Complete Technical Guide: Code P1679 – TCM/ECM Communication Error
Technical Definition
OBD-II Code P1679 is a manufacturer-specific diagnostic trouble code indicating a communication breakdown between the Transmission Control Module (TCM) and Engine Control Module (ECM) via the Controller Area Network (CAN bus). This code typically appears in General Motors, Ford, Chrysler, and European vehicles from 2003 onward.
Technical Overview of P1679
The P1679 error code represents a critical failure in the high-speed CAN bus communication network between two essential vehicle control modules. Modern vehicles utilize a multiplexed network system where modules communicate via serial data lines rather than individual wires for each signal.
1.1 CAN Bus Communication Protocol
The Controller Area Network (CAN) uses a message-based protocol where each module can broadcast and receive messages. The TCM and ECM exchange critical data including:
| Data Parameter | Transmission Rate | Criticality |
|---|---|---|
| Engine RPM | 10-50ms intervals | Critical |
| Throttle Position | 10-20ms intervals | Critical |
| Transmission Gear Position | 20-100ms intervals | Critical |
| Torque Request | 10-50ms intervals | Critical |
| Transmission Fluid Temp | 100-500ms intervals | Important |
| Vehicle Speed | 50-100ms intervals | Critical |
Detailed Symptoms Analysis
2.1 Primary Symptoms
When P1679 sets, the vehicle typically enters a default or “limp-home” mode to protect the transmission from damage due to lack of communication between control modules.
2.1.1 Transmission Limp Mode Characteristics
- Fixed Gear Operation: Transmission locks in 2nd or 3rd gear (varies by manufacturer)
- Torque Converter Lockup Disabled: Increased transmission temperature and reduced fuel economy
- Line Pressure Maximized: Harsh shifts when moving from neutral to drive
- Shift Schedule Disabled: Manual shifting may still operate via manual valve
2.1.2 Additional System Effects
| System | Effect | Reason |
|---|---|---|
| Speedometer | Inaccurate or zero reading | Vehicle speed data unavailable |
| Traction Control | System disabled | Requires torque management data |
| Cruise Control | Inoperative | Needs throttle and gear position data |
| Engine Performance | Reduced power output | Conservative default fueling maps |
Related Error Codes and Network Diagnostics
3.1 Common Companion Codes
P1679 rarely occurs in isolation. The following codes often accompany it, providing additional diagnostic clues:
| Error Code | Description | Diagnostic Significance |
|---|---|---|
P0700 |
Transmission Control System Malfunction | Generic indicator of TCM issues |
U0100 |
Lost Communication with ECM/PCM | Confirms network communication failure |
U0101 |
Lost Communication with TCM | Confirms network communication failure |
P0606 |
ECM/PCM Processor | Indicates possible ECM internal fault |
P0882 |
TCM Power Input Signal Low | Suggests TCM power supply issues |
P0562 |
System Voltage Low | Points to charging system problems |
3.2 CAN Bus Network Error Codes
U0001: High Speed CAN Communication Bus
U0002: High Speed CAN Communication Bus Performance
U0073: Control Module Communication Bus Off
U0121: Lost Communication with Anti-Lock Brake System
U0140: Lost Communication with Body Control Module
Comprehensive Diagnostic Procedure
4.1 Required Diagnostic Equipment
| Tool | Specification | Purpose |
|---|---|---|
| Professional Scan Tool | Bidirectional control, module programming | Module communication testing |
| Digital Multimeter | 10MΩ impedance, min 0.1V resolution | Voltage and resistance measurements |
| Oscilloscope | 100MHz bandwidth minimum | CAN signal waveform analysis |
| Breakout Box | Vehicle-specific connectors | Pin-to-pin continuity testing |
4.2 Step-by-Step Diagnostic Flowchart
-
Initial System Scan and Documentation
Record all DTCs, freeze frame data, and monitor live data parameters including:
- ECM and TCM communication status
- Network participation of other modules
- Battery voltage during cranking and running
- Ignition voltage at module connectors
-
Visual Inspection Protocol
Systematically inspect the following areas:
- Main Harness Routing: Near exhaust, sharp edges, moving components
- Module Connectors: Pins 1, 2, 19, 20 on TCM (GM), pins 6, 14, 26 on ECM
- Ground Points: G104 (engine block), G105 (chassis), G200 (dash)
- Termination Resistors: Typically at ECM and instrument cluster
-
CAN Bus Voltage Measurements
Test at the OBD-II connector pins 6 (CAN High) and 14 (CAN Low):
Condition CAN High (Pin 6) CAN Low (Pin 14) Differential Diagnosis Normal 2.5-3.5V 1.5-2.5V ~0.9V Network operational Short to Ground <0.5V <0.5V ~0V Check wiring for shorts Short to Power >4.5V >4.5V ~0V Check for 12V short Open Circuit ~2.5V ~2.5V ~0V Check termination resistors -
Resistance Testing Protocol
Disconnect battery negative cable. Measure between pins 6 and 14:
- Normal: 60 ohms (two 120-ohm resistors in parallel)
- One Module Disconnected: 120 ohms
- Open Circuit: Infinite resistance
- Short Circuit: <1 ohm
Detailed Repair Procedures
5.1 Wiring Repair Standards
All CAN bus wiring repairs must follow strict standards to prevent future communication errors:
| Repair Type | Procedure | Specifications |
|---|---|---|
| Wire Splice | Military splice with soldering | ROSIN core solder only, heat shrink tubing |
| Twisted Pair | Maintain original twist rate | 20-40 twists per meter (manufacturer specific) |
| Shield Repair | Braided shield continuity | 95% shield coverage minimum |
| Connector Repair | GM tool #J-38125 or equivalent | Weather-pack seals must be replaced |
5.2 Module Programming Procedures
1. Connect battery maintainer (14.0V minimum)
2. Launch GDS2 with SPS software
3. Select: “Program ECU” → “Transmission Control Module”
4. Enter VIN and calibration ID
5. Follow on-screen instructions (DO NOT INTERRUPT)
6. Perform: “Case Learn” and “Quick Learn” procedures
7. Clear all DTCs and test drive
Technical Specifications and Testing Values
6.1 Module Power Requirements
| Module | Ignition Voltage | Battery Voltage | Ground Resistance | Current Draw (Sleep) |
|---|---|---|---|---|
| ECM (GM) | 11.5-14.5V | 11.5-12.6V | <0.5Ω | 15-35mA |
| TCM (GM) | 11.5-14.5V | 11.5-12.6V | <0.5Ω | 8-25mA |
| ECM (Ford) | 10.8-15.0V | 10.8-12.8V | <1.0Ω | 20-50mA |
6.2 CAN Bus Specifications by Manufacturer
| Manufacturer | CAN Speed | Termination | Diagnostic Pins | Network Type |
|---|---|---|---|---|
| General Motors | 500 kbps | 120Ω at ECM & IPC | 6 & 14 | GMLAN |
| Ford | 500 kbps | 120Ω at BCM & PCM | 6 & 14 | MS-CAN |
| Chrysler | 125 kbps | 120Ω at PCM & TCM | 6 & 14 | PCI Bus |
| Toyota | 500 kbps | 120Ω at ECU | 6 & 14 | BEAN |
