P0622 – Alternator, field control -circuit malfunction

By Reinier (Contact Me)
Last Updated 2017-06-23
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0622 Mean?
2. What are the common causes of code P0622 ?
3. What are the symptoms of code P0622 ?
4. How do you troubleshoot code P0622 ?
5. Codes Related to P0622
6. Get Help with P0622

What Does Code P0622 Mean?

SPECIAL NOTES: Modern automotive alternators (and their control systems) have reached a high state of development and sophistication and today, almost all alternators and their outputs are controlled by dedicated circuits in the PCM, and sometimes by both the PCM and a dedicated control module that estimates and calculates the electrical load/current required to keep all systems sufficiently powered up. Non-professionals are therefore strongly urged to familiarize themselves with the specifics of the charging system on their vehicles in general and the operation of the alternator in particular before attempting a diagnosis of code P0622 – “Alternator, field control -circuit malfunction”.

Be aware that failure to gain at least a working knowledge of the charging system on the affected application before attempting a diagnosis and repair of this code  will almost certainly result in a misdiagnosis, wasted time, the unnecessary replacement of parts and components, and the distinct possibility of causing damage to the electrical system where there was none before. END OF SPECIAL NOTES.

OBD II fault code P0622 is a generic code that is defined as “Alternator, field control -circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects an abnormal, or the absence of the PWM (Pulse Width Modulated) signal that turns the field winding in the generator on and off. Take note that code P0622 specifically refers to the PWM signal that controls the alternator’s field windings. All other charging system faults will be indicated by their own dedicated codes. Also note that in modern automotive parlance, the words “alternator” and “generator” are increasingly being used interchangeably.

On modern applications, the PCM uses a dedicated alternator field duty cycle signal to continually monitor the alternators’ duty cycle, which is the output current that is being generated at the current engine speed, and which is expressed as a percentage of the alternator’s maximum rated output. Thus, a duty cycle of 50% represents 50% of the alternator’s maximum rated output.

In terms of operation, the duty cycle signal circuit is connected to the high-side of the field winding in the alternator, and depending on the load placed on the engine by various electrical consumers such as the A/C system, power windows, and others, the PCM can turn the field winding “ON” or “OFF”, by using a pulse width modulated signal that originates in the high-side driver of the voltage regulator. Being able to do this has the advantage that the PCM has the ability to estimate the load (parasitic drag factor) the alternator is placing on the engine, which serves as the basis for calculating required changes to ignition timing and fuel delivery to keep the engine running smoothly, and especially at low engine speeds, regardless of the load on the engine.

In practice, the PCM monitors the state of the field duty cycle circuit when the key is in the “ON” position, but with the engine not running. In this condition, the duty cycle should be zero since the engine is not running, and the alternator is therefore not generating a current. However, as soon as the engine starts and the alternator is generating a current, the duty cycle should read from about 5%, to 100%, depending on the required current. In an engine running condition, the PCM also begins to monitor the PWM signal (that controls the alternator’s field winding), and should it detect an abnormal, absent, or otherwise out-of-range signal it will set code P0622, and illuminate a warning light.

The image below shows the typical connections that commonly appear on most alternators today. Be aware however that connections can and do differ between applications, and that proper reference to the manual for the affected application MUST be made in order to identify connections correctly.

What are the common causes of code P0622 ?

Some common causes of code P0622 could include the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors, including damaged battery terminals
• Defective alternator
• Defective PCM. Note that unlike most other codes, a failed PCM, corrupted software, and memory errors are a relatively common cause of this code.

What are the symptoms of code P0622 ?

Common symptoms of code P0622 could include the following-

• Stored trouble code, and an illuminated charge indicator warning light
• Vehicle may exhibit signs of a flat, or run-down battery
• If the vehicle is being driven when the fault occurs, continued driving could cause some functions to stop working as the battery drains. Further driving may cause the engine to shut off when the battery reaches a critically-low state of charge.

How do you troubleshoot code P0622 ?

SPECIAL NOTES:  Diagnosing code P0622 requires a wiring diagram that clearly shows ALL the connections on the alternator, as well as a pin-out chart of the PCM connector on which ALL the pins/terminals are clearly marked, and especially  the pins that connect with the various alternator control/monitoring functions.

