How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask Mike S. Your Own Question

Mike S.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 5261
Experience:  ASE Certified Master Technician
20625509
Type Your Chevy Question Here...
Mike S. is online now
A new question is answered every 9 seconds

1996 Chevy: 1500 4x4..installation of distributor..timing..runs rough

Customer Question

I have a 1996 Chevy 1500 4x4 5.7L vortec 350. We have done all the steps stated below for installation of distributor and still can not get engine into timing, runs rough, have to keep foot in the gas to keep running. Can you help?
Submitted: 5 years ago.
Category: Chevy
Expert:  Mike S. replied 5 years ago.

Did you turn the engine over any at all when the distributor was out? If so you may be a tooth off. Since you already have the instructions, let me just add that the distributor will only go in 1 of 2 ways, depending on the alignment of the oil pump shaft, but turning that oil pump shaft with a big screwdriver or tool you can actually put that distributor anywhere you want, much more than just 2 spots. The easiest way is first of all find top dead center on the #1 piston on the compression stroke first. The only way to truly tell is remove the #1 spark plug and either feel with your finger over the hole, use a pressure tester or just take a wet piece of toilet paper and shove in the hole making a good seal. Then tap the engine over with the key untill it blows out that toilet paper. There will be 2 forces against that paper, a soft 1 and a hard 1. The hard 1 is what you want, the compression stroke, the soft 1 is the exhaust stroke. Now when it pushes out the paper you are coming up on compression stroke or maybe you just passed it. Now turn the crankshaft by hand with a socket and rachet on the front bolt untill the piston is at TDC. Don't depend on the timing mark, cause sometimes dampers slip. You will have to feel inside the cylinder. Stick something thin and a bendable plastic so it wont break and insert it into the cylinder untill you can feel the top of the piston. Now go back and forth on the crankshaft or forward and backward and feel the piston coming up, pausing a little bit then start going down. Work the crankshaft forward and backward a little at a time untill you feel the piston excactly in the middle of the flat spot at the top where it pauses. That is TDC. Now remove the distributor cap and look at where the rotor is facing. Then look down inside the distributor at the starwheel and pickup. There should be teeth on the starwheel and a piece of metal at the pickup. They have to lignup cause that is when it will fire the spark. All of has to be aligned now when the piston is at TDC. The starwheel firing at the piskup, the rotor also facing the #1 tower on the cap, too. If they are not all ligned up then pull the distributor back out just enough to clear the oil pump shaft and the camshaft gear. Then place the distributor where everything lines up, the starwheel with the pickup, the rotor facing the #1 on the cap. Go ahead and set the cap on for the moment and then take a piece of chaulk and make a mark somewhere on the engine where you can see it, then remove the cap and turn both the distributor and the shaft as 1 unit untill the rotor faces the #1 on the cap. Now set the distributor down into the hole. Don't suspect it is going to go all the way in cause it isn't aligned with the oil pump shaft and watch the rotor and starwheel as the distributor teeth align with the camshaft cause they have a tendancy to turn a little cause the teeth are curved. Now set the distributor down and if everything looks like it is going to fall where you want it, just place the cap back on momentarily and place a small brick over the cap for weight or press down on the cap while you turn the key and rotate the engine 1 trun untill the distributor shaft aligns with the oil pump shaft and drops into place. Best to remove the coil wire so you dont get shocked. If the timing mark was deadon when you searched for TDC go ahead and trust that for now and turn the engine back over by hand untill the rotor is facing #1 again on the cap and doublecheck that everything looks good. Now make a test plug and static time the engine.

 

You can make a test plug with a new spark plug and some wire, a mini hose clamp and an alligator clip. First open the spark plug gap to around 0.075" and strip both ends of a 16 gauge wire about 4 foot in length. Attach the mini hose clamp around the threads of the spark plug cause that is a ground and secure it with the mini hose clamp with the bare wire end in between to make a good connection. Next, take the other end of wire and secure to an alligator clip.

Now, you have a test plug.

