P1644

What Does Code P1644 Mean?

DTC P1644 signifies a detected malfunction within the fuel pump speed control circuit. In modern fuel systems, the Powertrain Control Module (PCM), also referred to as the Engine Control Module (ECM), does not directly power the fuel pump at a constant voltage. Instead, it communicates with a dedicated Fuel Pump Driver Module (FPDM) or Fuel Pump Control Module (FPCM). The PCM sends a Pulse Width Modulated (PWM) signal or a digital command to the FPDM/FPCM, instructing it on the desired fuel pressure or flow rate based on engine load, RPM, and other operating conditions. The FPDM/FPCM then modulates the voltage and/or current supplied to the fuel pump to achieve this commanded pressure, thereby controlling the fuel pump’s speed.

Code P1644 is set when the PCM monitors the feedback from the FPDM/FPCM and/or the fuel pressure sensor and detects an inconsistency. This could mean the FPDM/FPCM is failing to regulate the fuel pump speed as commanded, there is an electrical fault within the circuit (e.g., an open, short to voltage, or short to ground in the control circuit, power supply, or ground circuits), or the FPDM/FPCM is not reporting its status or output correctly. Essentially, the PCM has lost effective control over the fuel pump’s operation, impacting fuel delivery and engine performance.

Common Symptoms

• Hard Start or No Start Condition: Insufficient or no fuel pressure reaching the engine.
• Engine Hesitation or Stumbling: Especially noticeable under acceleration due to inadequate fuel delivery.
• Reduced Engine Performance/Loss of Power: Engine cannot receive enough fuel to generate full power.
• Engine Stalling: Engine may unpredictably shut off, particularly at idle or low speeds.
• Check Engine Light (CEL) Illumination: The primary indicator that a fault has been detected.
• Abnormal Fuel Pump Noise: A distinct whining or buzzing from the fuel tank area, possibly indicating the pump is running continuously at full speed or struggling.

What Causes the Code P1644?

• Faulty Fuel Pump Driver Module (FPDM)/Fuel Pump Control Module (FPCM): This is a very common cause, as the module itself can fail internally due to electronic component degradation or exposure to environmental elements (e.g., corrosion from road salt).
• Wiring Harness Issues: Opens, shorts to voltage, shorts to ground, or high resistance within the wiring circuits connecting the PCM to the FPDM/FPCM, or the FPDM/FPCM to the fuel pump. This includes power and ground wires.
• Corroded Electrical Connectors: Especially at the FPDM/FPCM or the fuel pump module, which are often exposed to moisture, road salt, or debris, leading to poor electrical contact.
• Faulty Fuel Pump: While the code points to the control circuit, an internal fault within the fuel pump (e.g., high current draw, internal short, mechanical binding) can overload or interfere with the FPDM/FPCM’s ability to control it, indirectly triggering P1644.
• Insufficient Voltage Supply to FPDM/FPCM: A weak power supply or high resistance in the battery feed circuit to the FPDM/FPCM can prevent it from functioning correctly.
• ECM/PCM Internal Fault: While less common, an internal malfunction within the PCM/ECM could lead to incorrect command signals or misinterpretation of feedback from the FPDM/FPCM.

How to Diagnose and Troubleshoot

Diagnosing P1644 requires a systematic approach, utilizing a digital multimeter (DMM), an advanced OBD-II scan tool, and thorough visual inspection.

