Toyota Tercel 1984-1994 Repair Guide

Computerized Engine Systems

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OPERATION



Engines equipped with a feedback carburetor use a simple system designed to keep the air/fuel ratio at an optimum of 14.7:1; excluding warm-up and acceleration. The carburetor is designed to run richer than it normally should. This sets up a rich limit of system operation. When a leaner operation is desired, the computer (ECM) energizes the electronic air bleed control valve (EBCV) to introduce additional air into the carburetor's main metering system and into the carburetor's primary bore. Once the air/fuel ratio is detected as being too lean by the oxygen sensor, the ECM will de-energize the EBCV and close both bleed ports. By shutting off the air, the mixture begins moving back towards the rich limit. The system is operating in the "closed loop'' mode, during which it will adjust itself and react to these adjustments. On these engines, the ECM receives information from the oxygen sensor, vacuum switches and the distributor.

The Electronic Fuel Injection (EFI) system precisely controls fuel injection to match engine requirements. This in turn reduces emissions and increases driveability. The ECM receives input from various sensors to determine engine operating conditions. These sensors provide the input to the control unit which determines the amount of fuel to be injected as well as other variables such as idle speed. These inputs and their corresponding sensors include:



Intake manifold absolute pressure - MAP or Vacuum Sensor
 
Intake air temperature - Intake Air Temperature Sensor
 
Coolant temperature - Water Temperature Sensor
 
Engine speed - Pulse signal from the distributor
 
Throttle valve opening - Throttle Position Sensor
 
Exhaust oxygen content - Oxygen Sensor
 

COMPONENT TESTING



Oxygen Sensor

Used on 3A-C, 3E and 3E-E engines.

The oxygen (O 2 ) sensor is located on the exhaust manifold to detect the concentration of oxygen in the exhaust gas. Using highly refined metals (zirconia and platinum), the sensor uses changes in the oxygen content to generate an electrical signal which is transmitted to the ECM. The computer in turn reacts to the signal by adjusting the fuel metering at the injectors or at the carburetor. More or less fuel is delivered into the cylinders and the correct oxygen level is maintained.

3A-C ENGINES

See Figure 1

  1. Warm up the engine to normal operating temperature.
  2.  
  3. Connect the voltmeter to the service connector. This round connector is usually located on the right fender apron below the wiper motor. Connect the positive probe to the OX terminal and the negative probe to the E terminal.
  4.  
  5. Run the engine at 2,500 rpm for 90 seconds or more. This allows the sensor to achieve a stable temperature and the exhaust flow to stabilize.
  6.  
  7. Maintain the engine at 2,500 rpm and check the meter. The meter needle should fluctuate within a 0-7 volt range at least 8 times in 10 seconds. This indicates that the sensor is working properly.
  8.  
  9. If the sensor fails the test, perform a careful inspection of all the wiring and connectors in the system. A loose connection can cause the sensor to fail this test. Repeat the voltage test after the inspection.
  10.  



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Fig. Fig. 1: Oxygen sensor test terminals on 3A-C engines

3E ENGINES

See Figure 2

  1. Warm up the engine to normal operating temperature.
  2.  
  3. Connect the voltmeter to the service connector. This is usually located on the left side of the firewall. Connect the positive probe to the OX terminal and the negative probe to the E1 terminal.
  4.  
  5. Run the engine at 2,500 rpm for 90 seconds or more. This allows the sensor to achieve a stable temperature and the exhaust flow to stabilize.
  6.  
  7. Maintain the engine at 2,500 rpm and check the meter. The meter needle should fluctuate 8 times or more in 10 seconds within a 1-5 volt range. This indicates that the sensor is working properly.
  8.  
  9. If the sensor fails the test, perform a careful inspection of all the wiring and connectors in the system. A loose connection can cause the sensor to fail this test. Repeat the voltage test after the inspection.
  10.  



