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Home >> Ford >> 1983 >> Bronco 4.9 Y, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> 4.9L MCU Testing >> Diagnosis & Testing >> Test 2 - Code 25/Knock Detection
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Test 2 - Code 25/Knock Detection

  1. Verify that knock simulation during "Functional Test" took place within 2" of knock sensor. Disconnect grounded jumper wire from Self-Test connector. If knock simulation was more than 2" from sensor, repeat "Engine Running Test". Otherwise, proceed with test.
  2. Turn ignition off and disconnect knock sensor harness. Verify that knock sensor is tightened to 12-18 ft. lbs. (16-24 N.m).
  3. Disconnect wiring harness from MCU and knock sensor. connect an ohmmeter between ground and MCU harness connector pin 13. If resistance is less than 1000 ohms, correct short to gourd. If resistance is 1000 ohms or more, proceed with test.
  4. Check continuity of wire from knock sensor to MCU harness connector pin 13. If resistance is more than 5 ohms, repair wire. If resistance is 5 ohms or less, proceed with test.
  5. Check continuity of wire from knock sensor to MCU harness connector pin 14. If resistance is more than 5 ohms, repair wire. If resistance is 5 ohms or less, reconnect MCU and proceed with test.
  6. Reconnect the ground jumper wire to the Self-Test connector. Connect a test lamp to positive terminal of battery. disconnect knock sensor.
  7. Repeat "Engine Running Test" noting the following: When the initialization pulses occur, tap wire that leads in pin 14 of MCU with test lamp for 5 seconds. If diagnostic code 25 is displayed, replace MCU. If any other diagnostic code is displayed, replace knock sensor.
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