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Home / DTC / P0703 — Torque Converter/Brake Switch B Circuit Malfunction

P0703 — Torque Converter/Brake Switch B Circuit Malfunction

Detailed page for trouble code P0703.

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P0703

MERCEDES-BENZ P — Powertrain

Torque Converter/Brake Switch B Circuit Malfunction

Brand: MERCEDES-BENZ
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Faulty brake switch (secondary/B switch)
  • Open, short-to-ground or short-to-power in the switch wiring or connector
  • Corroded or loose connector at switch or TCM/ECM
  • Blown fuse or bad power/ground for the circuit
  • Faulty TCM/ECM or internal driver fault
  • Intermittent wiring damage (chafing, water intrusion)

Symptoms

  • Malfunction Indicator Lamp (MIL) / Check Engine light on
  • Cruise control may be disabled or behave erratically
  • Torque converter lockup may not engage or disengage properly
  • Transmission may shift harshly, slip, or enter limp mode
  • Brake lights may not operate correctly (if switch is faulty)
  • Stored or pending transmission-related fault codes

What to check

  • Retrieve freeze frame and all stored codes; note related transmission or brake codes
  • Visually inspect brake switch, pedal linkage, connector, and nearby wiring for damage or corrosion
  • Verify proper brake light operation (press pedal and observe lights)
  • Check fuses and any related relays for this circuit
  • Backprobe the switch connector and the TCM/ECM input while operating the pedal and observe signal
  • Perform wiggle test along harness to try recreating the fault

Signal parameters

  • Switch toggles between open and closed states; expect continuity when pedal is pressed (closed) and open when released, or vice versa depending on design
  • Voltage at switch terminal typically toggles between approx. 0 V (ground) and battery voltage (~12 V) relative to chassis ground, or between 0 V and a reference input voltage used by the TCM/ECM
  • Resistance when closed should be near 0 ohms; open should be infinite (OL)
  • Signal should be stable (no rapid flicker) during steady pedal positions

Diagnostic algorithm

  1. Connect a scan tool, read and record P0703 and any related codes; check freeze frame data and live inputs.
  2. Visually inspect brake switch, pedal, connector and harness for physical damage, corrosion or loose pins. Check mounting/adjustment of the switch.
  3. Verify brake lights operate correctly by pressing pedal; if brake lights fail, focus on switch and power/ground feeding the switch.
  4. Check related fuses and power/ground circuits for the brake switch/TCM input.
  5. Backprobe the switch connector and measure voltage and continuity while operating the pedal. Confirm signal changes cleanly between open and closed states as expected.
  6. Backprobe the TCM/ECM input connector and confirm the same signal is present at the module. If the switch output is correct but module input is not, suspect wiring between or module fault.
  7. Perform wiggle tests along the harness (pedal area to firewall to TCM) while monitoring the input for intermittent changes.
  8. Repair or replace faulty wiring, connector, or brake switch as required. Secure and protect harness against future damage.
  9. Clear codes and perform a road test with live data monitoring to confirm the fault does not return and that transmission/torque converter behavior is normal.
  10. If problem persists after wiring and switch confirmation, consider TCM/ECM bench test or replacement as a last resort.

Likely causes

  • Brake light switch failed or out of adjustment
  • Connector pins corroded or bent at the brake switch
  • Broken wire or pinched harness near pedal or firewall
  • Aftermarket equipment or recent repair disturbed wiring
  • Failed TCM input (less common)

Fault status

⚠️ Status
TCM/ECM reports malfunction on torque converter/brake switch B circuit — open, short to power/ground, intermittent signal, or invalid input detected.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0700 P0702 P0705 P0715 P0720
856

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