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Fig. Fig. 2: Checking oxygen sensor operation on 3E engines

3E-E ENGINES

See Figure 3

  1. Warm up the engine to normal operating temperature.
  2.  
  3. Connect the voltmeter to the check connector on the left fender apron. Hook the positive probe to terminal VF and the negative probe to terminal E1 .
  4.  
  5. Run the engine at 2,500 rpm for at least 120 seconds.
  6.  
  7. With the engine speed being maintained at 2,500 rpm, use a jumper wire to connect terminals T and E1 at the check connector.
  8.  
  9. Watch the voltmeter and note the number of times the needle fluctuates in 10 seconds. If it moves eight times or more, the sensor is working properly.
  10.  

Perform a careful inspection of all the wiring and connectors in the system. A loose connection can cause the sensor to fail these tests.

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Fig. Fig. 3: Testing the oxygen sensor on 3E-E engines

Electronic Air Bleed Control Valve

See Figures 4, 5, 6 and 7

Used on 3A-C and 3E engines.

  1. Check for a short circuit. Using an ohmmeter, check that there is no continuity between the positive (+) terminal and the EBCV body. If there is continuity, replace the EBCV.
  2.  
  3. Check for an open circuit. Using an ohmmeter, measure the resistance between the two terminals. The resistance should be between 11-13- at 68°F (20°C). If the resistance is not within specification, replace the EBCV. Remember that the resistance will vary slightly with temperature. Resistance (ohms) will decrease as the temperature drops.
  4.  



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Fig. Fig. 4: Checking the EBCV for a short circuit on 3E engines



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Fig. Fig. 5: EBCV short circuit testing on 3A-C engines



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Fig. Fig. 6: EBCV open circuit testing on 3E engines



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Fig. Fig. 7: Testing the EBCV for an open circuit on 3A-C engines

Vacuum Switch (A)

See Figures 8 and 9

Used on 3A-C engines.

  1. Using an ohmmeter, check that there is continuity between the switch terminal and the switch body.
  2.  
  3. Start the engine and run it until normal operating temperature is reached.
  4.  
  5. Using an ohmmeter, check that there is NO continuity between the switch terminal and the switch body.
  6.  
  7. If either test is failed, replace the switch.
  8.  



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Fig. Fig. 8: With the engine off and cold, vacuum switch (A) should have continuity between its terminal and body



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Fig. Fig. 9: With the engine warm and running, vacuum switch (A) should not have continuity between its terminal and body

Vacuum Switch (B)

See Figure 10

Used on 3A-C engines.

  1. Using an ohmmeter, check that there is NO continuity between the switch terminal and the switch body.
  2.  
  3. Start the engine and run until normal operating temperature is reached.
  4.  
  5. Using an ohmmeter, check that there is continuity between the switch terminal and the body.
  6.  
  7. If either test is failed, replace the switch.
  8.  



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Fig. Fig. 10: With the engine off and cold, vacuum switch (B) should not have continuity between its terminal and body. It should have continuity when the engine is warm and running

Vacuum Switch

See Figures 11 and 12

Used on 3E engines.

  1. Using an ohmmeter, check for no continuity between the switch terminals.
  2.  
  3. Apply a vacuum of 5 in.Hg (12mm H 2 O) or greater to the port on the switch.
  4.  
  5. Using an ohmmeter, check that there is continuity between the switch terminals.
  6.  



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Fig. Fig. 11: On 3E engines, the switch should not have continuity between the terminals without vacuum applied



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Fig. Fig. 12: There should be continuity between the terminals with vacuum applied

Deceleration Fuel Cut-Off Systems

Used on 3A-C, 3E and 3E-E engines.

This system cuts off fuel flow to the idle circuit of the carburetor or to the fuel injectors to prevent overheating and afterburning in the exhaust system when the vehicle is decelerating from a certain engine speed.


WARNING
Perform these tests quickly to avoid overheating the catalytic converter. Also, on 3E engines, be careful not to damage the switch tip with the screwdriver.

