See Figures 1, 2 and 3
The SBEC II or PCM can detect certain faults in the fuel injection system. A fault code indicates that the engine controller has recognized an abnormal signal in the system. Fault codes indicate the result of a failure cannot identify the failed component directly. Any part of the component's circuit could be the cause of a fault and diagnosis must be approached with that in mind.
Fault codes can be obtained by cycling the ignition switch ON-OFF-ON-OFF-ON within 3-5 seconds. Upon activation of the trouble code read-out, fault codes will be flashed by the check engine light. Each of the flash represents a digit in the fault code. Please refer to the fault code description charts followed by the corresponding diagnostic tests, later in this section to help troubleshoot fault codes.
A Diagnostic Readout Box (DRB) may be necessary for diagnosis of certain engine malfunctions.
The first test performed by the SBEC II or PCM is for sensor output. If there is a problem with a circuit, the controller tests for an open circuit, short to ground and short to 12 volts. The second test determines if the oxygen sensor is functioning properly.Systems not monitored by the system self-diagnostics include the following:
See Figures 4 through 37
Coolant Temperature Sensor (CTS) Test
Disengage the wire harness connector from the CTS and measure the resistance of the sensor with a high input impedance (digital) volt-ohmmeter. With the engine warm, the resistance should be less than 1000 ohms for 1991-92 vehicles and less than 1340 ohms for 1993 and later vehicles. Refer to the resistance chart and replace the sensor if it is not within the range of resistance specified in the chart. Measure the resistance of the wire harness between engine controller wire harness connector terminal 2 and the sensor connector terminal, as well as between terminal 4 and the sensor connector terminal. Repair the wire harness if an open circuit is indicated.
Disengage the wire harness connector from the MAT sensor and measure the resistance of the sensor with a high input impedance (digital) volt-ohmmeter. The resistance should be less than 1000 ohms with the engine warm. Refer to the resistance chart and replace the sensor if it is not within the range of resistance specified in the chart. Measure the resistance of the wire harness between engine controller wire harness connector terminal 2 and the sensor connector terminal, as well as between terminal 4 and the sensor connector terminal. Repair the wire harness if the resistance is greater than 1 ohm.
- Inspect the MAP sensor vacuum hose connection at the throttle body and sensor, then repair as necessary.
- Test the MAP sensor output voltage at the sensor connector terminal B (as marked on the sensor body) with the ignition switch ON and the engine OFF . The output voltage should be 4-5 volts. The voltage should drop to 0.5-1.5 volts with a hot, neutral idle speed condition.
- Test engine controller terminal 5 for the same voltage as in Step 2 to verify the wire harness condition. Repair any problems, as necessary.
- Test the MAP sensor supply voltage at the sensor connector terminal C with the ignition ON . The voltage should be 4.5-5.5 volts. The same voltage should be present at terminal 6 of the engine controller wire harness connector. Repair or replace the wire harness, as necessary. If the engine controller is suspect, use Diagnostic Tester M.S.1700, or equivalent, to test engine controller function.
- Test the MAP sensor ground circuit at the sensor connector terminal A and engine controller connector terminal 4. Repair the wire harness as necessary.
- Test the MAP sensor ground circuit at the engine controller connector between terminal 4 and terminal 11 with an ohmmeter. If the ohmmeter indicates an open circuit, check for a defective sensor ground connection located on the right side of the cylinder block. If the ground connection is good, replace the engine controller.
If terminal 4 has a short circuit to 12 volts, correct this condition before replacing the engine controller.
Disengage the O 2 sensor connector, then install ohmmeter test leads to terminals A and B of the sensor connector for 1991 vehicles or across the white wire terminals of the connector for 1992-95 vehicles. The resistance should be 5-7 ohms. Replace the O 2 sensor if the ohmmeter displays an infinity reading.
Oxygen sensor operational testing requires the use of a special tester M.S.1700, or equivalent.
Disengage the speed sensor connector from the ignition control module, then install an ohmmeter between terminals
as marked on the connector. The ohmmeter should read 125-275 ohms on a hot engine for 1991 vehicles and no resistance for 1992-95 vehicles. Replace the sensor if the readings are not as stated.
A relay in the de-energized position should have continuity between terminals 87A and 30. Resistance values between terminals 85 and 86 is 70-80 ohms for resistor relays and 81-91 ohms for diode relays. Not all relays have battery voltage connected to terminal 30. Some may have battery voltage connected to terminals 87 or 87A.
- At the distributor connector, insert the positive (+) lead of a voltmeter into the gray with black tracer wire and the negative (-) lead into the brown with red tracer wire.
Do not disengage the distributor connector from the distributor. Instead carefully insert the voltmeter leads into the back side of the connector to make contact with the terminals.
- Set the voltmeter on the 15 volt AC scale and turn the ignition switch ON . The voltmeter should read approximately 5 volts. If there is no voltage, check the voltmeter leads for a good connection.
- If there is still no voltage, check for voltage at the white with black tracer supply wire. If there is still no voltage at the supply wire, remove the engine controller and check for voltage at pin 7 and ground with the harness connected. If there is still no voltage present, perform a vehicle test using a DRB II Service Diagnostic Tester.
- If voltage is present at the supply wire, replace the sync. sensor.
- If voltage is present at pin 7 but not at the supply wire, check for continuity between the white/black wire at the distributor connector and pin 7 at the engine controller. If there is no continuity, repair the wire harness as necessary.
- Check for continuity between the ground circuit wire at the distributor connector and ground. If there is no continuity, repair the wire harness as necessary.
- Crank the engine while observing the voltmeter; the needle should fluctuate back and forth between 0 and 5 volts while the engine is cranking. This verifies that the stator in the distributor is operating properly. If there is no sync pulse, stator replacement is necessary.
See Figure 15
The automatiic idle speed motor can be tested using exerciser tool 7558.
- With the gnition OFF , disengage the AIS motor wire connector at the throttle body. Plug the exerciser tool harness into the AIS motor. Connect the exerciser motor power leads to the battery. The red light on the exerciser should flash.
- Start engine. When the switch is in the HIGH or LOW position, the light on the exerciser will flash indicating that voltage pulses are being sent to the stepper motor.
Move the switch to the HIGH position; the engine speed should increase. Move the switch to the LOW position; the engine speed should decrease.
- If engine speed changes while using the exerciser, the AIS motor is functioning properly. Disconnect the tool and reconnect the AIS harness.
- If the engine speed does not change, turn the ignition OFF and proceed to Step 4.
- Remove the AIS motor from the throttle body.
When checking AIS motor operation with the motor removed, DO NOT allow the pintle to extend more than1/4in. (6mm). If the pintle is extended more than1/4in. (6mm), it may separate from the motor. The AIS motor must be replaced if the pintle separates from the motor.
With the ignition switch
, cycle the exerciser tool switch between the HIGH and LOW positions. Observe the pintle for movement.
- If the pintle does not move, replace the AIS motor. Start engine and test the replacement motor as described in Step 3.
- If the pintle operates properly, check the AIS motor bore in the throttle body for blockage and clean as necessary. Reinstall the AIS motor and retest.
One of the most common problems with electronic engine control systems results not from a faulty component, but from a bad circuit which is caused by loose, dirty or corroded connections. Always begin diagnosis by checking all wiring and hoses from proper connections, routing and lack of physical damage.
If a visual inspection does not turn up the problem, proceed with component testing in order to troubleshoot the condition. As necessary, refer to other areas in this guide for additional information on engine, fuel and ignition system components.