Engine Controls and Fuel - 6.6L (Lgh) - Description and Operation: Overview GMC Cutaway G3500

Engine Control Module Description

The Engine Control Module (ECM) interacts with many emission related components and systems, and monitors emission related components and systems for deterioration. OBD II diagnostics monitor the system performance and a diagnostic trouble code (DTC) sets if the system performance degrades. The ECM is part of a network and communicates with various other vehicle control modules.

Malfunction indicator lamp (MIL) operation and DTC storage are dictated by the DTC type. A DTC is ranked as a Type A or Type B if the DTC is emissions related. Type C is a non-emissions related DTC.

The ECM is the control center of the engine controls system. Review the components and wiring diagrams in order to determine which systems are controlled by the ECM.

The ECM constantly monitors the information from various sensors and other inputs, and controls the systems that affect engine performance and emissions. The ECM also performs diagnostic tests on various parts of the system and can turn on the MIL when it recognizes an operational problem that affects emissions. When the ECM detects a malfunction, the ECM stores a DTC. The condition area is identified by the particular DTC that is set. This aids the technician in making repairs.

Exhaust Gas Recirculation (EGR) System Description

The Exhaust Gas Recirculation (EGR) System is used to reduce the amount of nitrogen oxide (NOx) emission levels caused by high combustion temperatures. At temperatures above 1, 371°C (2, 500°F), oxygen and nitrogen combine to form NOx. Introducing small amounts of exhaust gas back into the combustion chamber displaces the amount of oxygen entering the engine. With less oxygen in the air/fuel mixture, the combustion pressures are reduced, and as a result, combustion temperatures are decreased, restricting the formation of NOx.

The EGR system on the LGH engine of a single or dual EGR coolers, an EGR valve and a Mass Air Flow (MAF) sensor.

The EGR system on the LML engine applications utilizes dual EGR coolers, an EGR valve, MAF sensor and an EGR cooler bypass valve, controlled by the ECM, to prevent coking of the EGR coolers during light load and idling.

The EGR and EGR cooler bypass valve motors are Direct Current (DC) motors utilizing a multi-stage gear drive connected to the valve stem. The motors are controlled by the ECM and actively force the valves open and closed. Each valve contains an integral valve position sensor which reflects the true position of the valve. The MAF sensor signal is used by the Engine Control Module (ECM) to detect the proper amount of EGR flow. The ECM continually does a consistency check on the indicated EGR flow by comparing the desired MAF to the actual MAF. If the actual MAF is less than or greater than a calibrated threshold level, the amount of EGR flow is determined to be out of range and a DTC will set.

Scheme 187

The EGR valve is controlled by the ECM through the EGR motor high control and EGR motor low control circuits. The ECM supplies voltage that is near ignition voltage to the high and low control circuits at all times. This voltage is used by the ECM as a reference voltage during non EGR operation in order to detect circuit failures. The ECM Pulse Width Modulates (PWM) the low control circuit to ground when the EGR valve is commanded open. A lower pulse width increases the open position of the valve. In order to close the EGR valve, the ECM will PWM the high control circuit to ground.

When the ignition is turned OFF, the EGR valve executes a series of clean and learn operations. The valve goes through a number of full sweeps, (cleaning operation), followed by a number of learn events that pushes the poppet just off the seat and then pulls it into the seat to determine the correct sensor position voltage. Certain environmental conditions, such as low temperature, may inhibit the clean and learn operations, so they may not occur during every ignition cycle.

The EGR valve has an integral position sensor that feeds back to the ECM. The sensor is located on the output gear of the actuator and is a non-contacting type sensor. The ECM uses the position sensor to determine the position of the valve. The ECM sends a reference voltage through the 5-volt reference circuit to the EGR position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the EGR valve position, is sent from the sensor to the ECM through the EGR position sensor signal circuit.

The ECM commands the required valve position based on engine requirements. The outlet of the EGR valve flows directly into the intake manifold.

Scheme 188

The EGR Cooler Bypass Valve (if equipped) is fed off of the passenger side rear exhaust bank. The ECM commands the Bypass Valve to one of two positions - HOT EGR or COLD EGR, depending on engine operating conditions. Under normal to high engine load and temperature conditions the intake system requests COLD EGR and the bypass valve is commanded to the COLD EGR position. The exhaust flow is directed into the two EGR coolers in series and then to the EGR Valve. Under light load/idle conditions, the intake system requests HOT EGR and the bypass valve is commanded to the HOT EGR position. The exhaust flow is directed through the bypass pipe directly into the EGR valve to prevent fouling of the EGR coolers.

When the ignition is turned OFF, the EGR Cooler Bypass Valve executes its own clean and learn operation. The bypass valve switches from seat to seat several times, effectively cleaning the stem while learning the seat position. Certain environmental conditions, such as low temperature, may inhibit the clean and learn operation, so it may not occur during every ignition cycle.

The EGR Cooler Bypass Valve has an integral position sensor that feeds back to the ECM. The sensor is located on the output gear of the actuator and is a non-contacting type sensor. The ECM uses the position sensor to determine the position of the valve. The ECM sends a reference voltage through the 5-volt reference circuit to the position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the valve position, is sent from the sensor to the ECM through the position sensor signal circuit.

Diesel Oxidation Catalyst (DOC) Operation

The DOC functions much like the catalytic converter used with gasoline fueled engines. As with all catalytic converters, the DOC must be hot in order to effectively convert the exhaust HC and CO into carbon dioxide (CO2) and water vapor. On cold starts, the exhaust gases are not hot enough to create temperatures within the DOC high enough to support full HC and CO conversion. The temperature at which full conversion occurs is known as light-off.