Put everything back together except just leave the distributor bolt loose enough to turn it and take off the # XXXXX spark plug wire. Insert your test plug into the #1 spark plug wire and with the key on the run position turn the distributor untill you get a spark. It may take a few times and you may have to do it pretty fast but when it sparks tighten down the distributor a little so it wont move. Put the #1 spark plug wire back on and start the engine then time it with a timing light and follow any directions such as plugging vacuum hoses or just let the engine warm up and rev it up a little and advanace the timing untill you just start to hear the engine pinging and knocking then retard or turn it back untill the pinging and knocking just disappears and that should be ok. Take it for a test drive and go up a couple of steep hills at full throttle, if it pings set back the timing a little more.

Customer: replied 5 years ago.

Thank you for you answer, but the problem is that for this truck (96 Chevy Z71) I have found posts on the web were people have noted that placing the pointer on the #1 will not work. I have read people saying something about a #8 cast number on the housing that the pointer should point to, and have tried this method as well. Can someone please give me the installation, step by step, on how to place the distributor in correctly on a 96 Chevy Z71, so I can start all over and get it right this time???

 

Thanks for the help,

 

Ian

Expert:  Mike S. replied 5 years ago.

You acn place that distributor anywhere you want it. If the engine was turned when the distributor was out than the origianl placement is off and the only way to get it back is like I explained before and set it where you want. If you want it facing #8 cast number than drop it in there.

From Chilton,

Distributor

Removal & Installation

  1. Disconnect the negative battery cable.

  2. Tag and remove the spark plug wires and the coil leads from the distributor.

  3. Unplug the electrical connector at the base of the distributor.

Mark the distributor and tag the spark plug wires

Mark the distributor and tag the spark plug wires

Click to Enlarge
A distributor wrench can be used to loosen hold-down bolt

A distributor wrench can be used to loosen hold-down bolt

Click to Enlarge
  1. Loosen the distributor cap fasteners and remove the cap.

  2. Using a marker, matchmark the rotor-to-housing and housing-to-engine block positions so that they can be matched during installation.

  3. Loosen and remove the distributor hold-down bolt and clamp.

  4. Remove the distributor from the engine.

Remove the distributor hold-down bolt and clamp

Remove the distributor hold-down bolt and clamp

Click to Enlarge
Be sure to mark the distributor position before removing it

Be sure to mark the distributor position before removing it

 

Timing Disturbed
  1. Remove the No. 1 cylinder spark plug.

  2. Turn the engine using a socket wrench on the large bolt on the front of the crankshaft pulley. Place a finger near the No. 1 spark plug hole and turn the crankshaft until the piston reaches Top Dead Center (TDC). As the engine approaches TDC, you will feel air being expelled by the No. 1 cylinder. If the position is not being met, turn the engine another full turn (360 degree). Once the engine's position is correct, install the spark plug.

  3. Align the pre-drilled indent hole in the distributor driven gear with the white painted alignment line on the lower portion of the shaft housing.

  4. Using a long screwdriver, align the oil pump drive shaft in the engine in the mating drive tab in the distributor.

  5. Install the distributor in the engine.

  6. When the distributor is fully seated, the rotor segment should be aligned with the pointer cast in the distributor base. The pointer will have a "6" or "8" cast into it indicating a 6 or 8-cylinder engine. If the rotor segment is not within a few degrees of the pointer, the distributor gear may be off a tooth or more. If this is the case repeat the process until the rotor aligns with the pointer.

  7. Install the cap and fasten the mounting screws.

  8. Install the hold-down clamp and bolt, then tighten the bolt to 20 ft. lbs. (27 Nm).

  9. Engage the electrical connections and the spark plug wires.

 

Timing Not Disturbed
  1. Install the distributor in the engine making sure that the matchmarks are properly aligned.

  2. If the rotor-to-housing and housing-to-engine marks are not aligned, the distributor gear may be off a tooth or more. If this is the case repeat the installation process until the marks are perfectly aligned.

  3. Install the hold-down clamp and bolt, then tighten the bolt to 20 ft. lbs. (27 Nm).

  4. Install the distributor cap and attach the electrical connector at the base of the distributor.

  5. Attach the spark plug wires and coil leads.

  6. Connect the negative battery cable.

Expert:  Mike S. replied 5 years ago.

Timing HEI

HEI Systems

  1. Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.

Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.

  1. Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.

  2. Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the under hood emission sticker for any other hoses or wires which may need to be disconnected.

  3. Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.

