The Caterpillar Electronic System is a complete electronic control system for gas engines. The following benefits are the most significant advantages of the electronic system:
- Air/Fuel ratio control
- Extensive system diagnostics
- Elimination of parts that are prone to mechanical wear
- Precise control of engine operation
- Engine protection
- Timing control of individual cylinders
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| Illustration 1 | g00527903 |
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Junction box | |
Most of the functions of the electronic system are provided by the Electronic Control Module (ECM). The ECM is a sealed unit that is located inside a junction box.
Five primary functions are supported by the ECM:
- Governing of the engine rpm
- Control of ignition
- Control of the air/fuel ratio
- Start/stop control
- Monitoring of engine operation
The ECM maintains the desired engine speed through electrical control of the throttle. The throttle regulates the throttle plate.
For more information on the desired speed input, see the Systems Operation/Testing and Adjusting manual.
Each cylinder has an ignition transformer that is located under the valve cover. To initiate combustion in each cylinder, the ECM sends a pulse to the primary coil of the ignition transformer. The transformer increases the voltage which creates a spark across the spark plug electrode.
The transformers are grounded through the valve cover. Use caution when a valve cover is removed.
Always disconnect the ignition harness from the transformer when a valve cover is removed.
An ignition harness connects each transformer to the ECM. The harness is routed inside the engine.
The ECM determines the desired volume for the flow rates of the air and fuel. The ECM sends information on the desired flow of air and fuel to the fuel metering valve. The ECM fine tunes the signal to the fuel metering valve in order to achieve the desired emissions. The process is repeated continuously during engine operation.
The ECM contains the logic and the outputs for controlling the starting and stopping of the engine. The logic for starting and stopping can be programmed by the customer.
The engine uses an energize-to-run system. The gas shutoff valve must remain energized in order to supply fuel to the engine. If power is removed from the gas shutoff valve, the fuel is shut off.
Sensors are used in order to monitor engine operation. Wiring harnesses connect the sensors to the ECM. The ECM uses information from the sensors in order to monitor the engine. The ECM also uses information from the sensors in order to control the engine. The information is also used to generate event codes, and diagnostic codes.
Note: The codes can be read with Caterpillar Electronic Technician (ET). The codes can also be read on the Machine Information Display System (MIDS) panel, if equipped.
Event - An event is a result of abnormal engine operation. If abnormal engine operation is detected, the ECM generates an event code. The ECM can generate an alarm or a shutdown for abnormal engine operation. These conditions are some examples of events: high inlet air temperature, low oil pressure and engine overspeed.
Diagnostic - A diagnostic code is a result of a problem with the operating system or with the monitoring system. The ECM uses sensors and internal circuitry to monitor the system components. If a problem develops in a component or a wiring harness, the control system will sense the problem. The control system will notify the operator by creating a diagnostic code. Some examples of conditions that activate diagnostics are a short in a circuit for a sensor, an open circuit, or a noisy signal.
Note: For detailed information on event codes and diagnostic codes, refer to the engine's Troubleshooting manual.
Sensors provide information to the ECM. The information enables the ECM to control the engine as efficiently as possible over a wide range of operating conditions. The information is used for monitoring engine operation and for engine protection.
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View of the left side of a typical Caterpillar 3520B Engine (1) Sensor for the pressure of the filtered oil (2) Sensor for the pressure of the unfiltered oil (3) Sensor for the pressure of the coolant at the outlet of the water jacket (4) Speed/timing sensor (5) Detonation sensors | |
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| Illustration 3 | g01449644 |
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View of the rear of a typical Caterpillar 3520B Engine (6) Sensor for the oil temperature (7) Sensor for the manifold air pressure (8) Sensor for nitrogen oxide | |
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View of the right side of a typical Caterpillar 3520B Engine (9) Sensor for the temperature of the coolant in the water jacket (10) Sensor for the temperature of the air at the inlet manifold | |
Note: The sensor for atmospheric pressure may be in the junction box on the left side of the engine. The sensor for atmospheric pressure may be at the rear of the engine above the exhaust housing.
The functions of the sensors are described below.
Oil pressure sensors (1) and (2) - The engine oil pressure is measured before the oil filters and after the oil filters. An alarm or a shutdown can be activated by any of the following occurrences: low filtered oil pressure, low oil filter differential pressure and high oil filter differential pressure.
Jacket water pressure sensor (3) - A pressure switch is located at the outlet for the engine jacket water. If the outlet pressure is too low, the ECM will activate a shutdown.
Speed/timing sensor (4) - The engine speed/timing sensor is located on the rear end of the left camshaft. The engine speed/timing sensor provides accurate information about the position of the crankshaft and the engine rpm to the ECM. The ECM uses the position of the crankshaft in order to determine ignition timing.
Detonation sensors (5) - The detonation sensors monitor the engine for detonation in each cylinder. To eliminate detonation, the ECM retards the timing of the cylinder. If excessive detonation continues, the ECM will shut down the engine.
Engine oil temperature sensor (6) - An oil temperature sensor monitors the engine oil temperature. A high oil temperature will activate an alarm or a shutdown. The ECM compares the oil temperature to the engine coolant temperature. A high difference between the two temperatures will activate an alarm or a shutdown.
Manifold air pressure sensor (7) - The manifold air pressure sensor is connected to the air inlet manifold. The sensor monitors the absolute manifold air pressure. The information is used by the ECM to determine the engine load.
Sensor for nitrogen oxide (8) - The sensor for nitrogen oxide and the buffer for nitrogen oxide work together in order to generate a signal that is proportional to the parts per million of nitrogen oxide in the exhaust manifold. The control system uses the signal to adjust the air/fuel ratio in order to achieve the level of emissions that is desired.
Sensor for the temperature of the jacket water coolant (9) - The temperature sensor is located in the water temperature regulator housing. To monitor the coolant temperature, the element must be in contact with the coolant. If overheating occurs due to low coolant level, the sensor will not function properly. A high coolant temperature will activate an alarm, a derating, or a shutdown. A low coolant temperature will only activate an alarm. The setpoints for the activation can be programmed with the ET. The engine can be restarted after a shutdown due to high engine coolant temperature. However, another shutdown will occur after one minute if the temperature remains high.
Inlet air temperature sensor (10) - A sensor for monitoring the air inlet temperature is located in the elbow before the number one cylinder head. Excessive inlet air temperature can activate an alarm, a derating, or a shutdown.