Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Under hood, center, upper engine area, mounted in front of intake manifold
See Figures 1 and 2
The EGR system's purpose is to control oxides of nitrogen which are formed during the peak combustion temperatures. The end products of combustion are relatively inert gases derived from the exhaust gases which are directed into the EGR valve to help lower peak combustion temperatures.
Fig. Fig. 1: Negative backpressure and positive backpressure EGR valve identification
Fig. Fig. 2: Linear EGR valve identification
The port EGR valve is controlled by a flexible diaphragm which is spring loaded to hold the valve closed. Vacuum applied to the top side of the diaphragm overcomes the spring pressure and opens the valve which allows exhaust gas to be pulled into the intake manifold and enter the engine cylinders.
The negative backpressure EGR valve has bleed valve spring below the diaphragm, and the valve is normally closed. The valve varies the amount of exhaust flow into the manifold depending on manifold vacuum and variations in exhaust backpressure.
The diaphragm on this valve has an internal air bleed hole which is held closed by a small spring when there is no exhaust backpressure. Engine vacuum opens the EGR valve against the pressure of a large. When manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes. This valve will open if vacuum is applied with the engine not running.
The linear EGR valve is operated exclusively by the control module command. The control module monitors various engine parameters:
Throttle Position Sensor (TPS) Manifold Absolute Pressure (MAP) Engine Coolant Temperature (ECT) sensor Pintle position sensor
Output messages are then sent to the EGR system indicating the proper amount of exhaust gas recirculation necessary to lower combustion temperatures.
Refer to the accompanying illustrations to identify the EGR valve used in your vehicle.
See Figures 3, 4, 5, 6, 7 and 8
Refer to the appropriate chart for diagnosis the EGR system. On linear EGR systems, an OBD-II compliant scan tool will be needed.
Fig. Fig. 3: Ported EGR system wiring diagram
Fig. Fig. 4: Ported EGR system check
Fig. Fig. 5: Negative backpressure EGR system test
Fig. Fig. 6: Negative EGR system wiring diagram
Fig. Fig. 7: Linear EGR system test
Fig. Fig. 8: Linear EGR system test (continued)
EGR Valveexcept 1996-98 models
See Figures 9, 10 and 11
Fig. Fig. 9: EGR valve mounting-4.3L engines, except 1996-98 models
Fig. Fig. 10: EGR valve mounting-5.0L and 5.7L engines, except 1996-98 models
Fig. Fig. 11: EGR valve mounting-7.4L engines, except 1996-98 models
See Figures 12 and 13
Do not try to disassemble the EGR valve.
Fig. Fig. 12: Linear EGR valve mounting-1996-98 4.3L, 5.0L and 5.7L engines
Fig. Fig. 13: Linear EGR valve mounting (1)-1996-98 7.4L engines
See Figures 14 and 15
Fig. Fig. 14: EGR valve and solenoid mounting-7.4L engine shown, others similar
Fig. Fig. 15: EGR control solenoid
Ok, what engine?
Fig. Fig. 52: 1995 (VIN Z) 4.3L
Are there any codes set?
Front seating area, driver side, driver side of steering column, mounted behind dash upper panel
Listings of the trouble for the various engine control system covered here are located in this section. Remember that a code only points to the faulty circuit NOT necessarily to a faulty component. Loose, damaged or corroded connections may contribute to a fault code on a circuit when the sensor or component is operating properly. Be sure that the components are faulty before replacing them, especially the expensive ones.
Fig. Fig. 1: ALDL connector-1988-92 models
The Assembly Line Diagnostic Link (ALDL) connector or Data Link Connector (DLC) may be located under the dash and sometimes covered with a plastic cover labeled DIAGNOSTIC CONNECTOR.
Fig. Fig. 2: ALDL connector-1993-95 models
The order of codes in the memory does not indicate the order of occurrence.
After making repairs, clear the trouble codes and operate the vehicle to see if it will reset, indicating further problems.
