What does it 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.