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Home >> Cadillac >> 2007 >> Escalade EXT >> Repair and Diagnosis >> External Pages >> Different car >> Section 201 (Charging System) >> Diagnostic Tests >> DTC P1638: Pulse Width Modulation Signal Out Of Range >> Diagnostic Procedure
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Diagnostic Procedure

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. If diagnostic system check was not performed, perform ENGINE ELECTRICAL DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM. If diagnostic system check was performed, go to next step.
  2. Connect scan tool to Data Link Connector (DLC). DLC is located below left side of instrument panel. Start engine. Observe GEN F-Terminal parameter in Powertrain Control Module (PCM) data list. If scan tool indicates GEN F-Terminal parameter is 5-95 percent, problem is intermittent. Check wiring and connections. If scan tool does not indicate GEN F-Terminal parameter is 5-95 percent, go to next step.
  3. Turn ignition switch to RUN position. Connect a test light to battery positive voltage. Repeatedly probe generator harness connector terminal "C" (Gray wire), while observing Generator Pulse Width Modulation (PWM) on scan tool. If Generator PWM display is affected, go to step  8 . If Generator PWM display is not affected, go to next step.
  4. Test Gray wire for a short or an open between generator harness connector terminal "C" and PCM harness connector C2 terminal No. 52. See WIRING DIAGRAMS  . If problem exists, repair as necessary. After repair, go to step  8 . If problem does not exist, go to step  6 .
  5. Inspect generator harness connector for poor connections. If problem exists, repair as necessary. After repair, go to step  8 . If problem does not exist, perform TEST A: CHARGING SYSTEM TEST  under SYSTEM TESTS.
  6. Inspect PCM harness connectors for poor connections. If problem exists, repair as necessary. After repair, go to step  8 . If problem does not exist, go to next step.
  7. Replace PCM. If equipped with a 6.6L (Diesel), replace ECM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article in ENGINE PERFORMANCE. Program PCM/ECM. After repair, go to next step.
  8. Review and record scan tool Fail Records data. Using scan tool, clear DTC's. Operate vehicle within Fail Records conditions as noted. Using scan tool, observe Specific DTC information for DTC P1638. If scan tool indicates DTC P1638 failed this ignition, repeat test beginning at step  2 . If an tool indicates DTC P1638 passed this ignition, system is okay.
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