Code
P0743
MERCEDES-BENZ
P — Powertrain
Torque Converter Clutch Circuit Electrical
Views:
UK: 19
EN: 23
RU: 34
AI status
Completed
Completed
100%
Causes
- Open or short in TCC solenoid wiring or connector
- Corroded, loose or damaged TCC solenoid connector
- Faulty TCC solenoid (coil open/short or intermittent)
- Blown fuse or faulty relay feeding TCM or solenoid power
- Poor ground at transmission/TCM
- Faulty TCM (output driver failure)
Symptoms
- Check Engine / MIL illuminated
- Reduced fuel economy (loss of lock-up)
- Transmission limp mode or reduced shift functionality
- Harsh or unusual shifts
- Shudder or vibration during attempted lock-up
- Possible stall or poor idle recovery when stopped with heavy load
What to check
- Read stored and pending DTCs, view freeze frame and live data (TCC status, commanded vs actual)
- Check battery voltage and charging system before electrical tests
- Visual inspection of transmission wiring harness, connectors, and ground points
- Inspect fuses and relays related to transmission/TCM circuits
- Measure resistance of TCC solenoid (with harness disconnected) and compare to spec
- Back-probe TCC connector with key on / engine running to verify control voltage/PWM and ground
Signal parameters
- Nominal solenoid coil resistance: typically ~10–30 ohms (vehicle-specific) — compare to OEM spec
- Supply voltage: ~12 V (battery) available to solenoid circuit when commanded or for reference
- Control signal: PWM from TCM, duty cycle 0–100% (varies with load and speed)
- Typical PWM frequency range: 10–200 Hz (manufacturer-specific)
- Open-circuit: OL on resistance meter; short to ground or B+ indicated by near 0 ohms
Diagnostic algorithm
- Retrieve all codes, freeze-frame and live data. Note related transmission or network codes.
- Verify battery voltage and good charging system (low voltage can cause false electrical faults).
- Perform a visual inspection of the TCC solenoid connector, wiring harness, and transmission grounds for damage, corrosion or looseness.
- Check relevant fuses and relays. Replace if blown or suspect.
- With ignition OFF, disconnect TCC solenoid connector and measure coil resistance across solenoid terminals. Compare to OEM spec. Replace solenoid if out of range or intermittent.
- With harness connected, back-probe connector with key ON (engine OFF) to verify reference power and ground. Record voltages.
- Start engine and command lock-up while monitoring live data. Back-probe and use an oscilloscope to observe PWM waveform, duty cycle and amplitude at the TCC control pin. Look for missing signal, stuck high/low, or noisy waveform.
- Wiggle the harness and connectors while monitoring for intermittent changes to recreate fault.
- Perform continuity checks from TCM harness pins to the solenoid connector and check for shorts to ground or battery. Repair wiring as needed.
- If wiring and solenoid test good, consider TCM output driver failure—confirm by swapping TCM with known-good unit only if service info allows, or consult dealer-level diagnostics.
- If electrical system checks OK but lock-up still fails, inspect transmission hydraulic circuits, valve body and fluid; check for stuck clutch or hydraulic fault.
- After repairs, clear codes and perform road test to verify repair. Re-scan for reappearance.
Likely causes
- Wiring/connectors (broken wire, chafed loom, corrosion) between TCM/ECM and TCC solenoid
- Failed TCC solenoid
- Blown fuse or bad relay supplying solenoid/TCM
- Poor ground at transmission harness
- TCM driver fault (less common) or internal transmission hydraulic issue
Fault status
Status
P0743 — Torque Converter Clutch Circuit Electrical. Electrical fault detected in the TCC control circuit; may prevent lock-up and cause drivability issues.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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