In addition to reducing emissions, the DOC also generates the exhaust heat needed by the SCR stage. Exhaust gas temperature (EGT) sensors are located upstream (EGT 1) and downstream (EGT 2) of the DOC. By monitoring the temperature differential between these two sensors, the ECM is able to confirm DOC light-off. Light-off is confirmed by a DOC output temperature that is greater than its input temperature.

In order to generate the high exhaust temperatures needed for regeneration, the aftertreatment system increases exhaust temperatures by injecting diesel fuel directly into the exhaust gases entering the DOC. This is accomplished by means of an ECM-controlled fuel injector, called the hydrocarbon injector (HCI), in the exhaust pipe upstream of the DOC. Injecting fuel directly into the exhaust rather than using a post-injection strategy greatly reduces oil/fuel dilution.

Proper DOC function requires the use of ultra-low sulfur diesel (ULSD) fuel containing less than 15 parts-per-million (ppm) sulfur. Levels above 15 ppm will reduce catalyst efficiency and eventually result in poor driveability and one or more DTCs being set.

Selective Catalyst Reduction (SCR) Operation

While diesel engines are more fuel efficient and produce less HC and CO than gasoline engines, as a rule they generate much higher levels of NOx. In order to meet today's tighter NOx limits, an SCR catalyst, along with DEF, is used to convert NOx into nitrogen gas (N2), carbon dioxide (CO2), and water vapor (H2O).

The ECM uses two smart NOx sensors to control exhaust NOx levels. The first NOx sensor is located in the turbocharger outlet and monitors the engine out NOx. The second NOx sensor is located in the exhaust pipe downstream of the SCR and monitors NOx levels exiting the aftertreatment system. The smart NOx sensors communicate with the ECM over the serial data line. Similar to the way the ECM uses oxygen sensor signals to maintain an optimum air/fuel ratio under various loads in gasoline applications, the ECM uses exhaust oxygen and NOx data from the NOx sensors to maintain the desired air/fuel ratio and to calculate the amount of DEF required to reduce exhaust NOx levels.

The NOx sensors incorporate an electric heater to quickly bring the sensors to operating temperature. As moisture remaining in the exhaust pipe could interfere with sensor operation, the ECM delays turning on the heaters until the exhaust temperature exceeds a calibrated value. This allows any moisture remaining in the exhaust pipe to boil off before it can effect NOx sensor operation. Depending on engine temperature at start up, the delay can be less than a minute or as long as two minutes. Typically, NOx sensor 1 will reach operating temperature faster than NOx sensor 2 as it's closer to the engine's hot exhaust. At idle or low engine speeds, NOx sensor 2 may require up to 5 minutes to reach operating temperature. The sensors must be hot before accurate exhaust NOx readings are available to the ECM.

DEF is a mixture of 66% deionized water and 34% urea. Within the SCR, exhaust heat converts the urea into ammonia (NH3) that reacts with NOx to form nitrogen, CO2, and water vapor. Optimum NOx reduction occurs at SCR temperatures above 250°C (480°F). At temperatures below 250°C, the incomplete conversion of urea forms sulfates that can poison the catalyst. To prevent this poisoning, the ECM suspends DEF injection when exhaust temperature falls below a calibrated limit.

The 6.6L (LML) engine uses exhaust gas temperature management to maintain the SCR catalyst within the optimum NOx conversion temperature range of 200-400°C (390-750°F). The ECM monitors EGT sensors located upstream (EGT 2) and downstream (EGT 3) of the SCR in order to determine if the SCR catalyst is within the temperature range where maximum NOx conversion occurs. The 6.6L (LGH) engine does not use exhaust gas temperature management; the ECM calculates SCR temperature based on the engine speed and load. For LGH applications, SCR temperatures are typically at the lower end of the temperature range under normal driving conditions; however, SCR temperatures will increase when hauling a trailer.

The smart NOx sensors provide a serial data message to the ECM with information on exhaust oxygen levels.

Glow Plug Operation

In the diesel engine, air alone is compressed in the cylinder. Then, after the air has been compressed, a charge of fuel is sprayed into the cylinder and ignition occurs, due to the heat of compression. Eight glow plugs are used as an aid to starting.

Control of the instant start system glow plugs is accomplished by a glow plug control module and 4.7 volt glow plugs, requiring 2 seconds to heat up to 1, 000°C (1, 832°F). The temperature and the power consumption is controlled between the engine control module (ECM) and the glow plug control module within a wide range to suit the engine's pre-heating requirements. Each glow plug is energized individually. This capability yields more optimum heat times for the glow plugs, thus pre-glow times can be kept to a minimum for short wait to crank times and maximum glow plug durability. A DTC will set if there is a glow plug system fault.

A normal functioning system operates as follows

1. Turn the ignition ON with the engine OFF, and at room temperature.
2. The glow plugs turn ON and heat up in 2 seconds and then are pulse-width modulated (PWM) for another 2 seconds.
3. The glow plug wait lamp is ON for 1 second during cold start.
4. The glow plug wait lamp may not illuminate during a warm engine start.
5. If the engine is cranked during or after the above sequence, the glow plugs may cycle ON and OFF after the ignition switch is returned from the start position, whether the engine starts or not. The engine does not have to be running to terminate the glow plug cycling.

The glow plug control module provides glow plug operation after starting a cold engine. This post-start operation is initiated when the ignition switch is returned to Run, from the Start position. This function helps clean up excessive white smoke and/or poor idle quality after starting.

The glow plug initial ON time will vary based on the system voltage and temperature. Lower temperatures cause longer ON times.