Ignition System: Description and Operation

NOTE: The Viper uses a distributorless electronic ignition system. Since the system does not use a distributor, ignition timing is not adjustable.

WARNING: The electronic ignition system generates approximately 40,000 volts. personal injury could result from contact with this system.

IGNITION SYSTEM
The system uses two epoxy filled coil packs mounted on the rear of the engine. The coil pack on the left side, contains three separate coils and fires cylinders 1, 2, 3, 4, 6, and 7. The coil pack on the right side, contains two individual coils and fires cylinders 5, 8, 9, and 10. When one of the coils discharges, it fires two cylinders at the same time (one cylinder on its compression stroke, the other on its exhaust stroke).

At the request of the Powertrain Control Module (PCM), it will energizes the individual coils in each coil pack. The PCM determines cylinder identification from the crankshaft position sensor and camshaft position sensor inputs.

The PCM operates the fuel injection system.

Fig 1 Powertrain Control Module:

POWERTRAIN CONTROL MODULE (PCM)
The PCM regulates the ignition system (Fig. 1). The PCM supplies battery voltage to the ignition coil through the Automatic Shutdown (ASD) Relay. The PCM also controls the ground circuit for the ignition coils. By switching the ground path for the coil ON and OFF, the PCM adjusts ignition timing to meet changing engine operating conditions.

During the crank-start period the PCM advances ignition timing a set amount. During engine operation the following inputs determine the amount of spark advance provided by the PCM.
- Intake air temperature
- Coolant temperature
- Engine RPM
- Available manifold vacuum
- Throttle position sensor

The PCM also regulates the fuel injection system.

Fig 2 Setting Plug Electrode Gap -- Typical:

SPARK PLUGS
The Viper 8.0L V-1O engine uses resistor spark plugs. They have resistance values of 6,000 to 20,000 ohms when checked with at least a 1,000 volt tester.

Remove the spark plugs and examine them for burned electrodes and fouled, cracked or broken porcelain insulators. Keep plugs arranged in the order in which they were removed from the engine. An isolated plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder. Replace spark plugs at the intervals recommended.

Spark plugs that have low mileage may be cleaned and reused if not otherwise defective. Refer to Testing and Inspection/Spark Plug Conditions. After cleaning, file the center electrode flat with a small flat file or jewelers file. Adjust the gap between the electrodes (Fig. 2) to the dimensions specified.

Always tighten spark plugs to the specified torque. Over tightening can cause distortion resulting in a change in the spark plug gap. Tighten spark plugs to 28 Nm (20 ft. lbs.) torque.

SPARK PLUG CABLES
Spark Plug cables are sometimes referred to as secondary ignition wires. The wires transfer electrical current from the electronic ignition coils to the individual spark plug at each cylinder. The nonmetallic spark plug cables have built in resistance. The cables provide suppression of radio frequency emissions from the ignition system.

Check the spark plug cable connections for good contact at the coil and spark plugs. Terminals should be fully seated over the coil or spark plug. The nipples and spark plug covers should be in good condition. Nipples should fit tightly on the coil and spark plug covers should fit tight around spark plug insulators. Loose cable connections permit corrosion that increases resistance.

Fig 3 Ignition Coil Packs:

Fig 4 Ignition Coil Packs:

IGNITION COILS
The electronic ignition system uses two epoxy filled coil packs mounted on a bracket at the rear of the engine (Fig. 3) and (Fig. 4). The coil pack on the left side, contains three separate coils and fires cylinders 1, 2, 3, 4, 6, and 7. The coil pack on the right side, contains two individual coils and fires cylinders 5, 8, 9, and 10. When one of the coils discharges, it fires two cylinders at the same time (one cylinder on its compression stroke, the other on its exhaust stroke).

CRANKSHAFT POSITION SENSOR
The Powertrain Control Module (PCM) energizes the individual ignition coils in each pack. The PCM can determine cylinder identification from the crankshaft position sensor and camshaft position sensor inputs.

Fig 5 Power Distribution Center:

AUTOMATIC SHUTDOWN RELAY
The relay is located in the Power Distribution Center (PDC) (Fig. 5). For the location of the relay within the PDC, refer to the PDC cover for location. Check electrical terminals for corrosion and repair as necessary.