Stored fault codes may be erased from memory at any time by removing power from the ECM for at least 30 seconds. It may be necessary to clear stored codes during diagnosis to check for any recurrence during a test drive, but the stored codes must be written down when retrieved. The codes may still be required for subsequent troubleshooting. Whenever a repair is complete, the stored codes must be erased and the vehicle test driven to confirm correct operation and repair.
WARNING The ignition switch must be OFF any time power is disconnected or restored to the ECM. Severe damage may result if this precaution is not observed.
Depending on the electrical distribution of the particular vehicle, power to the ECM may be disconnected by removing the ECM fuse in the fusebox, disconnecting the in-line fuse holder near the positive battery terminal or disconnecting the ECM power lead at the battery terminal. Disconnecting the negative battery cable to clear codes is not recommended as this will also clear other memory data in the vehicle such as radio presets
Fig. Fig. 2: Fuel injected engine trouble codes through 1995, except with 4L60E and 4L80E transmissions
Fig. Fig. 3: Fuel injected engine trouble codes through 1995 with 4L60E transmissions
Fig. Fig. 4: Fuel injected engine trouble codes through 1995 with 4L60E transmissions (continued)
Fig. Fig. 5: Fuel injected engine trouble codes through 1995 with 4L80E transmissions
Fig. Fig. 6: Fuel injected engine trouble codes through 1995 with 4L80E transmissions (continued)
okay thats not it. the check engine light is not on and i 've alrady check for codes could it be the computer and if so do u know how to test it
The only way I know of to test the computer is replace it. Napaonline has them.
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testing engine controls GM Full-Size Trucks 1988-1998
The ECM uses the camshaft signal to determine the position of the No. 1 cylinder piston during its power stroke. The signal is used by the ECM to calculate fuel injection mode of operation.
If the cam signal is lost while the engine is running, the fuel injection system will shift to a calculated fuel injected mode based on the last fuel injection pulse, and the engine will continue to run.
See Figure 2
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Fig. Fig. 2: Camshaft Position (CMP) sensor wiring schematic
1. Disconnect the CMP sensor wiring harness and connect an LED test light between CMP harness terminal C and battery ground.
2. With the ignition ON and the engine off, verify that the test light illuminates.
3. If not as specified, repair or replace the fuse and/or wiring.
4. Carefully connect the test light between CMP harness terminal A and C and verify that the test light illuminates.
5. If not as specified, repair the CMP harness ground circuit (terminal A).
6. Turn the ignition OFF and disconnect the test light.
7. Next, connect suitable jumper wires between the CMP sensor and CMP sensor harness. Connect a DC volt meter to the jumper wire corresponding to CMP terminal B and battery ground.
8. Start the engine and verify that the voltage signal is 5-7 volts.
9. If it is not as specified, the CMP sensor may be faulty.
Crankshaft Position (CKP) Sensor
See Figure 1
The Crankshaft Position (CKP) Sensor provides a signal through the ignition module which the ECM uses as a reference to calculate rpm and crankshaft position.
Fig. Fig. 1: Crankshaft Position (CKP) sensor
1. Disconnect the CKP sensor harness. Connect an LED test light between battery ground and CKP harness terminal A.
4. Carefully connect the test light between CKP harness terminal A and B. Verify that the test light illuminates.
Fig. Fig. 2: Crankshaft Position (CKP) sensor wiring diagram
1. If not as specified, repair the CKP harness ground circuit (terminal B).
2. Turn the ignition OFF and disconnect the test light.
3. Next, connect suitable jumper wires between the CKP sensor and CKP sensor harness. Connect a duty cycle meter to the jumper wire corresponding to CKP terminal C and battery ground.
4. Crank the engine and verify that the duty cycle signal is between 40-60%.
5. If it is not as specified, the CKP sensor may be faulty.
6. Next, connect a AC volt meter to the jumper wire corresponding to CKP terminal C and battery ground.
7. Crank the engine and verify that the AC voltage signal is at least 10.0 volts.
8. If not as specified the CKP sensor may be faulty.
Electronic Control Module (ECM)
When the term Electronic Control Module (ECM) is used here it refers to the engine control computer regardless that it may be a Vehicle Control Module (VCM), Powertrain Control Module (PCM) or Electronic Control Module (ECM).