3A-C ENGINES

See Figures 13 and 14

On 3A-C engines with a feedback carburetor system, follow this procedure:

  1. Connect a tachometer the engine.
  2.  
  3. Start the engine and check that it runs normally.
  4.  
  5. Unplug the electrical connector from vacuum switch (A).
  6.  
  7. Gradually increase the engine speed to 2,300 rpm. Check that the engine speed is fluctuating.
  8.  
  9. Reconnect the vacuum hose and again gradually increase the engine speed to 2,300 rpm. Check that the engine operation returns to normal.
  10.  

On 3A-C engines without a feedback carburetor system, follow this procedure:

  1. Start the engine.
  2.  
  3. Disconnect the hose from the vacuum switch and plug the hose end.
  4.  
  5. Check that you can feel a click from the fuel cut solenoid valve when the vacuum hose is connected and disconnected at idle.
  6.  
  7. Stop the engine and connect the hose.
  8.  



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Fig. Fig. 13: Testing the fuel cut-off system on 3A-C engines with a feedback carburetor



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Fig. Fig. 14: Testing the fuel cut-off system on 3A-C engines without a feedback carburetor

3E ENGINES

See Figures 15 and 16

  1. Connect a tachometer the engine.
  2.  
  3. Start the engine and check that it runs normally.
  4.  
  5. Gradually increase the engine speed to 2,100 rpm or less. Check that the engine speed is steady.
  6.  
  7. Raise the engine speed to 2,950 rpm or more. Using a screwdriver, press the tip of the throttle position switch. Check that the engine hesitates.
  8.  
  9. Release the throttle position switch and check that the engine speed returns to 2,950 rpm or more.
  10.  
  11. Unplug the solenoid valve connector at idle. Check that the idling becomes rough and that the engine eventually stops.
  12.  



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Fig. Fig. 15: Engine speed should fluctuate with the throttle position switch depressed on 3E engines



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Fig. Fig. 16: With the solenoid valve disconnected, the engine should run rough, then die out

3E-E ENGINES

See Figures 17 and 18

  1. Allow the engine to reach normal operating temperature.
  2.  
  3. On 1990 models, follow this procedure:
    1. Unplug the connector from the throttle position sensor.
    2.  
    3. Connect terminals IDL and E2 on the wiring harness side.
    4.  
    5. Gradually raise the engine speed and check that there is fluctuation between the fuel cut and fuel return points. The fuel cut speed is 2,300 rpm, fuel return speed is 1,700 rpm.
    6.  

  4.  
  5. On models after 1990, follow this procedure:
    1. Connect the test probe of a tachometer to terminal IG (-) of the check connector.
    2.  

  6.  


WARNING
Never allow the tachometer terminal to touch ground as it could result in damage to the igniter and/or ignition coil. As some tachometers are not compatible with this ignition system, we recommend that you confirm the compatibility of yours before use.

    1. Increase the engine speed to at least 2,500 rpm. Check the injectors for operating (clicking) sound.
    2.  
    3. Release the throttle lever, check that the injector sound stops momentarily and then resumes at approximately 1,300 rpm.
    4.  




Remove the tachometer.
 



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Fig. Fig. 17: Throttle position switch terminals on 1990 3E-E engines



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Fig. Fig. 18: Testing the fuel cut-off system on 1991 and later 3E-E engines

Manifold Absolute Pressure Sensor

See Figures 19, 20, 21 and 22

Used on 3E-E engines.

This sensor advises the ECM of pressure changes in the intake manifold. It consists of a semi-conductor pressure converting element which converts a pressure change into an electrical signal. The ECM sends a reference signal to the MAP sensor; the change in air pressure changes the resistance within the sensor. The ECM reads the change from its reference voltage and signals its systems to react accordingly.

Use only a 10 megaohm digital multi-meter when testing. The use of any other type of equipment may damage the ECM and other components.

