The atmospheric pressure sensor (arrow) is located adjacent to the Engine ECM. The Engine ECM uses the atmospheric pressure sensor as a reference for calculating boost and air filter restriction.
The sensor is also used for derating the engine at high altitudes. The ECM will derate the engine at a rate of 1% per kPa to a maximum of 20%. Derating begins at a specific elevation. The elevation specification can be found in the Technical Marketing Information (TMI) located on the Caterpillar Network. If the Engine ECM detects an atmospheric pressure sensor fault, the ECM will derate the fuel delivery to 20%. If the Engine ECM detects an atmospheric and turbocharger inlet pressure sensor fault at the same time, the ECM will derate the engine to the maximum rate of 40%.
The Engine ECM also uses the atmospheric pressure sensor as a reference when calibrating all the pressure sensors.
The atmospheric pressure sensor is one of the many analog sensors that receive a regulated 5.0 ± 0.5 Volts from the Engine ECM. The atmospheric pressure sensor output signal is a DC Voltage output signal that varies between 0.2 and 4.8 Volts DC with an operating pressure range between 0 and 111 kPa (0 and 15.7 psi).
To check the output signal of analog sensors, connect a multimeter between Pins B and C of the sensor connector. Set the meter to read "DC Volts." The DC Voltage output of the atmospheric pressure sensor should be between 0.2 and 4.8 Volts DC.
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The engine speed/timing sensor (1) is positioned near the rear of the left camshaft. The sensor signals the speed, direction, and position of the camshaft by counting the teeth and measuring the gaps between the teeth on the timing wheel which is mounted on the camshaft.
The engine speed/timing sensor is one of the most important inputs to the Engine ECM. If the Engine ECM does not receive an input signal from the engine speed/timing sensor, the engine will not run.
The engine speed/timing sensor receives a regulated 12.5 ± 1.0 Volts from the Engine ECM. To check the output signal of the speed/timing sensor, connect a multimeter between Pins B and C of the speed/timing sensor connector. Set the meter to read "Frequency." The frequency output of the speed/timing sensor should be approximately:
- Cranking: 23 to 40 Hz - Low Idle: 140 Hz - High Idle: 385 Hz
A passive (two wire) engine speed sensor (2) is positioned on top of the flywheel housing. The passive speed sensor uses the passing teeth of the flywheel to provide a frequency output. The passive speed sensor sends the engine speed signal to the Transmission/Chassis ECM and the Brake ECM.
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2
- Automatic Retarder Control (ARC) engine control speed - Shift time calculations
- Transmission Output Speed (TOS) ratification
The output signal of the passive speed sensor can also be checked by connecting a multimeter between the two pins of the speed sensor connector and setting the meter to read frequency.
NOTE: Turn ON the engine shutdown switch (see Visual No. 25) during the cranking test to prevent the engine from starting. The cranking speed and frequency output will vary depending on weather and machine conditions. When viewing engine speed in the ET status screen, cranking speed should be between 100 and 250 rpm.
The throttle position sensor (arrow) provides the desired throttle position to the Engine ECM. If the Engine ECM detects a fault in the throttle position sensor, the throttle back-up switch (see Visual No. 48) can be used to increase the engine speed to 1300 rpm.
The throttle position sensor receives a regulated 8.0 ± 0.5 Volts from the Engine ECM. The throttle position sensor output signal is a Pulse Width Modulated (PWM) signal that varies with throttle position and is expressed as a percentage between 0 and 100%.
To check the output signal of the throttle position sensor, connect a multimeter between Pins B and C of the throttle position sensor connector. Set the meter to read "Duty Cycle." The duty cycle output of the throttle position sensor should be:
- Low Idle: 16 ± 6%
- High Idle: 85 ± 4%
NOTE: The throttle position must be programmed to the 10 to 90% setting. The earlier trucks must be programmed to a 10 to 50% throttle position. The setting is changed in the Engine ECM configuration screen with ET.
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Shown is the top of a cylinder head with the valve cover removed. The most important output from the Engine ECM is the Electronic Unit Injection (EUI) injector solenoid (arrow). One injector is located in each cylinder head. The engine control analyzes all the inputs and sends a signal to the injector solenoid to control engine timing and speed.
Engine timing is determined by controlling the start and end time that the injector solenoid is energized. Engine speed is determined by controlling the duration that the injector solenoid is energized.
3500B injectors are calibrated during manufacturing for precise injection timing and fuel discharge. After the calibration, a four-digit "E-trim" code number is etched on the injector tappet surface. The E-trim code identifies the injector’s performance range.
When the injectors are installed into an engine, the trim code number of each injector is entered into the personality module (software) of the Engine ECM using the ECAP or ET service tool.
The software uses the trim code to compensate for the manufacturing variations in the injectors and allows each injector to perform as a nominal injector.
When an injector is serviced, the new injector’s trim code should be programmed into the Engine ECM. If the new trim code is not entered, the previous injector’s characteristics are used. The engine will not be harmed if the new code is not entered, but the engine will not provide peak performance.
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The 3500B engines have many improvements over the original 3500 engines. Some of the improvements are accomplished by adding additional switch and sensor inputs to the Engine ECM. Adding additional inputs allows the ECM to control the engine more precisely.
Additional inputs to the 3500B ECM are:
- Coolant flow is monitored (see Visual No. 74).
- Rear aftercooler temperature is measured (see Visual No. 78).
- Engine oil level is monitored (see Visual No. 23).
- Two turbocharger temperature sensors measure exhaust temperatures (see Visual No. 92).
- Two engine oil pressure sensors are located on the oil filter base to measure oil pressure and oil filter restriction (see Visual No. 81.
- Engine fan speed is measured (with variable fan speed attachment).
- Fuel filter restriction is monitored (see Visual No. 86).
- Air conditioner compressor pressure is monitored (for variable fan speed control) (see Visual No. 62).
- Engine crankcase pressure is measured (see Visual No. 63).