What does it mean?
OBD II fault code P0625 is a generic code that is defined as “Generator field terminal -circuit low”, and is set when the PCM (Powertrain Control Module) detects an abnormally low voltage in the generator field coil and its associated control circuit(s). NOTE: Note that in recent years, the word “generator” has become synonymous with “alternator”.
Also known as the “stator”, the field coil in an alternator is the most easily identifiable component in almost any alternator, due to the copper wire windings that are almost always visible through the alternator casing. This part of the alternator remains stationary; the rotor rotates inside this part, and the interaction between the electromagnetic fields of the field coils and the rotor is what creates an alternating current when the rotor rotates inside the field coils.
In the context of code P0625, “field current” refers to the current within the field coils only, and NOT to any current or general power supply in the rest of the application’s electrical system. In practice, the current within the field coils has two sources; from the battery when the alternator is not rotating and from the diodes in the bridge rectifier when the alternator is rotating fast enough to create a current.
In terms of operation, the filed coils are supplied with current from the battery through the ignition switch and the charging system warning light when the ignition is switched on. When the engine starts and the alternator starts to spin, the current it generates is fed to the voltage regulator through the rectifier (which converts the AC current to DC (direct current), and from there it serves as the power supply for the field coils.
At this point, the alternator is said to be self-sustaining, in the sense that the field coils no longer need to draw current from the battery. At this point, the charging system warning light is extinguished as an indicator that the alternator is generating sufficient current to meet the power demands of the application’s electrical consumers.
In a fully functional alternator, the voltage regulator maintains the current being generated in a range of between 13 and 14 volts, but note that on some applications, this can be as high as 15 – 16 volts, depending mainly on the type of battery used on the application.
Nonetheless, since the current in the field coils derives from the voltage regulator when the alternator is spinning, a malfunctioning voltage regulator, and by extension, defective diodes in the rectifier, can cause the voltage in the field coils to drop to below minimum allowable levels for that particular application. Thus, when the PCM (or other control module, such as a dedicated alternator control module detects an abnormally low voltage in the field coils, it will set code P0625.
Note though that whether or not a warning light is illuminated on the first failure depends on both the application, and the nature/severity of the problem.