant temperature exceeds 214°F (101°C) or A/C high side temperature is greater than 122°F (50°C).