P0620 – Alternator, control -circuit malfunction

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By Reinier (Contact Me)
Last Updated 2017-03-14
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0620 Alternator, control -circuit malfunction
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Wiring, alternator, battery, ECM

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Table of Contents

  1. What Does Code P0620 Mean?
  2. What are the common causes of code P0620 ?
  3. What are the symptoms of code P0620 ?
  4. How do you troubleshoot code P0620 ?
  5. Codes Related to P0620
  6. Get Help with P0620

What Does Code P0620 Mean?

SPECIAL NOTES: Since diagnosing OBD II code P0620 – “Alternator control circuit – malfunction” often involves, and requires, the testing of multiple systems and/or circuits in the CAN (Controller Area Network) bus system, non-professional mechanics should take note that this can generally NOT be done with cheap, generic code readers. Only professional grade diagnostic equipment has the ability to isolate, and interrogate individual “parts” of the CAN bus system, meaning that if such equipment is not available, the wiser option is to refer the application to the dealer, or other competent repair shop for professional assistance with diagnosing and repairing this code. 

However, while it may be possible to diagnose and repair this code on a DIY basis in some cases,  only generic information pertaining to diagnosing defective alternators can be provided here due to the complexities of CAN bus systems. Typically, the CAN bus system on an average application consists of a few dozen micro controllers and sensors that are all interconnected via thousands of circuits that are in their turn, all inter-connected with several thousand connections that include many dozens of connectors.

From the above, it should be obvious that diagnosing CAN bus related faults requires the use of advanced diagnostic equipment, as well as above average diagnostic skills in general, and expert-level knowledge of the affected application in particular. For these reasons, this guide can only provide a few generic diagnostic and repair steps that may or may not resolve the problem.

In cases where these steps do not resolve the problem, non-professional mechanics are strongly advised NOT to attempt additional diagnostic or repair steps without making proper reference to the manual for the application, or, referring the application to suitably qualified and experienced technicians for professional diagnosis and repair. END OF SPECIAL NOTES.

OBD II fault code P0620 is a generic code that is defined as “Alternator control circuit – malfunction”, and is set when the PCM (Powertrain Control Module), or any other affected control module detects a malfunction in one or more of the circuits that control or regulate either or both the operation, and/or control of the alternator’s output current. Note that the words “alternator” and “generator” are increasingly being used interchangeably, and in recent years, the two words have become virtually synonymous.

On most modern applications, both the operation, and output of the alternator is controlled by a dedicated control module that may or may not have diagnostics capabilities, and may or may not be incorporated into the PCM. We need not delve into the complexities of alternator control systems here, but suffice to say that the rapid development of automotive electronics has made it imperative that the alternators’ output be controlled very precisely to ensure that all controllers are able to perform their various functions efficiently.

As a practical matter, the alternator represents a significant parasitic drag factor on any engine, and part of the alternator control modules’ function is to regulate the alternator in such a manner that only the current that is required at any given moment is generated. This is accomplished by a circuit in the control module that estimates the applications’ total power requirements, which can vary considerably from one second to the next. The practical advantages of this is that reducing parasitic power losses ultimately saves some fuel, while the useful lifetimes of many and varied components are extended at the same time.

Based on this calculation, only the power that is drained from the battery during normal driving conditions at any given moment is replaced by the alternator. In practice, this means that every electrical consumer is supplied with only enough power to function; there is no, to, little, “spare capacity”, in a manner of speaking.

Thus, should a failure or malfunction occur in any circuit or component that is involved in controlling or regulating the operation of the alternator, one or more control modules such as the Transmission and ABS control modules (among others) may be deprived of sufficient power to function almost immediately; while controllers such as the Fuel and Ignition control modules (and others), may function until the battery is discharged or run down beyond a minimum threshold.

Note however that on some applications, the PCM will set code P0620 and illuminate a warning light immediately when the fault occurs, while on others, multiple fault cycles need to occur before this code will be set.

The image below shows the construction and components of a typical automotive alternator. Note that although all alternators follow this pattern, the performance of any particular alternator depends on the number of windings in the stator, as well as the speed at which the rotor is driven relative to the number of windings in the stator.


