2004 Mercedes-Benz ML500 Spark Plug Wire Replacement Cost

Spark plug wires are conduits for the transfer of high voltage electricity from the ignition coil to the spark plugs. Most vehicles typically have four to eight spark plug wires connected to the ignition distributor cap.

The distributor rotor receives the high voltage electricity from the ignition coil wire, and sends it through the distributor through an electrode on the tip of the rotor. As the rotor spins, electrical power is transferred from the rotor to individual electrodes inside the distributor cap. Once that high voltage reaches the distributor cap, the corresponding spark plug wire will carry that high voltage to the spark plug terminal. This action is timed for the moment the cylinder needs the spark plug to fire, and the sequence creates a combustion event inside the engine.

A failing ignition wire set will create misfires in the engine, and lead to hesitation, extreme power loss, engine stalling, poor fuel mileage, and illumination of the check engine light. The on-board diagnostics (OBD) trouble codes stored in the vehicle's computer will reflect engine misfires, catalyst inefficiency, and O2 sensor related errors. Under the hood there may be audible arching, that sounds like a rubber band snapping, coming from the ignition coil, distributor, or spark plug ends of any of the spark plug wires. Lastly, the ignition distributor cap and rotor may look burned, wet, corroded, or covered in a powder residue, and the spark plug wires may be cracked or degraded.

Ignition system components cause many problems when they fail, normally making the vehicle unpleasant or impossible to drive. Also, the amount of unburned fuel going into the catalyst could cause the catalyst to fail, requiring replacement. With severe ignition problems, the vehicle should be towed to a repair facility. Get this fixed by a RepairPal Certified Shop

The ignition wire set is a normal maintenance item, and the replacement schedule is listed in the owner's manual. If the ignition wires, or "spark plug" wires, wear out before the specified mileage, they must be replaced early. When not specified by the manufacturer, the average set of spark plug wires last between 60,000 and 100,000 miles, depending on application and operating environment.

Diagnosing any of the symptoms mentioned, for a distributor type ignition system, will normally being by checking all ignition components for abnormal conditions that may be indicative of failure. If nothing is found, the spark plug wires, distributor cap and rotor, and spark plugs, more or less in that order, will be removed, inspected, and diagnosed. If the spark plug wires are too resistant to electricity, the spark plugs are worn or fouled, or the distributor and rotor are worn, corroded, or oil fouled, those components would be diagnosed as faulty.

If the spark plug wires are being changed, the distributor and rotor will be changed, unless they have been changed according to scheduled maintenance intervals, or they appear to have service life remaining. Likewise, and with the same exceptions, if the distributor cap and rotor will be replaced, the spark plug wires will be replaced as well. The spark plug wires will be marked for their location, and the new spark plug wires will be assembled with the spark plugs and ignition distributor cap using those markings. After the ignition distributor rotor is popped into place, the distributor cap will be installed, and the spark plug wires attached to mounting hardware.

If completing this job as a DIY repair, mark the spark plug wires according to which position they sit on the distributor, and to which cylinder they supply power. When having these items changes at a repair facility, ask what type of parts are being used, if they conform to OEM standards, and specify if you prefer to use parts from the manufacturer.

Some spark plug wires are better than others at getting high voltage current to the spark plugs efficiently. Purchasing ignition parts that are built to exact OEM standards or better is recommended. Purchasing inferior parts may lead to a repeated repair.

This is a great task to select as a DIY job. It is an easy repair as long as you take your time, and put all of the spark plug wires in the correct place before removing the old spark plug wires. The distributor cap and rotor are normally replaced with spark plug wires. It is a relatively cheap and easy service on most vehicles.