  1. Unplug the vacuum sensor connector.
  2.  
  3. Turn the ignition switch ON.
  4.  
  5. Using a voltmeter, measure the voltage between terminals VCC and E2 of the vacuum sensor connector. It should be between 4-6 volts.
  6.  
  7. Push the electrical connector back into place.
  8.  
  9. Disconnect the vacuum hose from the sensor.
  10.  
  11. Connect a voltmeter to terminals PIM and E2 of the ECM. Measure and record the output voltage under ambient atmospheric pressure.
  12.  
  13. Apply vacuum to the sensor according to the segments show on the chart. Measure each voltage drop and compare to the chart.
  14.  



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Fig. Fig. 19: MAP sensor terminal identification



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Fig. Fig. 20: ECM terminal identification on 1990 3E-E engines



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Fig. Fig. 21: ECM terminal identification on 1991 and later 3E-E engines



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Fig. Fig. 22: MAP sensor voltage drop chart

Intake Air Temperature Sensor

See Figures 23 and 24

Used on 3E-E engines.

The IAT sensor advises the ECM of changes in intake air temperature (and therefore air density). As air temperature of the intake varies, the ECM, by monitoring the voltage change, adjusts the amount of fuel injection according to the air temperature.

  1. Unplug the electrical connector from the IAT sensor.
  2.  
  3. Using an ohmmeter, measure the resistance between both terminals. Refer to the chart for the proper resistance reading.
  4.  
  5. If the resistance is not as specified, replace the sensor.
  6.  



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Fig. Fig. 23: Testing the intake air temperature sensor



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Fig. Fig. 24: IAT sensor resistance chart

Water Temperature Sensor

See Figures 25 and 26

Used on 3E-E engines.

The water temperature sensor's function is to advise the ECM of changes in engine temperature by monitoring the changes in coolant temperature. The sensor must be handled carefully during removal. It can be damaged (thereby affecting engine performance) by impact.

  1. Unplug the electrical connector from the sensor.
  2.  
  3. Using an ohmmeter, measure the resistance between both terminals. Refer to the chart for the proper resistance reading.
  4.  
  5. If the resistance is not as specified, replace the sensor.
  6.  



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Fig. Fig. 25: Testing the water temperature sensor



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Fig. Fig. 26: Water temperature sensor resistance chart

Throttle Position Sensor

See Figures 27 through 31

Used on 3E-E engines.

  1. Unplug the sensor connector.
  2.  
  3. Insert a thickness gauge between the throttle stop screw and the stop lever.
  4.  
  5. Using an ohmmeter, measure the resistance between each terminal. Compare the readings to the chart.
  6.  



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Fig. Fig. 27: Throttle position sensor terminal identification on 1990 3E-E engines



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Fig. Fig. 28: TPS test chart for 1990 3E-E engines



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Fig. Fig. 29: Testing the throttle position sensor



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Fig. Fig. 30: TPS terminal identification on 1991 and later 3E-E engines



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Fig. Fig. 31: TPS test chart for 1991 and later 3E-E engines

Cold Start Injector Time Switch

See Figure 32

Used on 3E-E engines.

Some 3E-E engines utilize a cold start injector to improve starting ability. This switch controls the length of time the injector will stay on depending on engine temperature.

  1. Using an ohmmeter, measure the resistance between each of the terminals. They should be as follows:
    1. Terminals STA and STJ - 20-40- below 86°F (30°C) or 40-60- when above 104°F (40°C)
    2.  
    3. Terminal STA and ground - 20-80-
    4.  

  2.  
  3. If the resistance is not as specified, replace the switch.
  4.  



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Fig. Fig. 32: Testing the cold start injector time switch

EGR Gas Temperature Sensor

See Figure 33

Used on 3E-E engines.

  1. Using an ohmmeter, measure the resistance between the two terminals. It should be as follows:
    1. 69.40-88.50k- at 122°F(50°C)
    2.  
    3. 11.89-14.37k- at 212°F(100°C)
    4.  

