Fix P0620: Mechanic’s solutions that WORKED

The P0620: Generator Control Circuit Malfunction points to a malfunction in the generator control circuit, a key component of the vehicle’s charging system. This circuit is responsible for ensuring the alternator properly charges the battery and supplies power to the vehicle’s electrical systems. A fault in this circuit can lead to charging issues, potentially affecting the vehicle’s overall electrical functionality and battery health.

P0620 quick overview

Meaning	P0620: Generator Control Circuit Malfunction
Is it serious?	This code can be serious as it relates to the vehicle’s charging system, which is crucial for battery charging and overall electrical functionality.
Possible causes	– Faulty alternator or internal regulator – Damaged or corroded wiring in the charging system – Faulty or incompatible aftermarket alternator – Problems with the Powertrain Control Module (PCM) – Blown fuse affecting the charging system
How to diagnose?	Inspect Alternator and Wiring: Check the alternator and all related wiring for damage, corrosion, or poor connections. Check Electrical Connections: Ensure all connections to the alternator and battery are secure and free of corrosion. Verify Alternator Output: Use a multimeter to measure the voltage output of the alternator. Test Battery Health: Evaluate the condition of the battery, as a bad battery can affect charging system performance. Check Fuses and Circuitry: Look for blown fuses or circuit issues that could impact the charging system. Consider PCM Diagnosis: If other steps are inconclusive, consider diagnosing the PCM for any related issues.

How to fix P0620 – learn from mechanics

I looked into P0620 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chevrolet

Vehicle Information:
2003 Chevrolet Impala 3.4L

Issue + Repair History:

• Intermittent charging light flashing while driving.
• Code P0620 present; no problem found following Mitchell flow charts.
• Charging voltage at 14.2 volts; currently no light on.
• Alternator replaced twice, working well each time after leaving the shop.

Other OBD2 Codes + Test Results:

• P0620: Alternator circuit

Suggestions to Fix Issue:

• Check the battery for potential issues, including a cluster problem.
• Verify if the alternator is remanufactured or AC Delco, as reman alternators can have issues.
• Monitor the “turn on” circuit for 5 volts from the PCM to the generator.
• Inspect the alternator connector terminals for tightness and proper contact.
• Check wiring, particularly for the battery wire off the alternator rubbing to ground.

Final Fix:

• A bad wire was found at the starter terminal. After repairing this wire, the vehicle was road-tested and operated correctly. The issue with the P0620 code seems resolved.

Case #2: Ford

Vehicle Information:
2005 Ford Explorer XLS 4.0L

Issue + Repair History:

• Replaced alternator a month ago due to inoperative charging system.
• Two days prior to the current issue, the car was dropped off for no charging and check charging system display.
• Battery was nearly dead; charged battery and replaced alternator again, which lasted only one day.
• Both replaced alternators were aftermarket of different brands.
• Checked wiring from harnesses and found good connections.
• Inspected Instrument Panel Cluster (IPC), found all connections good with no corrosion or poor wires.
• Initially had code P0620.

Other OBD2 Codes + Test Results:

• P0620: Alternator circuit

Suggestions to Fix Issue:

• Some Ford alternators are controlled by the PCM, and aftermarket alternators may not be compatible.
• Check fuse F2.21 (5 amp) in the central junction box since the alternator is controlled by the PCM.
• Three fusible links come off the alternator; check them for any issues.
• Aftermarket parts may only repair what’s wrong, not fully rebuild; OEM parts are typically completely rebuilt.
• Consider having the alternator bench tested.
• The P0620 code indicates a PCM-related failure; Ford has a pinpoint test for this.

Final Fix:

• Based on code P0620, it was determined a new PCM was needed. However, after installation, the problem and the P0620 code persisted.
• Installing a Ford OEM remanufactured alternator finally cured the problem. The issue with non-OEM remanufacturers was noted as the cause of the problem.

Case #3: Buick

Vehicle Information:
2000 Buick LeSabre Custom 3.8L

Issue + Repair History:

• Replaced alternator as it wasn’t charging.
• After replacement, battery light stayed on and code P0620 appeared.

