1994 MERCEDES-BENZ S320 W140

Typical onset: any mileage, typically 25+ years old

Symptoms: Random electrical gremlins: windows, locks, gauges acting erratically, Check engine lights with multiple intermittent codes, Engine stalling or rough running from sensor failures, Insulation crumbling to dust when you touch harnesses

Fix: Mercedes used soy-based insulation that biodegrades. Full engine harness replacement is 12-16 hours, requires removing intake manifold and accessories. Chassis harness adds another 8-12 hours if affected. Many shops won't tackle it—find a W140 specialist.

Estimated cost: $2,500-5,000

Typical onset: 80,000-150,000 mi

Symptoms: Transmission fluid in coolant reservoir (looks like strawberry milkshake), Coolant in transmission causing slipping and overheating, Sudden transmission failure after coolant mixing, White smoke from exhaust if coolant reaches combustion chambers

Fix: The metal cooler lines rust through where they pass through subframe. If caught early, replace lines and flush both systems (6-8 hours). If mixed fluids circulate, you're looking at transmission rebuild AND potential head gasket failure from coolant contamination—easily 30+ hours combined.

Estimated cost: $800-1,200 (lines only) or $6,000-12,000 (full carnage)

Typical onset: 120,000-180,000 mi

Symptoms: Coolant loss with no visible leaks, White exhaust smoke on cold starts, Rough idle and misfires on cylinders 1-3, Oil cap shows milky residue, Overheat condition from failed temp sensors (harness issue)

Fix: Head gaskets fail partly due to undetected overheating from bad temp sensors (wiring harness again). Head gasket job on M104 is 18-22 hours—requires timing chain work, resurfacing heads. If you're in there, replace both gaskets, timing components, and the engine harness. Half-measures guarantee you're back in a year.

Estimated cost: $4,500-7,500

Typical onset: 100,000-150,000 mi

Symptoms: Rear end sags when parked, pumps up when started, Constant pump cycling or whining noise, SLS warning light on dash, Accumulators leak, struts weep green hydraulic fluid

Fix: Rear hydraulic struts and accumulators fail predictably. Each strut is 2-3 hours, accumulators another 2 hours. Pump failures add 4-5 hours. Many owners convert to Arnott coil springs ($800 kit, 4-6 hours labor) to eliminate the system permanently.

Estimated cost: $2,000-3,500 (OE repair) or $1,200-1,800 (coil conversion)

Typical onset: 90,000-130,000 mi

Symptoms: Clunk when shifting from Park to Drive, Excessive vibration at idle, Harsh or delayed shifts, especially 2-3 upshift, Transmission stays in limp mode or won't shift properly

Fix: Rubber transmission mounts collapse (2-3 hours to replace). The 722.6 electronic conductor plate inside trans fails from heat cycling—requires dropping transmission, 8-10 hours. Conductor plate symptoms often mimic other trans issues, so misdiagnosis is common.

Estimated cost: $400-700 (mounts) or $1,800-2,800 (conductor plate)

Typical onset: any mileage, heat-cycled to death

Symptoms: No-start condition, cranks but won't fire, Intermittent stalling when engine is hot, Random misfires and rough running, Check engine light with crank/cam correlation codes

Fix: These sensors are buried and heat-soaked. Crank sensor requires removing harmonic balancer (4-5 hours). Cam sensor on back of head near firewall (3-4 hours). Do both if one fails—you're already paying for access. Aftermarket sensors often fail quickly; use OE Bosch.

Estimated cost: $600-1,200

Typical onset: any mileage, age-related

Symptoms: Hard starting or extended cranking, Stumbling and hesitation under acceleration, Stalling at idle or when hot, No fuel pump prime when key turns on

Fix: The overvoltage protection relay (OVP, behind passenger kick panel) fails and kills fuel pumps. Relay is 0.5 hours. Fuel pumps are in-tank, dual setup—tank drop required, 4-6 hours. Distributor caps crack and rotors corrode (2 hours). Fuel filter is every 30k miles or it starves injectors.

Estimated cost: $150-300 (relay/filter) or $1,200-1,800 (pumps)