The R129 500SL with the M119 5.0L V8 is a solid platform hampered by complex hydraulics, aging wiring harnesses, and biodegradable engine components that fail predictably after 100k miles. The V12 is rarer and exponentially more expensive to maintain.
Engine Wiring Harness Deterioration
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Intermittent misfire codes across multiple cylinders, Hard starting when engine is hot, Check engine light with no consistent pattern, Rough idle that comes and goes
Fix: Mercedes used biodegradable insulation that turns to dust. Requires complete engine harness replacement, 8-12 hours labor depending on whether you pull the engine or work around it. Many techs pull the intake manifold for access.
Estimated cost: $2,000-3,500
Hydraulic Top Mechanism Failure
Common · medium severityTypical onset: 70,000-120,000 mi
Symptoms: Convertible top won't fully latch or unlatch, Top gets stuck mid-cycle, Hydraulic fluid leaking near trunk area, Slow or jerky top operation
Fix: Hydraulic cylinders and lines leak, pump gets weak, and microswitches fail. Often requires pump rebuild or replacement plus new cylinders. Labor is 6-10 hours depending on how many components need replacement. System must be bled properly.
Estimated cost: $1,800-4,000
Head Gasket Failure (M119 V8)
Occasional · high severityTypical onset: 100,000-150,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant consumption with no visible leaks, Oil in coolant reservoir or vice versa, Overheating under load
Fix: M119 head gaskets fail due to age and heat cycling. Requires heads off, resurfacing, new bolts, and all timing components while you're in there. 18-24 hours labor. Smart shops do valve stem seals, water pump, and thermostat at same time.
Estimated cost: $4,500-7,000
Transmission Valve Body and Conductor Plate
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Harsh or delayed 2-3 shift, Slipping between gears, Limp mode with transmission codes, No upshift past 2nd or 3rd gear
Fix: 722.6 five-speed has weak conductor plate connections and valve body wear. Requires transmission pan drop, valve body removal, and new conductor plate. Can often be done without removing transmission. 4-6 hours labor plus fluid.
Estimated cost: $1,200-2,200
Crankshaft Position Sensor and Cam Position Sensor Failure
Occasional · high severitySymptoms: No-start condition with cranking but no fire, Stalling at idle or while driving, Intermittent loss of spark, Engine cuts out then restarts normally
Fix: Heat-related failure of hall-effect sensors. Crank sensor is behind the starter, cam sensors are on valve covers. Diagnosis can be tricky because failure is often intermittent. 1-3 hours labor depending on which sensor.
Estimated cost: $400-900
Vacuum System Leaks
Common · medium severityTypical onset: 60,000-100,000 mi
Symptoms: HVAC blend doors stuck or won't change modes, Central locking intermittent or inoperative, Soft brake pedal despite good master cylinder, Hissing noise under dash or from engine bay
Fix: Hard plastic vacuum lines become brittle and crack. System controls door locks, climate control actuators, and brake assist. Requires methodical leak-down testing and line replacement. 3-6 hours depending on how many lines need replacement.
Estimated cost: $600-1,500
Automatic Soft Close Door Mechanism
Occasional · low severitySymptoms: Door won't soft-close or keeps trying to pull shut, Grinding noise from door latch, Battery drain from mechanism running continuously, Door ajar warning stays on
Fix: Soft-close motors and latches wear out or bind. Requires door panel removal and latch assembly replacement. Can temporarily disable by unplugging motor. 2-3 hours per door.
Estimated cost: $500-1,000
Buy one if you love the style and can wrench or have a trusted indie shop—budget heavily for deferred maintenance on any example over 80k miles, but the M119 drivetrain is fundamentally bulletproof once sorted.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.