1999 MERCEDES-BENZ SL500 R129

Typical onset: 80,000-140,000 mi

Symptoms: Pink or milky transmission fluid indicating coolant intrusion, Transmission slipping or delayed engagement after cooler fails, Engine overheating or coolant loss when trans fluid enters cooling system, Complete transmission failure if driven after contamination begins

Fix: Replace transmission oil cooler, flush both transmission and cooling system multiple times, replace radiator if contaminated. If caught early, trans survives; if driven contaminated, expect full 722.6 transmission rebuild. 8-12 hours labor for cooler/flush, add 20-30 hours if trans rebuild needed.

Estimated cost: $1,200-$2,500 preventive, $4,500-$7,500 with transmission rebuild

Typical onset: 100,000+ mi or age-related regardless of mileage

Symptoms: Check engine light with multiple random misfires or sensor codes, Rough idle, hesitation, or stalling due to injector/coil harness shorts, Insulation crumbling to dust when touched, exposing bare wires, Oil leaks accelerate harness degradation when soaked

Fix: Complete engine wiring harness replacement is the only proper fix. Aftermarket harnesses available but quality varies—OEM or quality reman preferred. 10-14 hours labor for thorough replacement including proper routing and connector cleaning.

Estimated cost: $1,800-$3,200

Symptoms: Convertible top moves slowly or stops mid-cycle, Hydraulic fluid leaks at cylinders, lines, or pump, Top won't latch or unlatch electrically, Pump runs continuously or makes grinding noise

Fix: Most often hydraulic cylinders leak and need rebuilding or replacement, or pump fails. Lines crack from age. Entire system uses special hydraulic fluid. Diagnosis 1-2 hours, cylinder replacement 4-6 hours, pump 3-4 hours. Full system overhaul if multiple components failed from neglect is 12+ hours.

Estimated cost: $800-$1,500 single cylinder, $2,500-$4,500 full system overhaul

Typical onset: 120,000-180,000 mi

Symptoms: Excessive oil consumption, 1 quart per 500-1,000 miles, Blue smoke on startup or acceleration, Fouled spark plugs, misfires on multiple cylinders, Low compression across multiple cylinders when tested

Fix: M113 piston rings wear prematurely if oil changes were extended or wrong oil used. Requires complete engine disassembly, cylinder honing, new pistons and rings, often new bearings while apart. 30-40 hours labor for proper rebuild. Some shops do ring-only jobs but short-lived without addressing bore wear. Many opt for low-mileage used engine swap instead at 15-20 hours labor.

Estimated cost: $6,500-$9,500 full rebuild, $4,000-$6,000 used engine swap

Typical onset: 70,000-120,000 mi

Symptoms: Clunk or thud when shifting from Park to Drive or Reverse, Excessive driveline vibration at idle in gear, Visible sag of transmission tailhousing when inspected on lift

Fix: Rear transmission mount (and often front engine mounts simultaneously) deteriorate from age and heat. Straightforward replacement, 2-3 hours labor for trans mount, recommend doing engine mounts at same time adds 2 hours.

Estimated cost: $400-$700 trans mount only, $800-$1,200 all mounts

Typical onset: 90,000+ mi or age-related

Symptoms: Rear suspension sags overnight or when parked, SLS warning light on dash, suspension-related fault codes, Hydraulic fluid leaks at rear shock absorbers or accumulator, Harsh ride when system fails and cannot level

Fix: SLS system uses hydraulic rear shocks with integrated leveling. Shocks leak internally, accumulators fail, pump can fail. Replacing rear SLS shocks 3-4 hours, pump 4-5 hours. Many owners convert to conventional coil-over setup (aftermarket kits available) to eliminate system entirely, 4-6 hours labor.

Estimated cost: $1,500-$2,800 OEM shock replacement, $1,200-$2,000 coilover conversion

Typical onset: 80,000-150,000 mi

Symptoms: No-start condition, engine cranks but won't fire, Intermittent stalling while driving, restarts after sitting, Check engine light with crank/cam correlation codes, Tachometer drops to zero while driving then car dies

Fix: Heat-related sensor failures common on M113. Crank sensor behind harmonic balancer requires removing balancer, 2-3 hours labor. Cam sensors on cylinder heads, 1-1.5 hours each. Carry spares if touring in this car—failure leaves you stranded.

Estimated cost: $300-$600 depending on which sensor