See Figure 1
There are 5 modes of testing required for the proper diagnosis of the system. They are as follows:
- Diagnostic Test Mode-This mode is used to access the fault codes from the PCM memory.
- Circuit Actuation Test Mode (ATM Test)-This mode is used to turn a certain circuit on and off in order to test it. ATM test codes are used in this mode.
- Switch Test Mode-This mode is used to determine is specific switch inputs are being received by the PCM.
- Sensor Test Mode-This mode looks at the output signals of certain sensors as they are received by the PCM when the engine is not running. Sensor access codes are read in this mode. Also this mode is used to clear PCM memory of stored codes.
- Engine Running Test Mode-This mode looks at sensor output signals as seen by the PCM when the engine is running. Also this mode is used to determine some specific running conditions necessary for diagnosis.
The following service procedures are intended for use with the Diagnostic Readout Box II (DRB II). Since each available diagnostic readout box may differ in its interpretation and display of the sensor results, refer to the instructional procedure that accompanies each tester unit.Obtaining Fault Codes
- Connect the readout box to the diagnostic connector located in the engine compartment near the PCM unit.
- Start the engine, if possible, cycle the transmission selector and the A/C switch if applicable. Shut off the engine.
- Turn the ignition switch ON, OFF, ON, OFF, ON within 5 seconds to access the read fault code data.
- Record all the fault code messages displayed on the readout box.
- Observe the CHECK ENGINE LIGHT on the instrument panel. The CHECK ENGINE LIGHT should illuminate for 3 seconds and the go out.
The PCM only recognizes 2 switch input states-HI and LOW. For this reason the PCM cannot tell the difference between a selected switch position and an open circuit, short circuit or an open switch. However, if one of the switches is toggled, the controller does have the ability to respond to the change of state in the switch. If the change is displayed, it can be assumed that entire switch circuit to the PCM is operational.Obtaining Circuit Actuation Test Mode (ATM Test)
The purpose of the circuit actuation mode test is to check for proper operation of the output circuits that the PCM cannot internally recognize. The PCM can attempt to activate these outputs and allow the technician to affirm proper operation. Most of the tests performed in this mode issue a audible click or visual indication of component operation (click of relay contacts, injector spray etc.) Except for intermittent conditions, if a component functions properly when it is tested, it can be assumed that the component, attendant wiring and driving circuit are functioning properly.Obtaining Sensor Read Test Mode Display Codes
The sensor mode allows the output of the 7 sensors and the state of 3 switches to be displayed on the readout box while the engine is not running. This provides a vehicle for testing each individual sensor and switch including harness wiring and PCM circuitry.Engine Running Test Mode
The engine running test mode monitors the sensors on the vehicle which check operating conditions while the engine is running. The engine running test mode can be performed with the engine idling in Neutral and with parking brake set or under actual driving conditions. With the diagnostic readout box READ/HOLD switch in the READ position, the engine running test mode is initiated after the engine is started.
Select a test code by switching the READ/HOLD switch to the READ position and pressing the actuator button until the desired code appears. Release actuator button and switch the READ/HOLD switch to the HOLD position. The logic module will monitor that system test results will be displayed.Exiting Diagnostic Test
By turning the ignition switch to the OFF position, the test mode system is exited. With a Diagnostic Readout Box attached to the system and the ATM control button not pressed, the computer will continue to cycle the selected circuits for 5 minutes and then automatically shut the system down.
There are two methods for obtaining or reading the diagnostic trouble codes from the vehicles covered in this information: with a Diagnostic Readout Box (DRB or DRB-II) or the equivalent type of scan tool, or with the CHECK ENGINE light, located in the instrument panel. Both methods will be covered in the following sections.Diagnostic Readout Box (DRB or DRB-II)
See Figures 1, and 2
Make sure to follow the manufacturer's instructions when using a scan tool.
- Attach the DRB-II scan tool , or equivalent, to the data link connector located in the engine compartment near the powertrain control module (an L shaped connector with 8 terminals-refer to the engine compartment component location illustrations earlier in this section).
