Electronic Control Module (ECM)
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| Illustration 1 | g03032197 |
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(1) Connector view from ECM side
(2) Connector view from harness side | |
The ECM bases decisions based on input information and memory information. After the ECM receives the input information, the ECM sends a corresponding response to the outputs. The inputs and outputs of the ECM are connected to the machine harness by two 70 contact connectors (J1 and J2). The inputs and outputs to the ECM can be viewed through the Caterpillar Electronic Technician (Cat ET).
Note: Only the complete ECM is serviced (no lower levels components). The ECM must be replaced if the ECM is damaged. Replace the ECM if a failure is diagnosed.
To aid in diagnostics of certain types of electrical circuits that are controlled by the ECM, an internal "pull up voltage" is connected to ECM switch and sensor signal input contacts. An above normal voltage is internally connected to the ECM signal input circuit through a resister.
During normal operation, the switch or sensor signal will hold the circuit low or at a certain signal amplitude, however, circuit conditions such as a loss of power to the component, a disconnection, or an open circuit will allow the circuit to be pulled high by the ECM pull up voltage. This condition will result in an above normal voltage condition at the ECM contact. As a result, the ECM will activate an FMI 03 (voltage above normal) diagnostic code for the affected circuit.
The types of ECM input circuits that have pull up voltage present are:
- Pulse Width Modulated (PWM) sensor input circuits
- Switch to Ground Input switch input circuits
- Active analog (voltage) input signal circuits
- Passive analog (resistance) input signal circuits
To aid in diagnostics of electrical circuits that are controlled by the ECM, an internal "pull down voltage" is connected to ECM switch to battery type input circuits.
During normal operation, the switch contacts that are allowing the connection to a voltage source will hold the circuit high. When circuit conditions such as a loss of power to the switch supply voltage, a disconnection in the switch circuit or an open circuit will allow the circuit to be pulled low by the ECM pull down voltage. This condition will result in a below normal voltage condition at the ECM contact. As a result, the ECM will activate an FMI 04 (voltage below normal) diagnostic code for the affected circuit.
The Cat Data Link is used to provide a connection for the service tool for troubleshooting, testing, and calibrations. The data link is bidirectional. The data link allows the ECM to receive information. The data link also allows the ECM to send information.
| Chassis ECM Connector J1
Contact Descriptions |
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|---|---|---|
| No. | Type | Function |
| 10 | CAN C Data Link - | CAN C Data Link - |
| 12 | Sourcing Driver Output | A/C Compressor Clutch Solenoid |
| 14 | Switch to Battery Input | A/C Manual |
| 22 | Analog Input | Cab Ventilation Air Temperature Sensor |
| 24 | Switch to Battery Input | A/C Auto |
| 27 | ECM Ground | Ground |
| 30 | Analog Input | A/C EVAP Coil Temperature Sensor |
| 36 | Cab Blower Speed Position | |
| 37 | Cab Air Temperature Control Position | |
| 44 | Sensor Power Output | +8 V sensor supply |
| 45 | Sensor Power Return | 8 V sensor return |
| 53 | Analog Input | Cab Air Temperature Sensor |
| Chassis ECM Connector J2
Contact Descriptions |
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|---|---|---|
| No. | Type | Function |
| 64 | CAN Data Link (+) | CAN B+ |
| 65 | CAN Data Link (-) | CAN B- |
| 67 | CAN Data Link (+) | CAN A+ |
| 68 | CAN Data Link (-) | CAN A- |
The machine has several different types of input devices. The ECM receives machine status information from the input devices and determines the correct output action that is needed to control machine operations based on memory and software parameters. The machine utilizes the following types of inputs: switch type and sensor type.
Switches provide signals to the switch inputs of the ECM. The possible outputs of a switch are listed: an open signal, a grounded signal and + battery signal.
