Connecting a 2011 Ford Fusion Alternator to a 2006 Mazda 3 Wiring Harness

The goal is to connect a 2011 Ford Fusion alternator to the existing wiring harness on a 2006 Mazda 3 using an Airtex 1P1243 Alternator Connector.

2006 Mazda 3 2.3L Alternator

The Mazda 3 uses a wye-connected stator generating three phases of AC from a rotating field coil. The field coil is energized by a transistor on the alternator that sources power (transistor collector) from the output of the alternator, and sinks current through the emitter to ground. The output of the alternator is directly connected to the battery through a 150 amp fuse, while the chassis of the alternator serves as the ground. DC output is provided by on-alternator rectifier diodes. Control over the alternator's output is provided by regulation of the field coil transistor. The base of the field coil transistor is connected to pin 2AQ of the powertrain control module (PCM). The PCM pulse width modulates (PWM) pin 2AQ (vs. pin 2AA ground) using 250 Hz (4 ms) pulses to pin L on the alternator connector. There is a capacitor between power and ground to reduce AC ripple in the output.

The GY connection at the alternator connects to pin 2AM on the PCM and is a phase pickup from one leg of the three stator windings in the generator. This 250 Hz (4 ms) PWM signal is used to determine the electrical load on the alternator. If the PCM senses greater than 20 amps from the alternator and senses the bus voltage to be below 8.5V while the engine is runing, DTC 2503 will be set.

2006 Mazda 3 2.3L Alternator Schematic

     2006 Mazda 3 Alternator Schematic      2001 Ford Fusion Alternator Schematic

2006 Mazda 3 2.3L PCM Output on pin 2AQ

     2006 Mazda 3 Alternator PCM PWM Output
The L pin on the alternator should still permit the alternator to be tested by removing the alternator wiring with the exception of the output to the battery and then connecting a 12v + lead through a test light (filament, not LED) to pin L. The voltage at the battery should rise to the maximum value.

2006 Mazda 3 2.3L Alternator Output on pin 2AM

     2006 Mazda 3 Alternator 2AM Output
The chart above was taken at 650 rpm idle conditions.

2011 Ford Fusion 2.5L Alternator Schematic

The Fusion alternator is similar to the Mazda 3 alternator in most respects. The following points highlight the differences:
  • Instead of powering the field coil from the output of the alternator, the Fusion runs a wire to the battery junction box and then the battery. This is done in cases where the wire to the battery would be too long, or there is a point in the electrical system that is identified as having the highest load. In the case of wiring the field coil pin A (pin 3) on the Mazda 3, the connection can probably be made either way. The battery is quite close to the alternator, so I ran a separate fused wire to the battery to power the field coil.

  • The Fusion alternator uses an on-alternator regulator instead of PCM control for regulation. The I (pin 2) connection looks to be a simple 12v + connection, while the S (pin 1) connection looks to be an output from the regulator. Although the Fusion alternator is internally regulated, the Fusion PCM commands a voltage level setting. The voltage level setting varies based on the temperature of the battery from the battery sensor and depending on the charging strategy employed by the PCM. The PCM will vary the voltage for the following reasons:

    • To improve the life of the battery by decreasing the charging voltage when the battery is warmer.
    • To improve the performance of the vehicle by reducing parasitic drag from the alternator under wide-open-throttle (WOT) conditions.
    • To reduce the load on the starter when starting the engine.
    • To reduce idle sag when electrical loads are increased at idle.

    The interesting thing about the on-alternator regulator is that in the absence of a commanded voltage level setting from the PCM, the alternator will start charging at 13.5v if the engine RPM reaches 2,000 rpm (4,500 rpm at alternator) for more than 3 seconds. So essentially, the alternator is able to self-excite the field coil under this condition. This makes the Fusion alternator behave nearly as a one-wire alternator if the A lead is connected to alternator output and the other two wires are kept disconnected. I discovered this after reading the pinpoints test for "alternator not charging."
     2001 Ford Fusion Alternator Schematic      2001 Ford Fusion Alternator Schematic

Fusion Alternator Connector Pinouts

  • Pin 1 - GENLI/GENMON
    This terminal is labeled S (Sense) on the alternator and is a 8.3v 125Hz (8 ms) PWM output from the alternator. See here.. According to the Fusion manual Pinpoint Test Step E7, the frequency needs to be in the range of 120-130 Hz.
  • Pin 2 - GENRC/GENCOM
    This terminal is labeled I on the alternator and is a 12v 125Hz (8 ms) PWM input to the alternator. See here.
  • Pin 3 -
    This terminal is labeled A on the alternator and is a constant 10 Amp Fused 12v+ line from the battery.

Current Wiring Connections

     Fusion S = Pin 1 = GENLI/GENMON to GY = 2AM of Mazda 3 PCM
     Fusion I = Pin 2 = GENRC/GENCOM to L = 2AQ of Mazda 3 PCM
     Fusion A = Pin 3 to Constant 10 Amp Fused 12v+ Mazda 3 Battery Terminal

Current Problems

  1. Charging System Warning Lamp is Lit
    The light is supposed to light if the charging system voltage drops to 12.5v or less. When I use Torque Pro to read the battery voltage, I get a value of 12.5v, even though the battery voltage at the terminals is 13.5v.

  2. Pending DTC 2503 - Charging System Voltage Too Low
    This pending code does not set the MIL indicator. The pending code is set because the charging amperage is greater than 20 amps, but the charging voltage is below 8.5v.

  3. Car is running lean, but isn't throwing a lean code
    The fuel injector reaction time varies depending on the battery voltage, which affects how much fuel is sprayed during the alotted time (injector pulsewidth). The PCM calibration contains a table or function for modifying the base air/fuel calculation based on battery voltage. Looking at the Mazda 3 alternator wiring diagram, you'll notice that there is a resistor on the voltage monitoring line going into the PCM. That resistor is housed inside the alternator.
Since the charging voltage is stuck at the default 13.5v, I must conclude that the PCM output from the Mazda 3 is incompatible with the Fusion regulator's expected command protocol. The Mazda PCM appears to expect a square wave of at least 8 volts. Looking at the Mazda 3 alternator wiring diagram, you'll notice that there is a resistor on the voltage monitoring line going into the PCM. My guess is that the resistor drops the output voltage from the charging voltage down to a safe input voltage for the PCM's microprocessor.

   Last update: Dec 7th, 2019