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3G (Ford 3rd-Generation Alternator) Voltage Regulator F1DZ10C359A Motorcraft GR821 (similar to 2G)

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All are wired similarly to this:


A terminal: receives battery power for Field and senses battery voltage
S terminal: detects Stator output (~1/2 B voltage) for ALT indicator control
I terminal: detects key-on to energize voltage regulator, and provides ground to ALT indicator in dash
A brush screw: connected directly to A terminal (thus to Battery positive) and Field winding
F brush screw: ground side of Rotor's Field winding, receives varying ground signal from regulator

S (out): sends ~1/2 B voltage to regulator S terminal to indicate normal operation & suppress the indicator
B terminal: sends current to the vehicle & battery
Mounting points: provide ground reference to voltage regulator, and ground path for Field input & Stator output.

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An alternator receives mechanical energy from the crankshaft through the belt (FEAD) and converts it to electrical energy, which is used to supply the vehicle's demands and charge the battery. To begin, the alternator must be supplied with electricity to generate the electromagnetic field within the Rotor windings. This is the main distinction between an alternator and a dynamo (which has permanent magnets & needs no electricity to begin working). An alternator might consume ~13A to produce 130A (net) output.

The case of the alternator must provide a solid ground to the voltage regulator & the stator, so its mounting points must be clean, and the engine block must be well-grounded to the battery (-) post.

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The voltage regulator detects the key on through the I (Indicator) terminal, and uses battery power from the A terminal (via the brushes, & the slip rings) to produce a magnetic field within the Rotor's Field windings.

The strength of this field is determined by the voltage regulator, based on the voltage between the A terminal and the alternator's case, which must be grounded to the battery (-) post to provide an accurate reading. The regulator controls output voltage by varying the ground signal applied to the F brush. Externally grounding this brush forces the alternator to its maximum output at that RPM (typically near 18V).

The pulley transfers mechanical energy from the belt to the Rotor's shaft (supported by 2 bearings), which causes the magnetic field to rotate through the Stator's 3 stationary windings in the case. This moving magnetic field induces an alternating current within the Stator windings, proportional to the strength of the field.

The Stator's alternating current passes through the Rectifier (a matrix of 6 or 12 high-current diodes) which lowers the voltage slightly and converts the AC to rough DC. The output is smoothed primarily by the battery (which is why a battery MUST be connected when the engine is running), and also by a capacitor (condenser) within the rectifier.

One of the Stator's windings is connected to the S terminal so the voltage regulator can monitor the alternator output.

The fans (integrated onto each end of the rotor) centrifugally force hot air out of each end of the case, causing cooler air to flow into the sides, cooling the stator & rotor.

The rear case & stator can be rotated (clocked) to 3 positions relative to the front case to orient the connectors for ease of installation. EITHER: the pulley must be removed first so the rotor can remain in the rear case while the stator is pushed back out of the front case; OR: the regulator must be removed, the brushes pinned, and then reinstalled after the case has been clocked. Since the stator wires are attached to the rectifier, and the rectifier is attached to the output stud, the stator MUST move with the rear case at all times.

Most failure modes result in the voltage regulator grounding the I terminal, which illuminates the ALT bulb in the dash. To maintain normal operation in the event of bulb failure (open I circuit), a 1/4W or larger 300 Ohm resistor must be wired in parallel with the bulb (typically a common 194).

130amp (2 holes between each pair of front ribs) Donors:
'94-95 Mustang 5.0L
'94-00 Mustang 3.8L
'94-97 Thunderbird/Cougar 3.8L
'90-95 Taurus/Sable 3.8L
'93-99 Taurus/Sable 3.0L
'95-98 Windstar 3.8L/3.0L
'91-94 Lincoln Continental 3.8L
'92-97 F-series (optional)
'92-96 E-series (optional)

There are at least 3 mounting bolt configurations among those, so check your mounting bracket carefully before choosing a donor.

See also:
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