Other OBD2 Codes + Test Results:

• P0620: Alternator circuit

Suggestions to Fix Issue:

• Have the alternator bench-tested to confirm its functionality.
• Follow the troubleshooting tree for code P0620.
• Check terminal L on the back of the alternator for a 5-volt signal.
• Confirm that the correct alternator model was installed.
• Inspect fuse F2.21 (5 amp) in the central junction box.
• Ensure there are close to 5 volts on one small wire to the alternator.
• Check for voltage drop between the underhood fuse and the alternator terminal.

Final Fix:

• The problem was a 10 amp dim fuse. Initially checked visually, but a test light should have been used. Repairing the fuse resolved the issue.

Case #4: Pontiac

Vehicle Information:
2004 Pontiac Bonneville SE 3.8L

Issue + Repair History:

• Car came in with a bad alternator, which was replaced.
• After replacement, the battery gauge reads normal for about 30 seconds, then drops to 0.
• Battery tests at 14v with a meter.
• Sets P0620 code but tests okay.
• Replaced the instrument cluster and alternator again, but the problem persisted.

Other OBD2 Codes + Test Results:

• P0620: Alternator circuit
• Battery Voltage: Consistently around 14v

Suggestions to Fix Issue:

• Check if the scan tool can read Body Computer Codes for additional diagnostics.
• Measure 5.0 volts on the red wire at the generator while the issue is occurring.
• Test the generator field duty cycle signal circuit for a short or open.
• Check all systems for codes, particularly the PCM, DIM, or wiring.
• Ensure the correct alternator was used.

Final Fix:

• The issue was a blown 10 amp dim fuse, which was not indicated in any trouble trees or the wiring chart for the charging gauge. Replacing this fuse resolved the problem.

Case #5: Ford

Vehicle Information:
2003 Ford Expedition Eddie Bauer 5.4L

Issue + Repair History:

• Customer came in with the battery light on.
• Initial charging system output was 14.1 volts.
• Replaced alternator with a new remanufactured one, but then it stopped charging (only battery voltage).
• Tried a second alternator with the same issue.
• Reinstalled the old alternator; charging resumed but the battery light remained on.
• Vehicle had a new battery installed in the last few months.

Other OBD2 Codes + Test Results:

• Alternator charging, but battery light on
• Fuses checked and found okay

Suggestions to Fix Issue:

• Review technical theory on Ford charging systems.
• Try a different brand of alternator.
• Check wiring from alternator to battery for chafing, especially near the AC line.
• Use an OEM alternator, ensuring the part numbers match.
• Consider that rebuilders may not use the same quality internal components.
• Check if the PCM controls the charging and monitors duty cycle (wrong amp alternator or pulley diameter could affect this).
• Investigate other issues with wiring or potentially faulty battery.

Final Fix:

• After finding no problem with wiring and trying a new Ford alternator, referred the customer to the dealer for a PCM reprogram.

Advice	Diagnostic Step Description
Check Alternator and Wiring	Inspect the alternator, wiring, and connections for damage, corrosion, or improper installation.
Use OEM or Quality Replacement Parts	Prefer OEM (Original Equipment Manufacturer) or high-quality replacement parts, especially for alternators.
Monitor Charging System Voltage	Use a voltmeter to check the charging system voltage at the battery and alternator to ensure proper operation.
Inspect and Replace Fuses as Needed	Check relevant fuses for damage or failure; a blown fuse can affect charging system operation.
Consider PCM or Electrical System Issues	Investigate if the Powertrain Control Module (PCM) or other electrical system components may be contributing to the issue.
Perform Continuity and Voltage Drop Tests	Conduct tests on the charging system circuits to identify any open circuits, high resistance, or voltage drops.
Verify Correct Alternator Model and Fitment	Ensure the alternator model is correct for the vehicle, as wrong specifications can lead to charging issues.
Examine Battery Condition	Evaluate the battery’s health, as a faulty battery can lead to charging system problems.