The W220 S600 with the M137 5.8L V12 is a technological masterpiece that becomes a financial nightmare around 100k miles. The engine's biodegradable wiring harness and weak piston ring design turn what should be luxury into a ticking time bomb requiring catastrophic repairs.
M137 Engine Wiring Harness Disintegration
Common · high severityTypical onset: 80,000-120,000 mi
Symptoms: Random misfires on multiple cylinders, Check engine light with dozens of codes stored, Hard starting or no-start conditions, Rough idle that moves between cylinder banks, White smoke from engine bay as insulation melts onto exhaust
Fix: Complete engine harness replacement requires 25-35 hours labor with engine partially dropped. Mercedes used soy-based insulation that literally crumbles to dust. Cannot be patched reliably—full harness replacement mandatory. Aftermarket improved harnesses available.
Estimated cost: $4,500-7,500
Piston Ring Failure and Cylinder Scoring
Common · high severityTypical onset: 90,000-150,000 mi
Symptoms: Excessive oil consumption—quart every 500-1000 miles, Blue smoke on cold start that persists, Loss of compression on multiple cylinders, Oil in intake manifold and throttle bodies, Eventually leads to catastrophic bearing failure
Fix: M137 has weak piston ring design with inadequate cylinder wall hardening. Requires complete engine rebuild with upgraded pistons/rings and cylinder honing or re-sleeving. Some shops recommend short block replacement instead. 60-80 hours labor for proper rebuild. This is THE killer on these cars—not if, but when.
Estimated cost: $12,000-18,000
ABC (Active Body Control) Hydraulic System Failures
Common · medium severityTypical onset: 70,000-110,000 mi
Symptoms: "ABC malfunction—visit workshop" message, Car drops to the ground when parked, Hydraulic fluid leaks under vehicle (green fluid), Bouncy or uneven ride height, Pump running continuously
Fix: ABC pump, accumulator spheres, or valve blocks fail. Pump replacement is 8-12 hours, valve block 10-15 hours, accumulator spheres 6-8 hours. Many owners delete ABC and convert to conventional coil springs ($3k-4k) rather than fix, but lose the magic carpet ride.
Estimated cost: $3,500-6,500
Transmission Valve Body and Conductor Plate Failure
Occasional · high severityTypical onset: 100,000-140,000 mi
Symptoms: Harsh or delayed shifts especially when cold, Limp mode with transmission warning light, Slipping between gears under load, No reverse or stuck in one gear, Metal shavings in transmission fluid
Fix: 722.6 five-speed transmission valve body and conductor plate (circuit board inside trans) fail. Requires transmission removal and rebuild or replacement. 18-24 hours labor. These transmissions are actually robust if maintained but conductor plate corrosion from moisture is the Achilles heel.
Estimated cost: $4,000-6,500
Airmatic Suspension Strut and Compressor Failure
Common · medium severityTypical onset: 80,000-120,000 mi
Symptoms: Suspension warning light with sagging corner, Compressor runs constantly, Car sits low in morning, rises when started, Clunking from front suspension, Air leak hissing sound when parked
Fix: Air struts develop leaks in bags or internal seals. Compressor motor burns out from overwork. Front struts 4-6 hours each, rears 3-4 hours. Compressor 3-5 hours. Budget for all four struts eventually—they age together. Relay (N51) also fails causing compressor not to run.
Estimated cost: $2,500-5,000
Fuel Pump and Fuel System Component Failures
Occasional · medium severityTypical onset: 90,000-130,000 mi
Symptoms: Hesitation or stumbling during acceleration, No-start with crank but no fire, Fuel smell in cabin or trunk area, Running rough under load at highway speeds, Fuel pressure codes stored
Fix: Dual fuel pumps in tank fail, fuel filter clogs (10k mile service interval often ignored), fuel pressure regulators leak. Pump replacement requires tank drop—8-10 hours labor for both. Fuel filter is 1 hour but often skipped. V12 is sensitive to fuel pressure variation.
Estimated cost: $1,800-3,200
Transmission Oil Cooler Line Corrosion and Leaks
Common · medium severityTypical onset: 60,000-100,000 mi
Symptoms: Transmission fluid dripping from radiator area, Low transmission fluid warnings, Slipping or erratic shifts from fluid starvation, Pink fluid visible under car, Burnt transmission smell
Fix: Steel transmission cooler lines rust through where they route near radiator and along frame rails. Must replace entire line assemblies, not just patch. 4-6 hours labor depending on which lines. Catastrophic if ignored—starves transmission and leads to failure.
Estimated cost: $800-1,500
Only buy if you have $15k cash reserve for the inevitable engine rebuild, love working on cars yourself, or can wrench at dealer-tech level—otherwise this V12 will bankrupt you with a smile.
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.