  2.  
  3. If the resistance is not as specified, replace the sensor.
  4.  



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Fig. Fig. 33: Testing the EGR gas temperature sensor resistance

Idle-Up Controls

See Figures 34, 35, 36 and 37

Used on 3E-E engines.

  1. Test the water temperature sensor as described earlier.
  2.  
  3. Test the VSV using an ohmmeter. Check that there is continuity between the terminals. The resistance should be between 30-36-.
  4.  
  5. If it is not as specified, replace the VSV.
  6.  
  7. Using an ohmmeter, check that there is no continuity between each terminal and the VSV body. If there is continuity, replace the VSV.
  8.  
  9. Check that air does not blow from pipe E to F.
  10.  
  11. Apply battery voltage across the terminals. Check that air flows from pipe E to F.
  12.  
  13. If operation is not as specified, replace the VSV.
  14.  



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Fig. Fig. 34: Inspecting the idle up VSV for an open circuit



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Fig. Fig. 35: Inspecting the idle up VSV for a short circuit



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Fig. Fig. 36: Air should not flow through the idle up VSV



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Fig. Fig. 37: Air should flow when battery voltage is applied to the idle up VSV

Fuel Pressure Controls

See Figures 38, 39, 40 and 41

Used on 3E-E engines.

  1. Test the water temperature sensor as described earlier.
  2.  
  3. Test the VSV using an ohmmeter. Check that there is continuity between the terminals. The resistance should be between 30-39-.
  4.  
  5. If it is not as specified, replace the VSV.
  6.  
  7. Using an ohmmeter, check that there is no continuity between each terminal and the VSV body. If there is continuity, replace the VSV.
  8.  
  9. Check that air flows from pipe E to P.
  10.  
  11. Apply battery voltage across the terminals. Check that air flows from pipe E to F.
  12.  
  13. If operation is not as specified, replace the VSV.
  14.  



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Fig. Fig. 38: Inspecting the fuel pressure VSV for an open circuit; throttle opener VSV is similar



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Fig. Fig. 39: Inspecting the fuel pressure VSV for a short circuit; throttle opener VSV is similar



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Fig. Fig. 40: No air should flow through the valve without battery voltage applied



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Fig. Fig. 41: Checking air flow on fuel pressure and throttle opener vacuum switching valves with battery voltage applied

Throttle Opener

Used on 3E-E engines.

  1. Test the throttle position sensor as described earlier.
  2.  
  3. Test the VSV using an ohmmeter. Check that there is continuity between the terminals. The resistance should be between 33-39-.
  4.  
  5. If it is not as specified, replace the VSV.
  6.  
  7. Using an ohmmeter, check that there is no continuity between each terminal and the VSV body. If there is continuity, replace the VSV.
  8.  
  9. Check that air flows from pipe E to P.
  10.  
  11. Apply battery voltage across the terminals. Check that air flows from pipe E to F.
  12.  
  13. If operation is not as specified, replace the VSV.
  14.  

REMOVAL & INSTALLATION



Oxygen Sensor

See Figure 42

Care should be used during the removal of the oxygen sensor. Both the sensor and its wire can be easily damaged.

  1. The best condition in which to remove the sensor is when the engine is moderately warm. This is generally achieved after two to five minutes (depending on outside temperature) of running after a cold start. The exhaust manifold has developed enough heat to expand and make the removal easier but is not so hot that it has become untouchable.Wearing heat resistant gloves is highly recommended during this repair.
  2.  
  3. With the ignition OFF, unplug the connector for the sensor.
  4.  
  5. Remove the two sensor attaching bolts.
  6.  
  7. Remove the oxygen sensor from the manifold.
  8.  

