The W140 S600 with M120 V12 is magnificent when sorted, but the engine has critical wiring harness and biodegradable insulation issues that can trigger catastrophic overheating and internal damage. Budget heavily for aging German complexity—this is a $150k car when new that now demands commitment.
Engine Wiring Harness Disintegration
Common · high severityTypical onset: all mileages—age-related, 20+ years
Symptoms: Check engine light with multiple random codes, Rough idle, misfires, or no-start conditions, Fuel smell from deteriorating injector seals caused by harness shorts, Oil leaks where harness passes through valve covers
Fix: Complete engine harness replacement requires removing intake manifold and valve covers. Biodegradable insulation turns to dust and shorts circuits. Aftermarket harnesses available but quality varies. 18-24 hours labor for proper job with valve cover gaskets and related seals.
Estimated cost: $3,500-6,000
Coolant Pipe O-Ring Failure Leading to Overheating
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Coolant loss with no visible external leaks, Overheating, especially under load, Steam from engine bay, Milky oil if overheating caused head gasket failure
Fix: Multiple coolant crossover pipes with O-rings fail between cylinder banks. Requires removing intake manifold to access. Often discovered after head gasket damage from unnoticed overheating. Preventive replacement smart. 12-16 hours labor, more if head gaskets compromised.
Estimated cost: $2,000-3,500 preventive, $8,000-15,000 if head gaskets blown
M120 Engine Internal Wear from Neglected Maintenance
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: Rod knock at startup or under acceleration, Low oil pressure warnings, Metal shavings in oil, Excessive oil consumption
Fix: V12 demands religious 3,000-mile synthetic oil changes and fresh coolant. Skipped services lead to bearing wear, scored cylinders. Full rebuild requires removing engine—pistons, rings, bearings, machine work. 60-80 hours labor. Some opt for used engine swap at 40-50 hours.
Estimated cost: $12,000-20,000 rebuild, $8,000-12,000 used engine swap
Transmission Valve Body and Conductor Plate Failure
Common · medium severityTypical onset: 100,000-150,000 mi
Symptoms: Harsh or delayed shifts, especially 2-3 upshift, Limp mode activation, Transmission slipping or flaring between gears, Fluid contamination from internal wear
Fix: 722.6 5-speed transmission valve body has fragile conductor plate connections that crack. Often combined with worn accumulator springs. Requires pan drop, valve body removal, new conductor plate and updated seals. 8-12 hours labor. Do transmission service simultaneously.
Estimated cost: $2,500-4,000
Airmatic Suspension Strut Leaks and Compressor Failure
Common · medium severityTypical onset: 80,000-130,000 mi
Symptoms: Vehicle sitting low, especially overnight, Compressor running constantly, 'Check Suspension' warning, Uneven ride height side-to-side
Fix: Airmatic struts develop leaks at airbag seals. Overworked compressor then fails. Front struts 3-4 hours each, rears 2-3 hours. Compressor 2-3 hours. Replace in pairs minimum. Aftermarket struts available but OE-spec recommended for longevity.
Estimated cost: $3,000-5,000 for pair of struts, $1,200-1,800 compressor
Fuel Pump and Accumulator Failure
Occasional · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Hard starting after sitting, Loss of power under acceleration, Stalling at idle or when hot, Fuel pump whine from rear
Fix: Dual fuel pumps with overvoltage relay and accumulator. System complexity means diagnosis critical. Tank drop required, 4-6 hours labor. Address fuel filter (often neglected) and check lines for corrosion. Pumps and accumulator together recommended.
Estimated cost: $1,500-2,800
Alternator and Voltage Regulator Issues
Occasional · medium severityTypical onset: 100,000+ mi
Symptoms: Battery warning light, Electrical gremlins—windows, seats, radio acting up, Dead battery after short sits, Voltage gauge reading high or low
Fix: 180-amp alternator works hard for V12 and electronics. Voltage regulator often culprit before alternator itself. Test charging system thoroughly. Alternator replacement 3-4 hours, regulator 2 hours. Remanufactured units widely available.
Estimated cost: $800-1,500 alternator, $300-600 regulator
Only for the dedicated enthusiast with a $10k+ reserve fund and access to a specialist—ultimate luxury when running, ultimate money pit when neglected.
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.