What are the common causes of code P0620 ?

Common causes of P0620 could include the following-

  • Defective alternator
  • Defective alternator control module
  • Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors. Note that in some cases, wiring may be damaged or shorted in parts of the CAN bus system that may not accessible unless the entire wiring harness is removed from the vehicle.
  • Voltage spikes caused by improper jump starting procedures can damage one or more control modules, including the PCM and alternator control module
  • Defects in batteries, such as internal short circuits, can damage one or more control modules, including the PCM and alternator control module

What are the symptoms of code P0620 ?

Typical symptoms of code P0620 could include the following, but note that many symptoms are make-and-model specific, meaning that the symptoms listed here do not represent a comprehensive or exhaustive listing of all possible symptoms on all applications-

  • Stored trouble code, and an illuminated warning light
  • Many additional codes may be present along with P0620
  • Depending on the nature of the problem, system voltages may be either abnormally high, or, abnormally low
  • Damage to the battery may be present. Note that both high and low system voltages can damage any automotive battery
  • One or more control modules (including the PCM) may have lost vital programming or memory
  • Depending on the nature of the problem, large sections of the wiring harness may have suffered damage caused by the over loading of circuits
  • Some instruments and/or gauges on the dashboard may appear to be “frozen”
  • Hard, or no start conditions can result from either high or low system voltages
  • Engine may idle roughly, or the idle speed may fluctuate
  • Automatic gearshifts may be harsh, erratic, or unpredictable. In some cases, the transmission may not shift at all
  • Some applications may experience varying degrees of power loss
  • In some cases, the PCM may force an automatic shut down of the engine to limit or prevent damage to both the engine and electrical system

How do you troubleshoot code P0620 ?

WARNING: Do NOT short out the battery terminals to see if there is “life” in the battery, even if the engine is not running. Doing this could cause the battery to explode, which can cause both serious personal injury, and extensive, if not always fatal damage to the vehicles’ electrical system.

Typical personal injuries include, but are not limited to, third degree acid burns, blindness, cuts, and abrasions. Typical damage to the vehicle could include the destruction of one or more control modules, or total destruction of the vehicle caused by the fire that started when the battery terminals were shorted out. NEVER test a battery in any way that is not prescribed by the manufacturer of the battery, or the manufacturer of the vehicle.

NOTE #1: As stated elsewhere, code P0620, or any other charging system related code for that matter, has the potential to trigger many diverse codes. In almost all cases, these additional codes are the result of P0620, as opposed to the cause of the code. The only time additional codes can be seen as the cause of this code is when P0620 was set and stored after any additional codes. All codes that follow P0620 are the result of the code, and resolving P0620 will almost always resolve the codes following it as well.

However, cheap, generic code readers may not display some additional codes, and particularly codes that pertain to faults in the CAN bus system. Therefore, if code P0620 (or symptoms) persists despite multiple repair attempts, refer the vehicle to a competent repair shop for a comprehensive diagnostic check and professional diagnosis.

NOTE #2: Diagnosing code P0620 almost always requires that the battery be disconnected at some point in the procedure. However, do NOT disconnect the battery without first installing a memory saving device to keep critical systems powered up during the time that battery power is not available. Refer to the manual for details on the specifications, and installation instructions of memory saving devices that are approved for that particular application.

Failure to install such a device or failure to install it correctly could cause one or more control modules to lose vital programming or memory, which requires that affected control modules be reprogrammed.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE: It is rare for code P0620 to be present without there being other codes present as well. Accompanying codes typically relate to abnormal system voltages (either high or low), and range/performance codes that could relate to a variety of sensors. Bear in mind though that all codes that follow P0620 are the result of this code. Codes that precede P0620 however, have likely caused, or have contributed to the setting of P0620, and as such, these codes must be investigated and resolved before attempting a diagnosis of code P0620.

Step 2

If no additional codes precede P0620, refer to the manual for details on the correct procedure to follow to test the battery to determine its state of charge, and general serviceability.