To install:
  1. During and after the removal, use great care to protect the tip of the sensor if it is to be reused. Do not allow it to come in contact with fluids or dirt. Do not attempt to clean it or wash it.
  2.  
  3. Apply a coat of anti-seize compound to the bolt threads but DO NOT allow any to get on the tip of the sensor.
  4.  
  5. Install the sensor in the manifold.
  6.  
  7. Reconnect the electrical connector and insure a clean, tight connection.
  8.  



Click image to see an enlarged view

Fig. Fig. 42: A common oxygen sensor

MAP Sensor

Replacing the MAP sensor simply requires unplugging the vacuum and electrical connections, then unbolting the sensor. Inspect the vacuum hose over its entire length for any signs of cracking or splitting. The slightest leak can cause false messages to be send to the ECM.

IAT Sensor

To replace the IAT sensor:

  1. Remove the air cleaner cover.
  2.  
  3. With the ignition OFF, unplug the electrical connector.
  4.  
  5. Push the IAT sensor out from inside the air cleaner housing.
  6.  

To install:
  1. Install the sensor, making sure it is properly placed and secure.
  2.  
  3. Connect the wiring harness, and install the air cleaner cover.
  4.  

Water Temperature Sensor

Perform this procedure only on a cold engine.

  1. Drain the cooling system as necessary.
  2.  
  3. With the ignition OFF, unplug the electrical connector to the sensor.
  4.  
  5. Using the proper sized wrench, carefully unscrew the sensor from the engine.
  6.  

To install:
  1. Coat the threads of the sensor with a sealant. Install the sensor and tighten it to 18 ft. lbs. (24 Nm).
  2.  
  3. Plug the electrical connector into the sensor.
  4.  
  5. Refill the coolant to the proper level. Road test the vehicle for proper operation.
  6.  

Cold Start Injector Time Switch

Perform this procedure only on a cold engine.

  1. Drain the cooling system as necessary.
  2.  
  3. With the ignition OFF, unplug the electrical connector to the switch.
  4.  
  5. Using the proper sized wrench, carefully unscrew the switch from the engine.
  6.  

To install:
  1. Coat the threads of the switch with a sealant and install it.
  2.  
  3. Plug the electrical connector into the switch.
  4.  
  5. Refill the coolant to the proper level. Road test the vehicle for proper operation.
  6.  

EGR Gas Temperature Sensor
  1. With the ignition OFF, unplug the electrical connector to the sensor.
  2.  
  3. Using the proper sized wrench, carefully unscrew the sensor from the engine.
  4.  

To install:
  1. Install the sensor.
  2.  
  3. Plug the electrical connector into the sensor.
  4.  
  5. Refill the coolant to the proper level. Road test the vehicle for proper operation.
  6.  

Throttle Position Sensor

See Figures 43, 44, 45, and 46

On 1990 models, follow this procedure:

  1. Secure the throttle valve opening at approximately 45 degrees. Be careful not to damage any components.
  2.  
  3. Remove the two screws and the sensor.
  4.  

To install:
  1. Place the TPS over the throttle valve shaft. Do not turn the TPS when it is being installed.
  2.  
  3. Temporarily install the two screws. Remove the device securing the throttle valve angle.
  4.  
  5. Adjust the throttle position sensor, refer to Fuel System .
  6.  

On 1991-1994 models, follow this procedure:

  1. Remove the two screws and the TPS.
  2.  
  3. To install, place the sensor on the throttle body as shown. Turn the sensor clockwise and install the two screws.
  4.  
  5. Adjust the TPS, refer to Fuel System .
  6.  



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Fig. Fig. 43: Removing the throttle position sensor



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Fig. Fig. 44: On 1990 models, it is necessary to secure the throttle valve at an angle before TPS removal



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Fig. Fig. 45: TPS installation on 1990 models



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Fig. Fig. 46: TPS installation on 1991 and later models

Vacuum Switches

Replacing the various vacuum switches simply requires unplugging the vacuum and/or electrical connections, then unbolting the switch. Inspect the vacuum hose over its entire length for any signs of cracking or splitting. The slightest leak can cause improper operation.

 
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