The Electronic Control Module (ECM) is required to maintain the exhaust emissions at acceptable levels. The module is a small, solid state computer which receives signals from many sources and sensors; it uses these data to make judgments about operating conditions and then control output signals to the fuel and emission systems to match the current requirements.
Engines coupled to electronically controlled transmissions employ a Powertrain Control Module (PCM) or Vehicle Control Module (VCM) to oversee both engine and transmission operation. The integrated functions of engine and transmission control allow accurate gear selection and improved fuel economy.
In the event of an ECM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although at a reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the ECM enters this mode.
Idle Air Control (IAC) Valve
The engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, (called counts).
This movement controls air flow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a scan tool. Zero counts corresponds to a fully closed passage, while 140 or more counts (depending on the application) corresponds to full flow.
Fig. Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor
See Figures 2, 3 and 4
1. Disengage the IAC electrical connector.
2. Using an ohmmeter, measure the resistance between IAC terminals A and B. Next measure the resistance between terminals C and D.
3. Verify that the resistance between both sets of IAC terminals is 20-80 ohms. If the resistance is not as specified, the IAC may be faulty.
4. Measure the resistance between IAC terminals B and C. Next measure the resistance between terminals A and D.
5. Verify that the resistance between both sets of IAC terminals is infinite. If the resistance is not infinite, the IAC may be faulty.
6. Also, with a small mirror, inspect IAC air inlet passage and pintle for debris. Clean as necessary, as this can cause IAC malfunction.
Fig. Fig. 2: Using an ohmmeter, backprobe terminals of the TPS sensor to check for proper resistances
Fig. Fig. 3: The TP sensor and IAC sensor are usually located at the side of the throttle body
Fig. Fig. 4: Idle Air Control (IAC) valve wiring and terminal identification
Intake Air Temperature (IAT) Sensor
the Intake Air Temperature (IAT) Sensor is a thermistor which changes value based on the temperature of the air entering the engine. Low temperature produces a high resistance, while a high temperature causes a low resistance. The ECM supplies a 5 volt signal to the sensor through a resistor in the ECM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the ECM calculates the incoming air temperature.
the IAT sensor signal is used to adjust spark timing according to incoming air density.
Fig. Fig. 1: Intake Air Temperature (IAT) sensor
See Figures 2 and 3
1. Remove the Intake Air Temperature (IAT) sensor.
2. Connect a digital ohmmeter to the two terminals of the sensor.
3. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the ambient air. Refer to the temperature vs. resistance illustration.
4. Repeat the test at two other temperature points, heating or cooling the air as necessary with a hair dryer or other suitable tool.
5. If the sensor does not meet specification, it must be replaced.
Fig. Fig. 2: Intake Air Temperature (IAT) sensor wiring diagram
Fig. Fig. 3: Intake Air Temperature (IAT) sensor temperature vs. resistance values
Manifold Absolute Pressure (MAP) Sensor
The Manifold Absolute Pressure (MAP) sensor measures the changes in intake manifold pressure, which result from the engine load and speed changes, and converts this to a voltage output.
A closed throttle on engine coastdown will produce a low MAP output, while a wide-open throttle will produce a high output. This high output is produced because the pressure inside the manifold is the same as outside the manifold, so 100 percent of the outside air pressure is measured.
The MAP sensor reading is the opposite of what you would measure on a vacuum gauge. When manifold pressure is high, vacuum is low. The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes.
The ECM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, the ECM knows the manifold pressure. A higher pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.
The ECM uses the MAP sensor to control fuel delivery and ignition timing.
Fig. Fig. 1: Common Manifold Absolute Pressure (MAP) sensor used on 4.3L, 5.0L and 5.7L engines
Fig. Fig. 2: Common Manifold Absolute Pressure (MAP) sensor used on 7.4L engines
See Figures 3, 4 and 5
1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C.
