The R129 300SL with M104 inline-six is generally reliable but suffers from known wiring harness biodegradation and a handful of transmission/engine issues that can get expensive. The engine itself is stout, but when major internal work is needed, costs escalate quickly.
Biodegradable Wiring Harness Insulation Failure
Common · high severityTypical onset: 25+ years age-related, any mileage
Symptoms: Intermittent no-start, rough idle, misfires, Check engine light with multiple random codes, Electrical gremlins: windows, gauges, accessories acting up, Visible cracking or sticky residue on underhood wiring
Fix: Replace engine and chassis harnesses. Engine harness alone is 8-12 hours; full engine + chassis harness runs 16-20 hours. Requires careful routing and connector work.
Estimated cost: $2,500-5,000
M104 Head Gasket Failure
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: White smoke from exhaust, especially on cold start, Coolant loss with no visible leaks, Overheating or fluctuating temperature gauge, Milky oil or oil in coolant reservoir
Fix: Head gasket replacement on M104 is 12-16 hours. Must resurface head, replace head bolts (torque-to-yield), timing chain components while in there. Often uncovers worn guides or tensioner.
Estimated cost: $3,000-5,500
722.5 Transmission Valve Body and Conductor Plate Issues
Common · medium severityTypical onset: 100,000-150,000 mi
Symptoms: Harsh or delayed 2-3 upshift, Limp mode (stuck in 2nd gear), Erratic shifting or no shift at all, Transmission slipping or flaring between gears
Fix: Valve body removal and overhaul, replace conductor plate and 13-pin connector. Trans doesn't need full removal—drop pan, 6-8 hours labor. Use OE Mercedes or reputable reman parts.
Estimated cost: $1,200-2,200
Transmission Oil Cooler Line and Radiator Leak
Common · medium severityTypical onset: 80,000-140,000 mi
Symptoms: Transmission fluid on ground or visible drips, Low trans fluid level, slipping under load, Pink or red residue around radiator or cooler lines, Burnt transmission smell after highway driving
Fix: Replace trans cooler lines and inspect radiator-integrated cooler for cross-contamination (coolant in trans fluid is catastrophic). Lines are 2-3 hours; if radiator is compromised, add 4-5 hours and flush transmission.
Estimated cost: $800-2,500
Engine and Transmission Mount Deterioration
Common · low severityTypical onset: 80,000-120,000 mi
Symptoms: Clunk on hard acceleration or deceleration, Vibration at idle, especially in Drive, Visible sagging or torn rubber on mounts, Excessive engine movement when revving in Park
Fix: Replace all three engine mounts and transmission mount. Engine mounts are 1.5 hours each; trans mount another 1.5 hours. Use OE or quality aftermarket (Lemforder, Corteco).
Estimated cost: $800-1,400
M104 Crankshaft Position Sensor and Cam Position Sensor Failure
Occasional · high severityTypical onset: Not mileage-specific, heat-related
Symptoms: Random no-start when engine is hot, Stalling at idle or while driving, restarts after cooling, Intermittent RPM gauge drop to zero, Check engine light with crank/cam correlation codes
Fix: Replace crank position sensor (behind harmonic balancer, 2-3 hours) and cam position sensor at distributor (1 hour). Common cause of roadside breakdowns; carry spares.
Estimated cost: $400-800
Convertible Top Hydraulic Cylinder and Pump Failure
Occasional · low severityTypical onset: Age-related, 20+ years
Symptoms: Top moves slowly or stops mid-cycle, Hydraulic fluid leaks visible at cylinders or pump, Top won't latch or unlatch properly, Whining or laboring pump motor
Fix: Rebuild or replace hydraulic cylinders (2 front, 2 rear) and pump assembly. Each cylinder is 1-2 hours; pump is 3-4 hours. Flush system and bleed thoroughly.
Estimated cost: $1,500-3,500
Buy one if the wiring harness has been replaced and you have service records showing transmission maintenance—otherwise, budget $3,000-5,000 for deferred maintenance in the first year.
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.