The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won't work with HEI.

 

WARNING

Never ground the HEI TACH terminal; serious system damage will result.

 

  1. Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley or damper notch isn't aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.

TDC or Top Dead Center corresponds to 0°B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.

  1. Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.

  2. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.

  3. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.

  4. Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.

  5. Stop the engine and disconnect the timing light.

Customer: replied 5 years ago.
I have repeated the process again, exactly. Still not working, we have read on different blogs that a scanner needs to be used to find out how far out the timing is...then adjust from the scanner to 0 (zero) in order to get the correct timing, due to the plus + or minus - of 2 degrees allows both electronic timing systems to accurately communicate, run the engine in correct timing order. Is the scanner that we need a standard diagnostic code reader? By the way, the engine is a v8.
Expert:  Mike S. replied 5 years ago.
  • With the ignition OFF install scan tool to the Data Link Connector (DLC).

  • Start the engine and bring the vehicle to operating temperature.

  • Monitor cam retard on the scan tool.

  • If cam retard is between -2° and +2° the distributor is properly adjusted.

  • If cam retard is not between -2° and +2° the distributor must adjusted.

  • With the engine OFF loosen the distributor hold-down bolt.

  • Start the engine and check the cam retard reading. Rotate the distributor counterclockwise to compensate for a negative reading and clockwise to compensate for a positive reading.

  • Momentarily raise the engine speed to over 1000 RPM and check the cam retard reading.

  • If the proper reading is not achieved repeat Steps 7 and 8.

  • When the proper reading has been achieved tighten the distributor hold-down bolt and disconnect the scan tool.

  •  

    Scan Tools

    On most models, the stored codes may be read with only the use of a small jumper wire, however the use of a hand-held scan tool such as GM's TECH-1® or equivalent is recommended. On 1996 models, an OBD-II compliant scan tool must be used. There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use. If you own a scan type tool, it probably came with comprehensive instructions on proper use. Be sure to follow the instructions that came with your unit if they differ from what is given here; this is a general guide with useful information included.

    The scan tool allows any stored codes to be read from the ECM or PCM memory. The tool also allows the operator to view the data being sent to the computer control module while the engine is running. This ability has obvious diagnostic advantages; the use of the scan tool is frequently required for component testing. The scan tool makes collecting information easier; the data must be correctly interpreted by an operator familiar with the system.

    Example of scan tool data and typical or baseline values

    Example of scan tool data and typical or baseline values

    Click to Enlarge

     

    An example of the usefulness of the scan tool may be seen in the case of a temperature sensor which has changed its electrical characteristics. The ECM is reacting to an apparently warmer engine (causing a driveability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the ECM may be viewed; comparison to normal values or a known good vehicle reveals the problem quickly.

     

    When you brought the number 1 piston up to TDC was the timing mark on 0? If not maybe the timing chain jumped.

    Expert:  Mike S. replied 5 years ago.
    Have you checked the timing when running or cranking with a timing light? If so what is it at?
    Expert:  Mike S. replied 5 years ago.

    Double check to make sure the firing order is correct.

    1996-98 5.0L, 5.7L and 7.4L Engines Firing Order: 1-8-4-3-6-5-7-2Distributor Rotation: Clockwise

    Expert:  Mike S. replied 5 years ago.
    1996-98 5.0L, 5.7L and 7.4L Engines Firing Order: 1-8-4-3-6-5-7-2Distributor Rotation: Clockwise
    Expert:  Mike S. replied 5 years ago.
    Check the wires on the cap compared to that picture.
    Expert:  Mike S. replied 5 years ago.

    Ok, I believe that is a distributor DI system.

    It appears that timing is not adjustable and the distributor only goes in 1 way.

