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Home / DTC / B29AB — Main cylinder close:open cycle

B29AB — Main cylinder close:open cycle

Detailed page for trouble code B29AB.

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Code

B29AB

MITSUBISHI B — Body

Main cylinder close:open cycle

Brand: MITSUBISHI
Type: B — Body
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Faulty main cylinder hydraulic actuator (sticking piston or internal leak)
  • Defective solenoid valve(s) or valve body inside ABS/hydraulic control unit
  • Faulty hydraulic pump or pressure relief device
  • Low brake fluid level or air in the brake lines
  • Wiring harness damage, poor connector contact or corrosion to valve/pump circuits
  • Faulty ABS/hydraulic ECU or software anomaly

Symptoms

  • ABS, brake or traction warning lamp illuminated
  • Brake pedal pulsation or inconsistent pedal feel
  • Unusual pump noise (continuous or intermittent) from ABS module
  • Reduced braking performance or longer pedal travel
  • Stored or pending hydraulic/valve related fault codes

What to check

  • Read and record DTCs and freeze frame data with a capable scan tool
  • Check and record brake fluid level and condition (contamination, dark fluid)
  • Visually inspect ABS/hydraulic control unit and master cylinder for leaks or physical damage
  • Inspect electrical connectors and wiring to ABS module, solenoids and pump for corrosion, looseness or damage
  • Listen for abnormal pump operation during start/self-test and braking tests
  • Perform basic ABS module self-tests / actuate valves via scan tool (if supported)

Signal parameters

  • Commanded valve state (open/close) — digital command from ABS ECU
  • Actual hydraulic pressure at accumulator or outlet (kPa or bar) — expected stable pressure when commanded closed
  • Pump motor voltage (V) and current draw (A) during cycle — excessive current suggests binding or short
  • Solenoid resistance (Ω) at ambient temperature — compare to factory spec
  • Duty cycle (%) if solenoid is PWM controlled
  • Cycle count and frequency of open/close events during fault capture

Diagnostic algorithm

  1. Connect a diagnostic scan tool, read and save all stored and pending codes and freeze frame data; note conditions when code set
  2. Clear codes and attempt to reproduce fault with monitored data active (test drive and/or controlled brake application)
  3. Check brake fluid level and condition; top up with correct fluid if low and inspect for leaks at master cylinder, lines and ABS unit
  4. Visually inspect and clean connectors at ABS/hydraulic control unit and master cylinder; repair any damaged wiring
  5. Using scan tool, command valves/pump while monitoring pressures and electrical parameters. Compare commanded state to actual pressure response
  6. Measure resistance of solenoids and compare to specification; check pump motor current while commanded to run — high current may indicate mechanical seizure or electrical fault
  7. If solenoids/pump fail bench or active tests, replace or repair ABS/hydraulic control unit or affected components per manufacturer procedure
  8. If mechanical binding of master cylinder is suspected, remove and inspect master cylinder for scoring, corrosion or internal leakage; replace or rebuild as required
  9. After repairs, bleed hydraulic system thoroughly (manufacturer method, often ABS-compatible procedure), clear codes and perform final road test to confirm fault not recurring

Likely causes

  • Intermittent solenoid operation in the ABS/hydraulic control unit
  • Contamination or internal leak in master cylinder causing unexpected pressure/vacuum changes
  • Open or intermittent electrical connection to valve or pump circuit
  • Worn or seized master cylinder piston causing abnormal cycling

Fault status

⚠️ Status
Abnormal main cylinder open/close cycling detected — intermittent or unexpected operation of main hydraulic cylinder or its control circuit.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

B29AA B29AC C1221 C1234 C1241
406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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