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DTC P1491: Rad Fan Control Relay Circuit (4.0L): Testing

  1. Turn ignition on, engine not running. With the DRBIII®, read DTCs. Is the DTC Good Trip counter displayed and equal to zero? If yes, go to next step. If no, go to INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM.
  2. Turn the ignition off. Remove the Radiator Fan Relay from the PDC. Using a 12-Volt test light connected to 12-Volts, probe the (A16) Fused B+ circuit in the PDC. Does the test light illuminate? If yes, go to next step. If no, repair the open or short to ground in the (A16) Fused B+ circuit. Inspect fuses and replace as necessary.
  3. Disconnect the PCM harness connectors. Measure the resistance of the (K173) Cooling Fan Relay Control circuit from PDC to PCM harness connector. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open in the (K173) Cooling Fan Relay Control circuit.
  4. Measure the resistance between ground and the (K173) Cooling Fan Relay Control circuit. Is the resistance above 100 k/ohms? If yes, go to the next step. If no, repair the short to ground in the (K173) Cooling Fan Relay Control circuit.
  5. Measure the resistance between the (Z307) ground circuit and a known good ground. Is the resistance below 5.0 ohms? If yes, go to the next step. If no, repair the open in the (Z307) ground circuit.
  6. Connect a jumper wire between the (C23) Radiator Fan Relay Output circuit and the (A16) Fused B+ circuit in the Relay harness connector. Did the Radiator Fan actuate? If yes, go to the next step. If no, replace the Radiator Fan Relay.
    7. NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.
  7. Using the wiring diagram as a guide, inspect the wire harness and connectors. Pay particular attention to all power and ground circuits. If there are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
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