Sensors provide an electrical signal to the ECM that constantly changes. The sensor input to the ECM can be one of several different types of electrical signals such as: pulse width modulated (PWM) signals, voltage signals, and frequency input signals. Each possible input to the ECM is listed in the tables for the 70-pin connectors.
Inputs provide information to the ECM in the form of sensors or switches.
Sensors provide information to the ECM about the intent of the operator or changing conditions. The sensor signal changes proportionally to the changing of operator input or changing conditions. The following types of sensor signals are used by the ECM.
Frequency - The sensor produces a signal and the frequency (Hz) varies as the condition changes.
Pulse width modulated - The sensor produces a signal. The duty cycle of the signal varies as the condition changes. The frequency of this signal is constant.
Analog - The ECM measures the voltage that is associated to a specific condition of the control.
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| Illustration 2 | g06225165 |
The operator temperature control sensor monitors the signal from the temperature control knob on the HVAC panel. This knob sets the operators desired temperature in automatic mode and adjusts the water valve in manual mode. In either mode, the ECM receives a pulse width modulated signal from the knob and HVAC panel that corresponds to a desired temperature or water valve position. The duty cycle is proportional to the position of the knob.
Switches provide an open signal, a ground signal, or a +battery signal to the inputs of the ECM. Switches are open or closed.
- When a switch is open, no signal is provided to the corresponding input of the ECM. The “no signal” condition is also called “floating”.
- When a switch is closed, a ground signal or a +battery signal is provided to the corresponding input of the ECM.
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| Illustration 3 | g06225171 |
The air conditioning mode switch is a two pole switch. The switch has two positions that control the operation of the air conditioning. In the automatic position the ECM controls the operation of the air conditioning to maintain a specified cab temperature. In the manual position, the air conditioning is manually engaged. The ECM has two contacts that pertain to this switch, one signals the ECM the switch is automatic mode, the other signals the ECM the switch is in manual mode. These contacts will float to a high voltage state unless the switch is closed to that contact. When the switch closes to a contact, the contact is pulled to a low or ground voltage state.
Air Conditioning Low Pressure Switches
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| Illustration 4 | g03350773 |
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(1) High-pressure side low-pressure switch
(2) Low-pressure side low-pressure switch (A) To ECM contact | |
The air conditioning low-pressure contact at the ECM monitors the voltage state of the high and low-pressure cutoff switches for the air conditioning. The switches are set up in series from a power source. The switches close and allow power to pass when the pressure in the high pressure and low pressures air conditioning loops is high exceeds the actuation pressure for the switch. If both switches have been actuated, both switches are closed and the ECM contact is pulled to a low or ground voltage state. If either switch has not been actuated, the ECM contact is pulled to a high voltage state.
The ECM responds to decisions by sending electrical signals to the outputs. The outputs can create an action or the outputs can provide information to the operator or the service technician.
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| Illustration 5 | g03091818 |
The water valve actuator controls the amount of heat that enters the HVAC system. When the water valve actuator is activated, the water valve opens and allows heat from the engine into the heater coils. The water valve actuator is controlled by a pulse width modulated signal from the Machine ECM.
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| Illustration 6 | g03352237 |
The AC compressor solenoid is an on/off solenoid. The ECM sends a signal to active the compressor when the ECM has determined that the AC compressor should be active. The solenoid controls the movement of a clutch engaging and disengaging the AC compressor.
Electronic communication between the ECM, Power Train ECM, Engine ECM, and the other control modules on the machine is conducted over data link circuits. The data link circuits allow the sharing of information with other electronic control modules. The data link circuits are bidirectional. The data link circuit allows the ECM to send information and to receive information.
The electronic communication system consists of multiple CAN datalink systems.
The SAE J1939 CAN Data Link circuit is separated into different groups. "CAN A" is connected to all the ECMs on the machine, and is used primarily for information and service purposes, such as the Caterpillar Electronic Technician (CAT ET). The other data link systems are high speed data links used for machine control purposes.