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Home >> Jeep >> 2000 >> Cherokee Limited, 4D Utility, RWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self Diagnostics >> Diagnostic Tests >> DTC P1391: Intermittent Loss Of CMP Or CKP >> Testing
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DTC P1391: Intermittent Loss Of CMP Or CKP: Testing

NOTE: CMP or CKP sensor signal loss can be detected by an RPM change, DTC or by observing oscilloscope pattern. Refer to manufacturer's operation manual for instructions in use of oscilloscope and procedure for pattern analysis.
  1. Turn ignition off. To diagnose CMP or CKP sensor signal loss using oscilloscope, go to next step. To diagnose CMP or CKP sensor signal loss without using oscilloscope, go to step  16 .
  2. Turn ignition off. Locate CMP sensor. On 2.5L, CMP sensor is located inside of distributor. On 4.0L, CMP sensor is located on right side of cylinder block. On all models, using oscilloscope's voltage lead, backprobe Tan/Yellow wire at CMP sensor connector. Start engine. If CMP sensor signal pattern is consistent, go to step  10 . If CMP sensor signal pattern is not consistent, go to next step.
  3. Turn engine off. Disconnect CMP sensor 3-pin connector. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connector is okay, go to next step (2.5L) or go to step  7 (4.0L).
  4. Remove distributor cap. Inspect distributor rotor and rotor drive for looseness. Repair distributor as necessary. If distributor rotor and rotor drive are okay, go to next step.
  5. Ensure ignition is off. Disconnect PCM connectors. PCM is located in engine compartment on left front fender, near radiator support. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connectors are okay, go to next step.
  6. At this time, CMP sensor is assumed to be defective. Replace CMP sensor.
  7. Ensure ignition is off. Disconnect PCM connectors. PCM is located in engine compartment on left front fender, near radiator support. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connectors are okay, go to next step.
  8. At this time, CMP sensor is assumed to be defective. Replace CMP sensor.
  9. Turn ignition off. Using oscilloscope's voltage lead, backprobe terminal No. 18 (Tan/Yellow wire) at PCM C1 connector. See Figure . PCM is located in engine compartment on left front fender, near radiator support. Start engine. While monitoring oscilloscope pattern, wiggle CMP sensor signal circuit from CMP sensor to PCM. If CMP sensor signal pattern is consistent, go to next step. If CMP sensor signal pattern is not consistent, repair wiring harness where wiggling caused oscilloscope pattern to become inconsistent.
  10. Turn ignition off. Using oscilloscope's voltage lead, backprobe Gray/Black wire at CKP sensor connector. Start engine. If CKP sensor signal pattern is consistent, go to step  15 . If CKP sensor signal pattern is not consistent, go to next step.
  11. Turn ignition off. Disconnect CKP sensor connector. CKP sensor connector is located on top of intake manifold, near firewall. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connector is okay, go to next step.
  12. Remove CKP sensor. CKP sensor is mounted to left side of transmission bellhousing. Inspect flywheel for damage or misalignment. Repair flywheel as necessary. If flywheel is okay, go to next step.
  13. Ensure ignition is off. Disconnect PCM connectors. PCM is located in engine compartment on left front fender, near radiator support. Visually inspect connectors for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connector is okay, go to next step.
  14. At this time, CKP sensor is assumed to be defective. Replace CKP sensor.
  15. Turn ignition off. Using oscilloscope's voltage lead, backprobe terminal No. 8 (Gray/Black wire) at PCM C1 connector. See Figure . PCM is located in engine compartment on left front fender, near radiator support. Start engine. While monitoring oscilloscope pattern, wiggle wiring harness from CKP sensor to PCM. If CKP sensor signal pattern does not remain consistent while wiggling wiring harness, repair wiring harness where wiggling caused CKP sensor signal pattern to become inconsistent. If CKP sensor signal pattern remains consistent while wiggling wiring harness, no problem is indicated at this time. Test is complete.
  16. Start engine and allow it to idle. Using scan tool, monitor engine RPM while wiggling CMP and CKP sensor wiring harness. If engine RPM changes while wiggling wiring harness, repair wiring harness where wiggling caused engine RPM change. If RPM does not change, go to next step.
  17. Turn engine off. Disconnect CKP sensor connector. CKP sensor connector is located on top of intake manifold, near firewall. Visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connector is okay, go to next step.
  18. Turn engine off. Disconnect CMP sensor connector. On 2.5L, CMP sensor is located inside of distributor. On 4.0L, CMP sensor is located on right side of cylinder block. On all models, visually inspect connector for corroded, damaged, pushed-out or miswired terminals. Repair connector as necessary. If connector is okay, go to next step.
  19. Ensure ignition is off. Disconnect PCM connectors. PCM is located in engine compartment on left front fender, near radiator support. Visually inspect connectors for corroded, damaged, pushed-out or miswired terminals. Repair connectors as necessary. If connector is okay, no problem is indicated at this time. Test is complete.
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