C-12, Cooling Fan Circuit Diagnosis: Description

Power is supplied to cooling fan motors through fusible links. Relays are energized when PCM completes ground path to relay coil. PCM Output Driver Module (ODM) "B" output No. 1 is grounded for fan No. 1 operation. PCM ODM "B" output No. 2 is grounded for fan No. 2 operation.

During fan No. 1 operation, PCM supplies a ground path for fan No. 1 relay through ODM "B" output No. 1. This closes fan No. 1 relay contacts and current flows from battery through relay and to puller cooling fan. Puller cooling fan is grounded at engine block. PCM commands fan No. 1 on when engine coolant temperature increases to 226°F (108°C). Fan No. 1 will remain on until engine coolant temperature decreases to less than 271°F (133°C). PCM also commands fan No. 1 on when A/C refrigerant pressure increases above 190 psi (13.4 kg/cm² ). Fan No. 1 will remain on until A/C refrigerant pressure decreases below 117 psi (8.2 kg/cm² ).

During fan No. 2 operation, PCM keeps fan No. 1 energized and supplies a ground path for fan No. 2 relay through ODM "B" output No. 2. This closes fan No. 2 relay contacts and current flows to pusher cooling fan. Pusher cooling fan is grounded at chassis. PCM commands fan No. 2 on when engine coolant temperature increases to 232°F (111°C). Fan No. 2 will remain on until engine coolant temperature decreases to 226°F (108°C). PCM also commands fan No. 2 on when vehicle speed is less than 48 mph. and A/C refrigerant pressure increases above 240 psi (16.9 kg/cm² ). Fan No. 2 will remain on until A/C refrigerant pressure decreases to 190 psi (13.4 kg/cm² ).