The diagnostic procedure requires that some circuits between the alternator and the PCM be tested. So if the required diagram and pin-out chart are not available, or if you are not comfortable with the idea of testing charging system circuits, the wiser option would be to refer the application to the dealer or other competent repair facility for professional diagnosis and repair of the code. END OF SPECIAL NOTES.

WARNING: Bear in mind that most, if not all, PCM-controlled alternators can NOT be tested on a test bench in the conventional way, since these alternators do not have integrated voltage regulators. Thus, if the few generic steps outline below shows the alternator itself to be defective the best option is to simply replace the alternator with an OEM replacement to ensure proper operation. Repairing this type of alternator, or using rebuilt units is often not worth the possible saving since as has been stated elsewhere, these alternators cannot be tested, which means that proper operation of such an alternator can never be guaranteed.

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: If other codes are present along with P0622, refer to the manual to determine the possible relationship(s) between P0622 and the additional codes. This required because the code setting parameters of code P0622 vary between manufacturers, and in some cases, P0622 will set as the result of other, unrelated failures.

Step 2

If no other codes are present, locate the alternator, and refer to the manual to determine the exact function, routing, and color-coding of all wiring that is connected to the alternator. Use this opportunity to locate and inspect all related fuses, and/or fusible links. However, do NOT replace any blown fuses/fusible links until the possible short circuit that had caused them to blow had been found and repaired.

Step 3

Once all wiring is located and correctly identified, inspect the battery terminals for signs of looseness, corrosion, or other defects/conditions that could inhibit the proper flow of current through the terminals. Make repairs, or replace terminals as required if the terminals are in a less than perfect condition.

WARNING: If the battery is to be disconnected to make repairs, refer to the manual to determine if any special precautions are required, such as fitting a memory saving device. Failure to follow prescribed procedures exactly could cause the PCM and/or other control modules to lose vital programming or memory, which will require reprogramming, and sometimes replacement of affected control modules.

Step 4

If the battery terminals are fine, locate the wire on the alternator that supplies the battery with current. This will always be heavy-gauge wire: test the current (DCV) between the point where it attaches to the alternator, and a suitable ground. The voltage in this wire should always be equal to battery voltage. If it is not, check the battery voltage: if the battery voltage checks out as per the manufacturers’ specifications replace the cable with an OEM replacement.

Step 5

If this voltage checks out, refer to the manual to identify the alternator field duty cycle circuit. This wire will often be in a connector that plugs into the alternator, but this is not always the case.  However, be absolutely sure that you identify the correct circuit, since you need to test this circuit’s resistance and continuity between where it connects to the alternator, and the PCM connector.

Disconnect this wire (or connector) from the alternator; also disconnect the PCM connector, and making sure that you have the correct pin on the PCM connector, proceed to test the continuity and resistance of the wire as per the instructions in the manual. Compare the obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

Step 6

If the duty cycle circuit checks out, refer to the manual to locate and identify the PWM (Pulse Width Modulated) signal wire, and proceed to test its resistance and continuity as per the instructions in the manual. Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the manufacturers’ specified ranges.

Step 7

If all circuits pass the above tests suspect a defective alternator, which is a likely cause of the code. If the code persists after replacing the alternator, it is likely that the PCM is defective; however, since the replacement PCM will require programming, do not condemn the PCM out of hand until the possibility of an intermittent wiring fault had been investigated fully.

Be aware though that faults of this nature can be extremely challenging and time consuming (even for professional technicians) to find and repair. In some cases, it is more cost effective to simply replace all relevant wiring than to try and find an intermittent fault, so if an intermittent fault is suspected, the better option would be to refer the vehicle to the dealer or other competent repair facility for  professional diagnosis and repair.

Codes Related to P0622

• P0625 – Relates to “Generator Field/F Terminal Circuit Low”
• P0626 – Relates to “Generator Field/F Terminal Circuit High”

NOTE: While the above codes are not strictly related to P0622 – “Alternator, field control -circuit malfunction”, they nevertheless involve the alternator field control system, and often produce the same or similar symptoms as code P0622.

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