Tip & How-To about Mercedes-Benz E-Class
3) Spark: From battery to spark plug
The battery is the source of stored electrical power in the vehicle. A voltmeter between the terminals of a healthy battery should read 12.5 volts. A reading of 12.2 volts indicates a flat battery and a reading of 10.4 volts indicates a dead cell (each cell beingabout 2 volts). A battery should be capable of sustaining high current, as needed by the starter motor when turning the engine over. When the car is running the voltage measured across the terminals shouldbe between 14.2 and 14.5 volts. The value is the voltage being supplied to the battery by the output charge from the alternator. THis charging voltage should not alter much when additional load is applied to the battery (head lights on etc).
When the ignition key is turned to the second position,often referred to as 'key on engine off', all car functions are powered up including lights and fuel pump. When the key is turned to position 3 many ancillary functions are temporarily interrupted to allow the battery to meet the extra needs of the starter motor. Electrical charge from the starter switch passes through a master fuse and starter relay before energizing the starter solenoid. The starter solenoid moves the pinion gear on the starter motor to engage with peripheral teeth of the flywheel. Simultaneously, heavy copper contacts are bridged by this same movement causing the starter motor coils to become energized thereby turning the engine over.
During the ignition process electrical charge passes through the primary windings of the ignition coil(s). The engine control unit (ECU), taking signalsfrom the engine crank position sensor, collapses the field of the coil(s) appropriate to the cylinder requiring ignition. As the field collapses the secondary coil windings discharge very high voltage through the high tension lead to the spark plug. The charge is so high that at the spark plug gap the charge jumps across with a spark. The compressed fuel air mixture in the cylinder is ignited by this spark.
Checking the spark process starts with the plugs. The condition of the removed plugs reveals a lot about the nature of the ignition within the cylinders. Dark sooty deposits indicate overly rich fuel air mix and glazed plugs indicate overly hot lean mixtures. Removing the plugs (with the fuel pump relay unplugged) and holding the screw threads to the cylinder head whilst turning the engine over will reveal the quality of spark. The spark holders and leads should be examined for discharge through the insulation to the cylinder head. Removing the leads from the coil should reveal sparks emanating from the exposed connectors. Failure to see sparks at this point indicatesa failed coil (especially if neighboring coils show good spark) or a failed crank sensor if all coils show no spark.
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Posted by Joseph... on
Mar 07, 2013 | Cars & Trucks
Jan 07, 2013 | 2000 Kia Spectra
Use your crescent wrench to disconnect the positive and negative battery
cables. Be careful not to touch both battery posts together with your wrench,
as this will damage the battery and possibly harm you.
Use your spark plug socket to gently grab the insulated end of the spark
plug wire; pull it away from the tip of the plug. The spark plug socket looks
like a regular set of pliers, except that the jaws have rubber inserts to grip
the insulation without damaging it. The plug wire itself looks like a 15-gauge
wire with a rubber boot on each end: One end is connected to the spark plug
present in each of the eight cylinders, while the other end is connected to a
coil pack terminal, usually located on the right rear side of the engine
compartment. The coil pack is a square, black, solid-state component that has
eight terminals for each of the eight plug wires.
Use your spark plug socket to gently pull the coil pack end of the plug
wire away from the terminal. Repeat the last two steps until all eight wires
Consult the service manual for your vehicle's year model and determine
the exact firing order. You will need to connect each new wire to the numbered
coil pack terminal and the corresponding engine cylinder. Failure to replace
the wires in the right order will result in failure of engine operation.
Use your crescent wrench to reconnect the battery cables (black wire to
black terminal, red wires to red terminal).
Start the vehicle and make sure it is running smoothly. If you hear
misfiring in the engine, use the spark plug wrench to make sure all the
connections are properly seated. If the condition continues, you will need to
further diagnose the electrical system.
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