Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1998SUBURBAN K2500, 7.4 J, PART TIMEREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 143 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.1L ENGINE)DIAGNOSTIC TESTSDTC P1639: 5-VOLT REFERENCE 2 CIRCUITDIAGNOSTIC PROCEDURES
1998 Chevrolet Suburban K2500, 7.4 J, Part Time
Diagnostic Procedures
1998 Chevrolet Suburban K2500, 7.4 J, Part TimeSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix, 2002 Chevrolet Malibu, and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
- Observe the Freeze Frame/Failure Records data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC or as close to the Freeze Frame/Failure Records data that you observed. See CONDITIONS FOR RUNNING DTC . Does the DTC fail this ignition? If yes, go to next step. If no, go to INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.
- Inspect the PCM and engine grounds. Ensure that the grounds are clean and secure. If a problem is found, repair as necessary. Did you find and correct the condition? If yes, go to step 12. If no, go to next step.
- Turn ignition off. Disconnect the A/C pressure sensor connector. Sensor is located on refrigerant line, above the condenser. Connect a Digital Multimeter (DMM) between the 5-volt reference and a good ground. Turn ignition on, with engine off. Does the voltage measure about 5 volts? If yes, go to step 10. If no, go to next step.
- Does the voltage measure more than 5 volts? If yes, go to next step. If no, go to step 7.
- Turn ignition off. Disconnect the PCM. Inspect for a short to voltage on the 5-volt reference circuit. Repair as necessary. Did you find and repair a problem? If yes, go to step 12. If no, go to step 11.
- Turn ignition off. Using a DMM inspect for continuity between the 5-volt reference circuit and low reference circuit at the A/C pressure sensor connector. Does the resistance measure infinity or OL? If yes, go to next step. If no, go to step 9.
- Leave ignition off. Using a DMM, inspect for continuity between the 5-volt reference circuit and a good chassis ground. Does the resistance measure infinity or OL? If yes, go to step 11. If no, go to next step.
- Repair the circuit for a short to chassis ground or a short to A/C pressure sensor low reference. After repairs, go to step 12.
- Replace A/C pressure sensor. After repairs, go to step 12.
- Replace PCM. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE under PROGRAMMING. After replacing PCM, go to next step.
- Use the scan tool in order to clear the DTCs. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step 2.
- Using scan tool, observe stored information and Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.