What does it 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 occ