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.
SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 327 (CHARGING SYSTEM)DIAGNOSTIC TESTSDTC P0620: FAULT DETECTION CIRCUIT MALFUNCTIONDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
- If diagnostic system check was performed, go to next step. If diagnostic system check was not performed, go to ENGINE ELECTRICAL DIAGNOSTIC SYSTEM CHECK under ON-VEHICLE TESTING.
- Connect scan tool to Data Link Connector (DLC). DLC is located under instrument panel, near steering column. Start engine. Using scan tool, observe ignition 1 signal parameter in PCM data list. If scan tool indicates voltage is less than 13 volts, go to next step. If scan tool indicates voltage is greater than 13 volts, go to step 6 .
- Using scan tool, command GEN-L terminal ON and OFF. If voltage level does not change with each command, go to next step. If voltage level changes with each command, go to step 8 .
- Turn ignition switch to LOCK position. Disconnect generator connector. Start engine. Measure voltage between generator turn-on signal circuit and ground. Using scan tool, command GEN-L Terminal ON and OFF. If voltage is not as specified, go to next step. If voltage is 0 volt with generator OFF and 5.0 volts with generator ON, go to step 7 .
- Test generator turn-on signal circuit for a short or an open. If short or open was not found, go to step 8 . Repair circuit as necessary, then go to step 10 .
- Test generator turn-on signal circuit for a short to battery positive voltage. If short was not found, go to step 8 . Repair circuit as necessary, then go to step 10 .
- Inspect generator harness connector for poor connections. Repair as necessary, then go to step 10 . If connections are okay, see TEST A: CHARGING SYSTEM TEST under SYSTEM TESTS.
- Inspect PCM harness connector for poor connections. If connections are okay, go to next step. Repair as necessary, then go to step 10 .
- Replace PCM. After replacement, reprogram PCM. See REMOVAL & INSTALLATION article in ENGINE PERFORMANCE. Go to next step.
- Review and record scan tool Fail Records data. Using scan tool, clear DTCs. Operate vehicle within Fail Records conditions. Using scan tool, observe Specific DTC Information for DTC P0620. If scan tool indicates DTC P0620 failed this ignition, go to step 2 . If scan tool indicates DTC P0620 passed this ignition, system is okay.
RENDER: 1.0x
NO RELATED
Recommended Tools & Savings
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
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.