P1637

What Does Code P1637 Mean?

The OBD-II diagnostic trouble code P1637 indicates a malfunction within the Controller Area Network (CAN) communication link between the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), and the Anti-lock Brake System Control Module (ABSCM). Specifically, the ECM/PCM has detected that it is not receiving expected data messages or specific signals from the ABSCM over the high-speed CAN bus within a predetermined timeframe. This signifies a loss of communication integrity or a complete failure of the communication pathway between these two critical control units. The ECM/PCM relies on data from the ABSCM, such as wheel speed information, for various functions including vehicle speed calculation, cruise control operation, and potentially transmission shift strategies. When this communication is disrupted, the ECM/PCM flags the P1637 code, indicating a critical network issue.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will illuminate.
• ABS warning light illumination: The ABS system warning light will be on.
• Traction Control System (TCS) / Stability Control System (SCS) warning light illumination: These warning lights may also illuminate, indicating system disablement.
• Loss of Anti-lock Braking System (ABS) functionality: The ABS will not operate, and brakes may lock up under hard braking.
• Loss of Traction Control System (TCS) functionality: The vehicle’s traction control will be inoperative.
• Inoperative cruise control: The cruise control system may not engage or may disengage unexpectedly.
• Harsh or erratic transmission shifting: In some applications, the transmission control module relies on wheel speed data from the ABSCM, and its absence can cause shifting irregularities.
• No communication with the ABSCM via diagnostic scanner: A scan tool may be unable to establish communication with the ABSCM.

What Causes the Code P1637?

• Damaged or corroded CAN bus wiring: Opens, shorts, or excessive resistance in the CAN High (CAN-H) or CAN Low (CAN-L) circuits between the ECM/PCM and ABSCM.
• Faulty or corroded electrical connectors: Loose pins, corrosion, or poor terminal tension at the ECM/PCM or ABSCM connectors.
• Insufficient power or ground supply to the ABSCM: A lack of proper voltage or ground can prevent the ABSCM from powering on and communicating.
• Internal ABS Control Module (ABSCM) failure: A defective ABSCM unable to transmit data or process network communication.
• Internal Engine Control Module (ECM/PCM) failure: While less common, a faulty ECM/PCM could be unable to send or receive CAN messages correctly.
• High resistance or open circuit in CAN bus terminating resistors: Though less common specifically for this code, incorrect bus termination can disrupt communication.

How to Diagnose and Troubleshoot

Diagnosing P1637 requires a systematic approach using a diagnostic scanner, a digital multimeter (DMM), and potentially an oscilloscope.

1. Verify DTCs and Freeze Frame Data: Connect an OBD-II scanner and confirm P1637 is present. Check for any other related communication or module-specific codes. Review freeze frame data to understand the conditions (engine speed, vehicle speed, temperature, etc.) when the code was set, which can provide clues.
2. Attempt Communication with ABSCM: Using the diagnostic scanner, attempt to establish direct communication with the ABSCM. If communication is impossible, it strongly suggests a power, ground, or CAN bus issue specific to the ABSCM or the module itself.
3. Visual Inspection: Disconnect the battery. Carefully inspect the wiring harness and connectors leading to both the ECM/PCM and the ABSCM. Look for signs of chafing, cuts, corrosion, bent pins, or loose connections. Pay close attention to the twisted-pair CAN bus wires (often specific colors, e.g., yellow/blue, green/white). Inspect ground points for corrosion or looseness.
4. Check ABSCM Power and Ground: Reconnect the battery. With the ignition ON (engine OFF), use a DMM to measure voltage at the ABSCM power input terminals (consult wiring diagram for correct pins). Verify battery voltage (approx. 12.6V). Then, measure resistance between the ABSCM ground terminals and a known good chassis ground point. Resistance should be less than 0.5 ohms. If power or ground is missing or compromised, trace the circuit back to the fuse box or ground point.
5. CAN Bus Circuit Resistance Check (Ohm Check): Disconnect the battery. Disconnect both the ECM/PCM and the ABSCM connectors. Using a DMM, measure the resistance between the CAN High and CAN Low pins on the vehicle harness side, at the ABSCM connector. The typical resistance across the entire CAN bus segment should be approximately 60 ohms (representing two 120-ohm terminating resistors in parallel). A reading significantly higher (e.g., > 70 ohms or an open circuit “OL”) indicates an open in one of the CAN wires or a faulty terminating resistor. A reading significantly lower (e.g., < 50 ohms) suggests a short within the CAN bus wiring.
6. CAN Bus Voltage Check (Live): With the ignition ON (engine OFF), use a DMM to measure the voltage on the CAN High and CAN Low wires at the ABSCM connector (harness side). Typically, CAN Low will idle around 2.5V and fluctuate lower to about 1.5V when communicating. CAN High will idle around 2.5V and fluctuate higher to about 3.5V when communicating. If one or both lines are stuck at 0V, 5V, or battery voltage, it indicates a short to ground, power, or an open circuit. An oscilloscope provides a more detailed view of the waveform integrity.
7. Continuity and Short-to-Ground/Power Tests: With the battery disconnected and modules unplugged, check for continuity of each CAN wire from the ABSCM connector to the ECM/PCM connector. Also, check each CAN wire for shorts to ground and shorts to battery voltage by probing the wire and then a good chassis ground or battery positive. Resistance should be infinite.
8. ABSCM Self-Test/Replacement: If all wiring, power, and ground checks pass, and the scanner still cannot communicate with the ABSCM, the module itself is likely faulty.

Recommended Repairs and Solutions

• Repair or Replace Damaged Wiring: If the visual inspection or electrical tests reveal damaged, corroded, or open/shorted CAN bus wiring, perform precise repairs using appropriate connectors, terminals, and soldering techniques, or replace the affected harness section.
• Clean or Replace Corroded Connectors: Address any corrosion or poor terminal tension in the ECM/PCM or ABSCM connectors. Use electrical contact cleaner and a small pick to restore proper contact, or replace the connector housing if severely damaged.
• Restore Power and Ground to ABSCM: Repair any issues found with the power supply (e.g., blown fuse, open circuit) or ground connection to the ABSCM. Ensure all battery cables and ground straps are clean and tight.
• Replace the ABS Control Module (ABSCM): If the ABSCM is determined to be internally faulty after all other checks pass, it will need to be replaced. Important Note: New ABSCMs often require programming or coding to the vehicle’s specific VIN and options using a factory-level diagnostic tool. Failure to program the module correctly will likely result in further DTCs or system malfunctions.
• Replace the ECM/PCM: In rare cases where all other components (wiring, ABSCM) are verified good, the ECM/PCM may be at fault. This is typically a last resort, as ECM/PCM replacement also requires programming and can be costly.
• Clear Codes and Test Drive: After any repair, clear all stored DTCs from the ECM/PCM and ABSCM. Perform a thorough test drive to ensure the communication link is stable and the code does not return.