Engine Controls - ECM Testing and Notes
Classification:EL93-028
Section:
Electrical System
Reference:
TECHNICAL BULLETIN NTB93-161
Models:
All Models
Date:
November 5, 1993
ELECTRONIC CONTROL UNITS
Electronic control units (ECU's) control the operation of numerous electrical systems and components on Nissan vehicles. The ECU's do this by either controlling the supply voltage or ground path of an electrical component. The following is an explanation of how these systems work and what precautions should be taken while performing diagnostic procedures.
Systems controlled by electronic control units have three (3) basic requirements, Inputs, Controller Logic and Outputs.
Inputs
Sensors and switches provide electrical signals to the control unit. These signals are provided to the ECU to inform it of various vehicle operating conditions. The electrical inputs can be either analog or digital signals. However, all of the signals are converted to digital signals before the ECU can process the information.
Control Logic
The ECU uses the electrical input signals to determine what the system should do. This decision may be based on a few or many inputs. After processing the information the ECU will provide a signal (output) to various electrical components.
Outputs
The signal that the ECU puts out to an electrical component (actuator) is called the output. These output signals are what operate and control the system.
Digital Input Signals
Digital signals are ON-OFF (Hi-Low) voltage pulses. This signal is a typical digital signal.
Examples of common digital signals are:
Battery Voltage
The ignition switch in the ACC or ON position provides a battery voltage signal to several control units. This informs the control unit of the ignition position.
Switch to Ground
A door switch is a common example of a switch to ground. When the door is opened the door switch completes the circuit. The interior light is turned on. In addition, the time control unit is sent an OFF signal indicating the door is open.
Pulse Voltage
A pulse voltage is made up of alternating high and low voltage signals of varying duration. These signals are counted by the control unit within a predetermined time frame.
Analog Input Signals
An analog signal is a voltage that varies over a period of time. Analog signals are produced by sensors in the vehicle. Sensors change resistance to deliver variable voltage signals. The Oxygen Sensor sine wave signals shown are typical analog signals. These charts are example signals printed using CONSULT.
^ The first chart shows an Oxygen Sensor lean (low voltage) condition.
^ The second chart is a normal Oxygen Sensor voltage printout.
^ The third chart illustrates an Oxygen Sensor rich (high voltage) condition.
Examples of common sensors are:
Temperature Sensors
Temperature sensors change resistance with a change in temperature. This temperature is converted into an analog voltage range.
Vehicle Speed Sensor
The vehicle speed sensor produces an alternating voltage signal. This alternating signal (analog) is converted to a digital signal for processing.
The control unit can only process digital signals. Analog signals must be converted to a digital form before the information can be used by the control unit.
Control Unit Logic
The control unit processes digital signals in two forms. To the control unit the signal is either ON or OFF. In computer logic an ON or high voltage signal is a "1" and an OFF or low voltage signal is a "O". The control unit uses combinations of these signals to control its outputs. The controller uses inputs from many sensors to determine what outputs it will make and at what time it will make them. These outputs control a variety of functions from automatic speed control to engine driveability.
Outputs
An output is simply an action the control unit tells an electrical component to make. Based on the calculations the control unit makes of the inputs, it will signal an action. The output can tell a component to perform a function for a period of time.
Power and Ground Control
The ECU controls either a component's power or ground. To understand this, refer to the diagrams.
If the ECU is between the electrical component (load) and the power source, the ECU controls its power.
^ If the component is between the power source and the ECU, the ECU controls the component ground.
VOLTAGE DROP TEST
Service and Diagnosis Precautions
Before diagnosing any electronic controlled circuits the following cautions should be followed:
1. Make sure the following parts and systems are in good condition and are operating properly.
^ Battery output
^ Battery terminals
^ Grounds
^ System connectors are properly positioned and connected
2. Never use a test lamp to check an electronically controlled circuit.
3. Do not drop, jar or shake electronic components.
Follow the trouble diagnosis charts in the appropriate section of the Service Manual to isolate which component may be the incident part.
Voltage Drop Tests
Voltage Drop Tests are often used to find components or circuits which have excessive resistance. A voltage drop in a circuit is caused by a resistance when the circuit is in operation. Part of the available voltage is used by the resistance. When there is excessive resistance, less voltage is available for other loads (lights, motors, etc.) in the circuit. Since each resistance in a circuit uses voltage, a voltmeter can be used to isolate the cause of the problem.
A voltage drop across closed contacts can indicate excessive resistance. This can cause the circuit to operate incorrectly. Remember, a switch is not a load. During diagnosis, use a voltmeter to measure the voltage drop across each switch contact while the circuit is in operation.
Check the wire in the illustration. If an ohmmeter is used to measure resistance, the single strand of wire still making contact would give an ohmmeter reading of 0 ohms. This would indicate a good circuit. When the circuit operates, this single strand of wire is not able to carry the current. It will become hot and have a high resistance to the current. This will be picked up as a voltage drop.
Unwanted resistance can be caused by many situations:
-Undersized wiring (single strand)
-Corrosion on switch contacts
-Loose wire connections or splices
If repairs are needed always use wire that is of the same or larger gauge (lower number).
Measuring Voltage Drop
1. Connect the voltmeter across the connector or part of the circuit you want to check. The positive lead of the voltmeter should be closer to power and the negative lead closer to ground.
2. Operate the circuit
3. The voltmeter will indicate how many volts (voltage drop) are being used to "push" current through that part of the circuit. Note in the illustration that there is an excessive 4.1 volt drop between the battery and the bulb, or a 4.0 v. drop between the switch and the bulb.
Relationship between Open/Short circuit and the ECU pin control
System description: When the switch is ON, the ECU lights up the lamp.
The VOLTAGE VALUE is based on the body ground.
This chart indicates appropriate pin voltages for the ASCD system: