Code
P1748
DAEWOO
P — Powertrain
Tcu - ecu comm line #2 mal
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UK: 15
EN: 28
RU: 24
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in the TCU↔ECU communication circuit (#2)
- Corroded or damaged connectors or terminals at TCU, ECU, or intermediate junctions
- Poor or missing power or ground to TCU or ECU
- Blown fuse or relay supplying communication/logic power
- Faulty TCU or faulty ECU (internal transceiver or processor)
- Bus interference from a shorted node or high-resistance connection
Symptoms
- Diagnostic trouble code P1748 stored and MIL/TCU warning lamp may be illuminated
- Transmission enters limp/backup mode or shifts abnormally
- No or limited data from TCU visible on a scan tool
- Other modules reporting communication faults or related U-codes
- Intermittent faults that may occur with vibration or after moisture exposure
What to check
- Read and record all stored DTCs and freeze frame data from both engine ECU and TCU
- Check for related U-codes that indicate network/bus problems (loss of communication)
- Visually inspect wiring harness and connectors for damage, corrosion, or water ingress
- Verify battery voltage and charging system; inspect fuses and relays for TCU/ECU power
- Verify good chassis and engine/transmission grounds
- Use a scan tool to request live data from the TCU and note if the module responds
Signal parameters
- Idle/rest voltage on the communication line depends on vehicle type — may be a single-wire serial (~0–12 V idle) or a differential bus (CAN ~2.5 V idle per line).
- Active data appears as digital pulses or differential transitions; look for clean, repeating data frames when the engine/ignition is on and modules awake.
- Abnormal signs: no activity (stuck idle), constant low or high (short to ground or battery), excessive noise, or distorted waveform edges.
- Line continuity: very low resistance between modules when powered off (no unintended shorts to ground or battery).
Diagnostic algorithm
- Retrieve and document all DTCs from engine ECU and TCU. Note when codes set and any freeze frame data.
- Inspect both TCU and ECU connectors for corrosion, bent pins, water intrusion, or loose terminals. Repair or replace connectors as needed.
- Check fuses and power feeds to both modules. Verify battery voltage at module power pins with key ON and engine running.
- Verify ground connections for engine, transmission, and body. Clean and tighten grounding points.
- With ignition ON, use a scan tool to attempt communication with the TCU. If the TCU does not respond, verify wiring between the scan tool junction and TCU.
- Perform a wiggle test on the harness while observing live data or watching for code reappearance. Note any change when harness is moved.
- Measure resistance and continuity of the communication line(s) with key OFF. Look for unexpected shorts to ground or battery. Repair any damaged wiring.
- Use an oscilloscope or suitable scope adapter to observe the communication waveform during key ON/engine running. Compare to a known-good waveform or specification: check for activity, correct idle voltage, and clean transitions.
- Isolate the fault by disconnecting other modules on the same bus one at a time (where safe and documented) to see if a single node causes the bus failure.
- If wiring, connectors, power and grounds check good and the bus waveform/activity remains abnormal, consider reflashing software or replacing the module showing no communication—replace only after wiring is confirmed good.
- After repairs, clear codes and perform road test to confirm code does not return and transmission behavior is normal.
Likely causes
- Broken or pinched harness where it passes through the vehicle body or near suspension/engine mount
- Corrosion at the multi-pin connector for the TCU or ECU
- Intermittent ground connection at engine/gearbox grounding strap
- Accessory or aftermarket device tied into the communication wiring causing noise or loading
- Failed TCU transceiver while ECU still functional (or vice versa)
Fault status
Status
TCU-ECU communication fault detected on communication line #2. Intermittent or no data exchange; check wiring, connectors, power/ground, and module responsiveness.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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