1. Visual Inspection:
   • Begin by inspecting the FPDM/FPCM’s physical location. Many FPDMs are mounted externally (e.g., under the vehicle, near the spare tire) and are prone to corrosion. Check the module housing and its electrical connectors for signs of damage, corrosion, or looseness.
   • Carefully inspect the entire wiring harness leading to and from the FPDM/FPCM, as well as the fuel pump connector, for chafing, cuts, pinches, or signs of rodent damage.
2. Retrieve and Analyze DTCs and Freeze Frame Data:
   • Connect an OBD-II scan tool. Record any other stored DTCs, as they might provide additional context (e.g., fuel pressure sensor codes).
   • Review freeze frame data associated with P1644. This data captures engine parameters at the moment the fault occurred, which can be invaluable in recreating the conditions for diagnosis.
3. Monitor Live Data:
   • With the scan tool, monitor relevant live data parameters such as “Fuel Pump Duty Cycle Commanded,” “Actual Fuel Pump Duty Cycle” (if available), “Fuel Pressure Commanded,” and “Actual Fuel Pressure.”
   • Observe if the actual fuel pressure responds to commanded changes, or if it’s consistently too high, too low, or erratic. If the commanded duty cycle does not translate to an appropriate actual fuel pressure, it points to a control circuit or pump issue.
4. Verify FPDM/FPCM Power and Ground:
   • Access the FPDM/FPCM connector. Using a DMM, check for constant battery voltage and a solid ground connection at the appropriate pins, according to the vehicle’s wiring diagram. Key Off and Key On checks may be required depending on the circuit.
   • Perform a voltage drop test on the power and ground circuits to ensure they can carry the necessary current without excessive resistance.
5. Check Control Signal from PCM:
   • With the FPDM/FPCM still connected (or back-probed), use an oscilloscope or a DMM capable of reading PWM signals to verify the control signal from the PCM to the FPDM/FPCM. This signal should vary in duty cycle or voltage depending on engine operating conditions and commanded fuel pressure.
6. Test FPDM/FPCM Output to Fuel Pump:
   • With the FPDM/FPCM receiving proper input from the PCM and having good power/ground, test the output signal to the fuel pump. This will often be a PWM signal. Compare the output signal to the input signal and specifications. A missing or incorrect output indicates an internal FPDM/FPCM fault.
7. Inspect Fuel Pump Circuit Resistance:
   • Disconnect the battery. Disconnect the FPDM/FPCM and the fuel pump connectors. Using the DMM, measure resistance in the wires from the FPDM/FPCM connector to the fuel pump connector. Look for opens (infinite resistance) or shorts to ground or power.
8. Direct Fuel Pump Test (Caution Advised):
   • If all FPDM/FPCM inputs and outputs appear correct, but the fuel pump is not operating or fuel pressure is incorrect, the fuel pump itself may be at fault. Briefly apply direct 12V power to the fuel pump (following manufacturer guidelines to avoid damage and ensure safety) to confirm it runs. Also, check its current draw; excessively high current indicates an internal pump fault, which could overstress the FPDM.

Recommended Repairs and Solutions

Addressing P1644 typically involves repairing or replacing components within the fuel pump speed control system.

• Replace the Fuel Pump Driver Module (FPDM)/Fuel Pump Control Module (FPCM): This is frequently the most common and effective repair, especially if diagnostics point to internal module failure or corrosion on the module itself. When replacing, ensure the new module is correctly mounted and sealed, particularly in areas prone to environmental exposure. Some replacement modules may require programming or calibration with a factory-level scan tool.
• Repair or Replace Damaged Wiring: If the diagnosis reveals opens, shorts, or high resistance in the wiring harness, perform precise repairs using appropriate connectors, heat-shrink tubing, and soldering techniques. For extensive damage, replacing the entire harness section may be more reliable. Always ensure proper routing and secure mounting to prevent future damage. After repairing, apply dielectric grease to connectors to prevent corrosion.
• Replace Corroded Electrical Connectors: If corrosion is present on the FPDM/FPCM or fuel pump connectors, replace the affected connector pigtails. Cleaning heavily corroded pins is often insufficient for long-term reliability.
• Replace the Fuel Pump Assembly: If diagnostic steps confirm that the fuel pump itself is drawing excessive current, is mechanically binding, or has an internal electrical fault despite proper input from the FPDM/FPCM, then the fuel pump assembly (often including the fuel level sender) will need replacement. It’s often prudent to replace the fuel filter concurrently.
• ECM/PCM Software Update or Replacement: Only consider an ECM/PCM replacement as a last resort, after definitively ruling out all other possibilities and confirming an internal PCM fault through dedicated diagnostic procedures, which typically involve dealer-specific tools and expertise. Sometimes, a software update from the manufacturer can resolve communication glitches or incorrect diagnostic logic.

Mechanic’s Tips:

• Always check for any manufacturer-issued Technical Service Bulletins (TSBs) related to P1644 for the specific vehicle make and model, as these can highlight known design flaws or common failure points.
• When replacing an FPDM mounted externally, ensure its mounting surface is clean and that any thermal pads or gaskets are correctly installed to prevent heat buildup and corrosion.
• After any repair, clear the DTCs from the PCM and perform an extended drive cycle under various load conditions to verify the repair and ensure the code does not return. Monitor fuel pressure live data during the drive to confirm proper operation.