- Start the engine, if possible, cycle the transaxle selector and the A/C switch (if applicable). Shut the engine OFF .
- Turn the ignition switch ON , access Read Fault Screen. Record all of the fault messages shown on the DRB-II or other scan tool. Observe the malfunction indicator lamp (CHECK ENGINE lamp on the instrument panel). The lamp should light for 3 seconds, then go out (bulb check).
The switch inputs used by the PCM have only 2 recognized states, high and low. For this reason, the PCM cannot recognize the difference between a selected switch position versus an open circuit, a short circuit, or a defective switch. If the change is displayed, it can be assumed that the entire switch circuit to the PCM is functional. From the state display screen access either State Display Inputs and Outputs or State Display Sensors.State Display Inputs and Outputs-
Connect the DRB-II scan tool, or equivalent, to the vehicle (data link connector). access the State Display screen, then access Inputs and Outputs. Select one of the engine control system functions displayed in the Input and Output screen.State Display Sensors-
Connect the DRB-II scan tool, or the equivalent, to the vehicle (data link connector) and access the State Display screen. Access the Sensor Display and select one of the engine control system functions available in this window.CIRCUIT ACTUATION TEST MODE
The circuit actuation test mode checks for proper operation of output circuits or devices which the PCM cannot internally recognize. The PCM can attempt to activate these outputs and allow an observer to verify proper operation. Most of the tests provide an audible or visual indication of device operation (click of relay contacts, spray fuel, etc.). Except for intermittent conditions, if a device functions properly during testing, assume the device, its associated wiring and the driver circuit are working correctly.Obtaining Circuit Actuation Test
Connect the DRB-II scan tool , or the equivalent, to the vehicle and access the Actuators screen. Select one of the numerous engine control system functions accessible through this screen.Check Engine Light
The early Premier vehicles can only have their fault codes read by a scan tool.
The CHECK ENGINE light has 2 modes of operation: diagnostic mode and switch test mode. If a DRB II diagnostic tester is not available, the PCM can show the technician fault codes by flashing the CHECK ENGINE light on the instrument panel in the diagnostic mode. In the switch test mode, after all codes are displayed, switch function can be confirmed. The light will turn on and off when a switch is turned ON and OFF. Even though the light can be used as a diagnostic tool, it has the following restrictions:
Be advised that the CHECK ENGINE light can only perform a limited amount of functions and it is a good idea to have the system checked with a scan tool to double-check the circuit function.READING THE CHECK ENGINE LIGHT
The "CHECK ENGINE'' lamp has two modes of operation. They are as follows:
Diagnostic Mode-The PCM can show fault codes by means of flashing the check engine lamp on the instrument cluster. To activate this function turn the ignition key on-off-on-off-on within five seconds. The check engine lamp will then come on for two seconds as a bulb check. Immediately following this it will display a fault code by flashing on and off. There is a short pause between flashes and a longer pause between digits. All codes displayed are two digit numbers with a four second pause between codes. An example of a code is as follows:
- Lamp on for two seconds, then turns off-lamp check.
- Lamp flashes 5 times, pauses, and then flashes once-code 51.
- Lamp pauses for four seconds, flashes five times, pauses and then flashes five times-code 55.
The two codes are 51 and 55. Any number of codes can be displayed as long as they are in memory. The lamp will flash until all of them are displayed.
- Switch Test Mode-After all codes are displayed, switch function can be verified. The lamp will turn on or off when a switch is turned on or off.
After the component failure has been repaired, the engine computer will erase the fault codes after 50-100 ON/OFF turns of the ignition key.Limp-In Mode
The limp-in mode is the attempt by the PCM to compensate for the failure of certain components by substituting information from other sources. If the engine computer senses incorrect data or no data at all from the MAP sensor, throttle position sensor or coolant temperature sensor, the system is placed into limp-in mode and the CHECK ENGINE light on the instrument panel is activated. Although at the expense of performance and emission control efficiency, this mode will keep the vehicle drivable until service is obtained.