    Engine Disturbed

    1. Remove the No. 1 cylinder spark plug.
    2. Turn the engine using a socket wrench on the large bolt on the front of the crankshaft pulley. Place a finger near the No. 1 spark plug hole and turn the crankshaft until the piston reaches Top Dead Center (TDC). As the engine approaches TDC, you will feel air being expelled by the No. 1 cylinder. If the position is not being met, turn the engine another full turn (360 degree). Once the engine's position is correct, install the spark plug.
    3. Align the pre-drilled indent hole in the distributor driven gear with the white painted alignment line on the lower portion of the shaft housing.
    4. Using a long screwdriver, align the oil pump drive shaft in the engine in the mating drive tab in the distributor.
    5. Install the distributor in the engine.
    6. When the distributor is fully seated, the rotor segment should be aligned with the pointer cast in the distributor base. The pointer will have a "6" or "8" cast into it indicating a 6 or 8-cylinder engine. If the rotor segment is not within a few degrees of the pointer, the distributor gear may be off a tooth or more. If this is the case repeat the process until the rotor aligns with the pointer.
    7. Install the cap and fasten the mounting screws.
    8. Install the hold-down clamp and bolt, then tighten the bolt to 20 ft. lbs. (27 Nm).
    9. Engage the electrical connections and the spark plug wires.

    The Distributor Ignition (DI) system consists of the distributor, hall effect switch (camshaft position sensor), ignition coil, secondary wires, spark plugs, knock sensor and the crankshaft position sensor. The system is controlled by the Vehicle Control Module (VCM). The VCM using information from various engine sensors, controls the spark timing, dwell, and the firing of the ignition coil. It is used on the 1993-98 models.

     

    FIRING ORDERS

    To avoid confusion, remove and tag the spark plug wires one at a time, for replacement.

    If a distributor is not keyed for installation with only one orientation, it could have been removed previously and rewired. The resultant wiring would hold the correct firing order, but could change the relative placement of the plug towers in relation to the engine. For this reason it is imperative that you label all wires before disconnecting any of them. Also, before removal, compare the current wiring with the accompanying illustrations. If the current wiring does not match, take note how your engine is wired.

     

    1996-98 5.0L, 5.7L and 7.4L Engines Firing Order: 1-8-4-3-6-5-7-2 Distributor Rotation: Clockwise

    Click image to see an enlarged view

     

    IGNITION TIMING

    1996 Chevrolet Truck C1500 1/2ton P/U 2WD 5.7L SFI 8cyl

    The Ignition Timing



    Non-adjustable.

     

    OPERATION



    See Figure 1

    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.

    TESTING



    See Figure 2



    Click image to see an enlarged view

    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

    OPERATION



    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.



    Click image to see an enlarged view

    Fig. Fig. 1: Crankshaft Position (CKP) sensor

    TESTING



    See Figure 2

    1. Disconnect the CKP sensor harness. Connect an LED test light between battery ground and CKP harness terminal A.
    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 CKP harness terminal A and B. Verify that the test light illuminates.




    Click image to see an enlarged view

    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)

    OPERATION



    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

    OPERATION



    See Figure 1

    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.



    Click image to see an enlarged view

    Fig. Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor

    TESTING



    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.




    Click image to see an enlarged view

    Fig. Fig. 2: Using an ohmmeter, backprobe terminals of the TPS sensor to check for proper resistances



    Click image to see an enlarged view

    Fig. Fig. 3: The TP sensor and IAC sensor are usually located at the side of the throttle body



    Click image to see an enlarged view

    Fig. Fig. 4: Idle Air Control (IAC) valve wiring and terminal identification

     

    Intake Air Temperature (IAT) Sensor

    OPERATION



    See Figure 1

    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.



    Click image to see an enlarged view

    Fig. Fig. 1: Intake Air Temperature (IAT) sensor

    TESTING



    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.




    Click image to see an enlarged view

    Fig. Fig. 2: Intake Air Temperature (IAT) sensor wiring diagram



    Click image to see an enlarged view

    Fig. Fig. 3: Intake Air Temperature (IAT) sensor temperature vs. resistance values

     

    Manifold Absolute Pressure (MAP) Sensor

    OPERATION



    See Figures 1 and 2

    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.



    Click image to see an enlarged view

    Fig. Fig. 1: Common Manifold Absolute Pressure (MAP) sensor used on 4.3L, 5.0L and 5.7L engines



    Click image to see an enlarged view

    Fig. Fig. 2: Common Manifold Absolute Pressure (MAP) sensor used on 7.4L engines

    TESTING



    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.




