5033372 JOHNSON *DIAPHRAGM SET

Illustration 1 g00905946
Schematic of the air/fuel ratio control
(1) ECM in a remote panel
(2) Manifold air pressure sensor
(3) Oxygen buffer
(4) Oxygen sensor
(5) Inlet manifold temperature sensor
(6) Actuator
(7) Fuel valve
(8) Caterpillar Electronic Technician (Cat ET)
(9) Digital Diagnostic Tool (DDT) ECM (1) is the main component of the system. For the air/fuel ratio control, three inputs are provided to the ECM by sensors on the engine: inlet manifold air pressure, inlet manifold air temperature and exhaust oxygen level. The ECM uses the inputs to calculate the desired fuel flow.Manifold air pressure sensor (2) is mounted on the engine. The sensor measures the engine's inlet manifold air pressure (absolute pressure). The ECM uses the information to calculate the engine load.Oxygen buffer (3) is mounted on the engine. The buffer provides an interface between oxygen sensor (4) and the ECM. The buffer controls the sensor's heater and the supply of voltage to the sensor. The buffer also converts the sensor's output current into a signal that is sent to the ECM. The ECM converts the duty cycle into a percent of oxygen.Oxygen sensor (4) is mounted in an adapter ring on the exhaust elbow. The sensor measures the percent of oxygen in the engine's exhaust. The exhaust oxygen level is an indication of the exhaust emissions. The sensor has a heater that is used during calibration. Voltage is input to the sensor and a current signal is output by the sensor to the oxygen buffer.The ECM is programmed with a map of desired exhaust oxygen versus the inlet manifold air pressure. The map is used to adjust the percent of desired exhaust oxygen according to the calculated engine load.Inlet manifold temperature sensor (5) monitors the temperature of the air/fuel mixture after the aftercooler. The ECM compares information from the inlet manifold temperature sensor to a programmed map. The map is used to offset the percent of desired exhaust oxygen for various inlet manifold temperatures in order to maintain a constant level of NOx.For example, the desired exhaust oxygen can be increased by 0.016 percent for each 1 °C (1.8 °F) of temperature that is greater than 45 °C (113 °F). The desired exhaust oxygen can be decreased by 0.016 percent for each 1 °C (1.8 °F) of temperature less than 45 °C (113 °F). In either case, the maximum adjustment is approximately 0.7 percent.The ECM sends a command to actuator (6) in order to move fuel valve (7). The fuel valve is located between the gas pressure regulator's outlet and the carburetor's fuel inlet. The quantity of fuel that is delivered to the carburetor is determined by the position of the fuel valve. The actuator sends a signal to the ECM in order for the ECM to monitor the position of the actuator.The ECM monitors the electrical systems of each component. The ECM will generate a diagnostic code and a warning if there is a problem with an electrical circuit. The warning is indicated by an LED on the display of the ECM.The diagnostic code can be read on the ECM or with either of these electronic service tools: Cat ET (8) and DDT (9).The signal of the oxygen sensor's duty cycle must be calibrated in order for the ECM to monitor the actual percent of oxygen in the exhaust. The Cat ET or the DDT is used for calibration of the oxygen sensor. The desired percent of exhaust oxygen can be programmed with either electronic service tool.The air/fuel ratio control is a closed loop system. The inlet manifold air pressure, the inlet manifold air temperature, and the exhaust oxygen are continuously monitored. The information is compared to the programmed map of desired oxygen versus inlet manifold air pressure. The map is offset by the map for the inlet manifold temperature. The flow of fuel is controlled in order to maintain the desired exhaust emissions. The system maintains a consistent level of NOx under various operating conditions.Generally, as the air/fuel mixture becomes more lean the exhaust emissions and temperatures are reduced. The fuel consumption can increase slightly.As the air/fuel mixture is richened, the exhaust emissions, temperatures, and power are increased. If the air/fuel mixture is too rich, detonation can occur.A change in the fuel energy content requires changes in the air/fuel mixture in order to maintain the desired exhaust oxygen. If the fuel energy content becomes greater, the air/fuel ratio control will move the fuel valve in order to maintain the desired exhaust emissions.