If this test proves that the battery is discharged or damaged, recharge or replace the battery as required (but strictly as per the instructions in the manual) to ensure that full system voltage is available during subsequent diagnostic testing. Refer to NOTE #2 above for details on how to prevent damage to control modules during this step.

NOTE: Do not connect the battery at this point, since the probable cause of the problem has not been identified. Should there still be a short circuit somewhere in the wiring for instance, connecting the battery now could damage the replacement battery,  as well as cause further damage to the vehicle’s electrical system.

Step 3

Assuming that the battery is now serviceable, refer to the manual to locate and identify the alternator and its associated wiring, as well as the routing, function, and color-coding of all associated wiring. Also, locate all fuses that are in any way related to the charging system. Check all relevant fuses, and replace all damaged or suspect fuses.

Perform a thorough visual inspection of all wiring leading away from the alternator. Look for obvious signs of damage caused by overheating (discoloration), or other evidence of poor electrical contacts. Also, look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required, or replace wiring to ensure that the load in this wiring can be carried efficiently.

Step 4

If no visible signs of short circuits or other damage in the wiring is found, consult the manual on the correct procedures to follow to test the continuity, resistance, and ground connectivity of all relevant wiring. Note that the wiring must be disconnected from the PCM and other controllers to prevent damage to these modules.

Compare all obtained readings with the values stated in the manual, and replace wiring to ensure that all electrical values fall within the ranges specified by the manufacturer.

Step 5

If all wiring checks out, consult the manual for details on whether the alternator itself can be tested with a multimeter or not.  This is critically important, especially on applications where the alternators’ output is controlled by a dedicated control module, since resistance or continuity testing on these units can destroy the control module.

WARNING: In these cases, the better option is to refer the vehicle for professional diagnosis and repair, since the problem might not be in the alternator, but inside a control module or worse, buried deep in the CAN bus system wiring.

Step 6

If the alternator can be tested with a multimeter, perform all prescribed tests, and compare obtained readings with the values stated in the manual. In a large percentage of instances of P0620 where “test-able” alternators are used, the problem can be resolved simply by replacing the alternator, but make sure that obtained test results do indeed indicate a defective component in the alternator.

If tests confirm a defect in the alternator, replace the alternator with a new or rebuilt unit, since this is almost always more cost effective than repairing an alternator. However, make absolutely sure that the replacement unit is identical to the original in all respects, including mounting brackets, features, and ratings.

Follow the directions provided in the manual to remove the old unit and reinstall the replacement. Make sure that all wiring terminations are tightened properly and that all wiring is routed and secured away from hot engine components.

NOTE: Double check that the alternator drive belt is fully engaged on all pulleys, recheck all connections, and ensure that the ignition is in the “OFF” position before reconnecting the battery terminals in the order specified in the manual.

Step 7

Do NOT start the engine.

Instead, clear all codes, and rescan the system to see which codes (if any) return. At this point, it is likely that some codes may still be present, but depending on the codes and the application, some codes will clear automatically within a few drive cycles.

Start the engine, and monitor both the warning lights on the dashboard, and the scanner for confirmation that the alternator is working, as it should do. The warning light should extinguish within a few seconds; once it does, check the scanner display to confirm that the alternator is developing full system voltage, which should be between 14.2, and about 14.8 volts on most applications.

NOTE: On some applications, full system voltage can be as high as 15.5 volts and even higher, but this largely depends on the type of battery in the application, as well as on the state of charge of that battery. Refer to the manual for details on the system voltage that applies to that application before drawing any conclusions as to the serviceability (or otherwise) of the charging system.

Step 8

Operate the vehicle for at least one complete drive cycle before scanning the system again to see if any codes have returned. At this point, it is unlikely that P0620 will be present, but clearing some other codes may require performing the relevant relearning procedures.

Thus, if there are still codes present that have not cleared automatically, or cannot be cleared manually, consult the manual for details on how to proceed to clear these codes.

There are no known generic codes that are directly related to P0620. However, since there may be make and model specific codes that are related, refer to the manual for the application for details on the possible relationship between P0620 and any manufacturer specific codes that may be present.

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