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Home >> GMC >> 1997 >> Forward Control 4.3 W, L35, Automatic >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 10 (Engine Controls - Tests W/Codes - 6.5L Diesel (P Series)) >> Diagnostic Trouble Codes >> DTC P0251 - Injection Pump Cam System >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different variant/trim than selected.
  1. Perform Powertrain ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. See the BASIC TESTING article. Then go to next step.
  2. Install scan tool. Start and operate engine at idle. Using scan tool, with throttle closed, observe CAM REF MISSED display. If scan tool displays at least "8", go to step 4). If scan tool does not display at least "8", go to next step.
  3. DTC is intermittent. If no additional DTCs are set, see DIAGNOSTIC AIDS. If any additional DTCs are set, go to applicable DTC test.
  4. Turn ignition off. Disconnect optical/fuel temperature sensor harness connector. Turn ignition on, with engine off. Using DVOM, check voltage between 5-volt reference circuit of optical sensor connector and ground. If voltage reading is 5 volts, go to next step. If voltage reading is not 5 volts, go to step 7).
  5. Using a test light connected to battery voltage, probe ground circuit of optical sensor connector. If test light illuminates, go to next step. If test light does not illuminate, go to step 8).
  6. Reconnect optical/fuel temperature sensor connector. Start and idle engine. Using scan tool, command engine speed to 900 RPM. Using DVOM on Hz scale, backprobe cam signal circuit at PCM. If DVOM reading is 57-63 Hz, go to step 12). If DVOM reading is not 57-63 Hz, go to step 11).
  7. Remove electrical harness filter from vehicle. Using DVOM, check resistance of filter harness. If resistance is greater than 2 ohms, go to step 15). If resistance is less than 2 ohms, go to next step.
  8. Turn ignition off. Leave electrical filter removed from vehicle. Disconnect PCM harness connector. Check 5-volt reference circuit for open, short to ground, or short to sensor ground circuit. Repair as necessary. After repairs, go to step 16). If circuit is okay, go to step 10).
  9. Check for open or poor connections at sensor ground terminals of PCM. Repair as necessary. After repairs, go to step 16). If connections are okay, go to step 14).
  10. Check for poor connections at 5-volt reference terminals of PCM. Repair terminals as necessary. After repairs, go to step 16). If terminals are okay, go to step 14).
  11. Turn ignition off. Check for open or short to ground in cam signal circuit. Repair as necessary. After repairs, go to step 16). If circuit is okay, go to step 13).
  12. Check for poor connections at PCM. Repair terminals as necessary. After repairs, go to step 16). If terminals are okay, go to step 14).
  13. Replace injection pump. If fuel injection pump is replaced, new pump must be timed. See the ADJUSTMENTS article. After replacing injection pump, go to step 16).
  14. Replace PCM. After replacing PCM, go to step 16).
  15. Replace electrical filter. After replacing filter, go to next step.
  16. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0251. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
  17. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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