Chart C-12D Coolant Fan Functional Check (ON at All Times)

Chart C-12D - Coolant Fan Functional Check (On At All Times):

Wiring Diagram for Chart C-12D - Coolant Fan Functional Check (On At All Times):

CHART C-12D - COOLANT FAN FUNCTIONAL CHECK (ON AT ALL TIMES)

Circuit Description:

On VO8 (Heavy Duty Cooling) systems, the two coolant fans are energized through one low speed and two high speed relays. On standard cooling applications, one fan, one low speed relay and one high speed relay are used.

Power for the fan(s) comes through the fusible link to terminal "1" on all relays. The relays are energized when current flows to ground through the activation of the A/C, coolant switches, and/or the ECM.

LO Speed Relay - The LO Speed Relay is energized by the ECM or the A/C pressure switch. The ECM energizes the relay through Terminal "D2" when the coolant temperature reaches 98°C (208°F) and vehicle speed is below 45 mph. The LO speed relay is also energized through the A/C pressure switch through Terminal "B" when refrigerant pressure reaches 150 psi (1034 kPa).

High Speed Relay - The High Speed Relay is energized by the A/C high pressure and coolant override switches. If the A/C refrigerant pressure reaches 275 psi (1896 kPa) or the coolant temperature reaches 108°C (226°F) the high speed fan relay is energized. The ECM has no control of the High Speed Fan Relay.

Pusher Fan Relay - The Pusher Fan is installed as part of the heavy duty cooling package (VO8) and is turned on anytime the high speed fan is running. When the A/C pressure reaches 275 psi (1896 kPa) or the coolant temperature reaches 108°C (226°F) both fans are energized. The ECM has no control of the pusher fan relay.



Test Description: The following numbered steps correspond with the step numbers on the diagnostic chart.

1. Checks to see if CKT 535 is shorted to ground which would allow the low speed relay to be energized at all times.

2. If relay is faulty, test light won't be on. If test light is on, temperature, A/C high pressure or CKT 536 is shorted to ground.

3. Checks to see if ECM is faulty. If test light goes out then ECM is faulty.

4. If test light is Off the relay is faulty. If test light is On the A/C. high pressure fan switch, ECM or CKT 535 is shorted to ground.