2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
3. If the voltage is not as specified, either the wiring to the MAP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
4. Backprobe with the high impotence voltmeter at MAP sensor terminals B and A.
5. Verify that the sensor voltage is approximately 0.5 volts with the engine not running (at sea level).
6. Record MAP sensor voltage with the key ON and engine off.
7. Start the vehicle.
8. Verify that the sensor voltage is greater than 1.5 volts (above the recorded reading) at idle.
9. Verify that the sensor voltage increases to approximately 4.5. volts (above the recorded reading) at Wide Open Throttle (WOT).
10. If the sensor voltage is as specified, the sensor is functioning properly.
11. If the sensor voltage is not as specified, check the sensor and the sensor vacuum source for a leak or a restriction. If no leaks or restrictions are found, the sensor may be defective and should be replaced.
Fig. Fig. 3: Location of the MAP sensor-TBI system shown
Fig. Fig. 4: Probe the terminals of the MAP sensor to check for proper reference voltage
Fig. Fig. 5: Manifold Absolute Pressure (MAP) sensor wiring diagram
Mass Air Flow (MAF) Sensor
The Mass Air Flow (MAF) Sensor measures the amount of air entering the engine during a given time. The ECM uses the mass airflow information for fuel delivery calculations. A large quantity of air entering the engine indicates an acceleration or high load situation, while a small quantity of air indicates deceleration or idle.
Fig. Fig. 1: Exploded view of the Mass Air Flow (MAF) sensor
1. Backprobe with a high impedance voltmeter between MAF sensor terminals C and B.
2. With the ignition ON engine off, verify that battery voltage is present.
3. If the voltage is not as specified, either the wiring to the MAF sensor, fuse or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
4. Disconnect the voltmeter and backprobe with a frequency meter between MAF sensor terminals A and B.
5. Start the engine and wait until it reaches normal idle speed and verify that the MAF sensor output is approximately 2000 Hz.
6. Slowly raise engine speed up to maximum recommended rpm and verify that the MAF sensor output rises smoothly to approximately 8000 Hz.
7. If MAF sensor output is not as specified the sensor may be faulty.
Fig. Fig. 2: Mass Air Flow (MAF) sensor wiring diagram
Throttle Position Sensor (TPS)
The Throttle Position Sensor (TPS) is connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ground.
A third wire is connected to the ECM to measure the voltage from the TPS. As the throttle valve angle is changed (accelerator pedal moved), the output of the TPS also changes. At a closed throttle position, the output of the TPS is low (approximately .5 volts). As the throttle valve opens, the output increases so that, at wide-open throttle, the output voltage should be approximately 4.5 volts.
By monitoring the output voltage from the TPS, the ECM can determine fuel delivery based on throttle valve angle (driver demand).
Fig. Fig. 1: Common Throttle Position Sensor (TPS) found on GM trucks
1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
4. Backprobe with a high impedance voltmeter at terminals C and B.
5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
7. If the sensor voltage is not as specified, replace the sensor.
Fig. Fig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage
Fig. Fig. 3: Using the DVOM, backprobe terminals C and B of the TPS sensor, open and close the throttle and make sure the voltage changes smoothly
Fig. Fig. 4: Throttle Position Sensor (TPS) wiring diagram
Vehicle Speed Sensor (VSS)
The vehicle speed sensor is made up of a coil mounted on the transmission and a tooth rotor mounted to the output shaft of the transmission. As each tooth nears the coil, the coil produces an AC voltage pulse. As the vehicle speed increases the number of voltage pulses per second increases.
Fig. Fig. 1: Vehicle Speed Sensor (VSS) and vehicle speed signal buffer wiring diagram
1. To test the VSS, backprobe the VSS terminals with a high impedance voltmeter (set at the AC voltage scale).
2. Safely raise and support the entire vehicle using jackstands. Make absolutely sure the vehicle is stable.
3. Start the vehicle and place it in gear.
4. Verify that the VSS voltage increases as the drive shaft speed increases.
5. If the VSS voltage is not as specified the VSS may be faulty.
Sorry the pictures didn't show up, try this.
everything u just sent me do u have the wires colors