Fig 6 Timing Slots On Crankshaft:

CRANKSHAFT POSITION SENSOR
The crankshaft position sensor detects slots cut into a disk in the middle of the crankshaft (Fig. 6). There are a 5 sets of slots. Each set contains 2 slots, for a total of 10 slots. Basic timing is determined by the position of the last slot in each group. Once the Powertrain Control Module (PCM) senses the last slot, it determines crankshaft position (which piston will next be at TDC) from the camshaft position sensor input. It may take the PCM up to one engine revolution to determine crankshaft position during cranking.

The PCM uses the camshaft reference sensor to determine injector sequence. Once crankshaft position has been determined, the PCM begins energizing the injectors in sequence. The PCM determines ignition timing from the crankshaft timing sensor.

Fig 7 Crankshaft Position Sensor:

The crankshaft position sensor is located in the passengers side of the engine block, below the exhaust manifold (Fig. 7).

Fig 8 Camshaft Position Sensor:

CAMSHAFT POSITION SENSOR
The camshaft position sensor provides cylinder identification to the Powertrain Control Module (PCM) (Fig. 8). The sensor generates pulses. The PCM determines crankshaft position from the camshaft position sensor and crankshaft position sensor inputs. The PCM uses the input to determine fuel injection synchronization and to determine which electric ignition coil to energize.

Fig 9 Camshaft Sprocket:

The camshaft position sensor detects when a step in the camshaft sprocket passes beneath it (Fig. 9). When the sensor detects the step, the input voltage from the sensor to the PCM switches from low (0.3 volts) to high (5 volts). As the step returns away from the sensor, the input voltage switches back to low (0.3 volts).

Fig 10 Camshaft Position Sensor Location:

The camshaft position sensor is mounted in the front of the timing case cover (Fig. 10). The bottom of the sensor is positioned in front of the camshaft sprocket.

Fig 11 Engine Coolant Temperature Sensor:

ENGINE COOLANT TEMPERATURE SENSOR
The engine coolant temperature sensor threads into the cylinder head behind the Power Steering Pump (Fig. 11). The sensor provides an input to the Powertrain Control Module (PCM). As coolant temperature varies, the sensor resistance changes, resulting in a different input voltage to the PCM.

Until the engine reaches normal operating temperature, the PCM demands slightly richer air-fuel mixtures and higher idle speeds.

This sensor is also used for cooling fan control.

INTAKE AIR TEMPERATURE SENSOR
The Intake Air Temperature (IAT) sensor measures the temperature of the air as it enters the engine. The sensor supplies one of the inputs the PCM uses to determine injector pulse-width.

MANIFOLD ABSOLUTE PRESSURE SENSOR
The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor. Intake manifold pressure is a function of barometric pressure. The MAP sensor converts pressure into voltage. The PCM monitors the MAP sensor output voltage. As vacuum increases, MAP sensor voltage decreases proportionately. Also, as vacuum decreases, MAP sensor voltage increases proportionately.

During Key-On, before cranking the engine, the PCM determines atmospheric air pressure from the MAP sensor voltage. While the engine operates, the PCM determines intake manifold pressure relative to atmospheric pressure. Also, atmospheric pressure is updated at wide-open-throttle. Based on MAP sensor voltage and inputs from other sensors, the PCM adjusts spark advance and air/fuel mixture.

Fig 12 MAP Sensor:

The MAP sensor (Fig. 12) mounts to the drivers side intake manifold plenum.

Fig 13 Throttle Position Sensor:

THROTTLE POSITION SENSOR (TPS)
The TPS mounts to the side of the driver side throttle body (Fig. 13). The TPS connects to the throttle blade shaft. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage). The signal represents throttle blade position. As the position of the throttle blade changes, the resistance of the TPS changes.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the powertrain control module) represents throttle blade position. The TPS output voltage to the PCM varies from approximately 0.38 volts to 1.03 volts at minimum throttle opening (idle) to a maximum of 3.1 volts to 4.0 volts at wide open throttle.

Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. The PCM also adjusts fuel injector pulse width and ignition timing based on these inputs.

Fig 14 Ignition Lock Cylinder Detents:

LOCK KEY CYLINDER
The lock cylinder is inserted in the end of the housing opposite the ignition switch. The ignition key rotates the cylinder to 5 different detents (Fig. 14):
- Accessory
- Off (lock)
- Unlock
- On/Run
- Start