    Click image to see an enlarged view

    Fig. Fig. 3: Location of the MAP sensor-TBI system shown



    Click image to see an enlarged view

    Fig. Fig. 4: Probe the terminals of the MAP sensor to check for proper reference voltage



    Click image to see an enlarged view

    Fig. Fig. 5: Manifold Absolute Pressure (MAP) sensor wiring diagram

     

    Mass Air Flow (MAF) Sensor

    OPERATION



    See Figure 1

    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.



    Click image to see an enlarged view

    Fig. Fig. 1: Exploded view of the Mass Air Flow (MAF) sensor

    TESTING



    See Figure 2

    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.




    Click image to see an enlarged view

    Fig. Fig. 2: Mass Air Flow (MAF) sensor wiring diagram

     

    Throttle Position Sensor (TPS)

    OPERATION



    See Figure 1

    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).



    Click image to see an enlarged view

    Fig. Fig. 1: Common Throttle Position Sensor (TPS) found on GM trucks

    TESTING



    See Figures 2, 3 and 4

    1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
    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 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.




    Click image to see an enlarged view

    Fig. Fig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage



    Click image to see an enlarged view

    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



    Click image to see an enlarged view

    Fig. Fig. 4: Throttle Position Sensor (TPS) wiring diagram

     

    Vehicle Speed Sensor (VSS)

    OPERATION



    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.

    TESTING



    See Figure 1



    Click image to see an enlarged view

    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.

     

     

    Expert:  Mike S. replied 5 years ago.
    Once you get the distributor in and set then if it still dont run right check these sensors.
    Customer: replied 5 years ago.
    We are in receipt of your advice and will get back to you with results.
    Expert:  Mike S. replied 5 years ago.
    ok

    JustAnswer in the News:

     
     
     
    Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
    JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
    Web sites like justanswer.com/legal
    ...leave nothing to chance.
    Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
    Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
    I will tell you that...the things you have to go through to be an Expert are quite rigorous.
     
     
     

    What Customers are Saying:

     
     
     
    • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
    < Last | Next >
    • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
    • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
    • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
    • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
    • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
    • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
    • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland
     
     
     

    Meet The Experts:

     
     
     
    • Tim's Auto Repair

      mechanic

      Satisfied Customers:

      7641
      Have owned a repair shop for 25 yrs.
    < Last | Next >
    • http://ww2.justanswer.com/uploads/TI/tim37857/2011-10-28_23053_S6301814.64x64.JPG Tim's Auto Repair's Avatar

      Tim's Auto Repair

      mechanic

      Satisfied Customers:

      7641
      Have owned a repair shop for 25 yrs.
    • http://ww2.justanswer.com/uploads/DT/dt69nova/2012-6-15_234154_pic1.64x64.jpg Dave Nova's Avatar

      Dave Nova

      Chevrolet Technician

      Satisfied Customers:

      6870
      ASE Master Certification. GM World Class Certification
    • http://ww2.justanswer.com/uploads/ggrote1/2009-06-27_031011_me.jpg Gary's Avatar

      Gary

      Chevy Technician

      Satisfied Customers:

      5404
      15 years GM master tech
    • http://ww2.justanswer.com/uploads/testedtuff/2009-09-26_170108_pictures.jpg John's Avatar

      John

      Master A.S.E Automotive technician

      Satisfied Customers:

      4071
      A.S.E MasterTechnician A1-A8 w/L1,38 years experience General Motors Master Technician
    • http://ww2.justanswer.com/uploads/RO/rodcuda/2011-7-25_224114_Donhoff1.64x64.JPG Neal's Avatar

      Neal

      Chevy Mechanic

      Satisfied Customers:

      2306
      Shop Owner and Mechanic on multiple makes.
    • http://ww2.justanswer.com/uploads/goodwrench9124/2008-11-09_220838_dscf0007.jpg goodwrench9124's Avatar

      goodwrench9124

      ASE Certified Technician

      Satisfied Customers:

      2041
      Ase Master Certified, GM Master Certified Tech. Awarded GM Top performer, Hybrid certified
    • http://ww2.justanswer.com/uploads/DR/drivefast1971/2012-1-12_172422_0009051tQKh.64x64.jpg Ron Z.'s Avatar

      Ron Z.

      - Chevy Tech -

      Satisfied Customers:

      1912
      18+yrs experience. State Inspector and Chevy Diagnostics
     
     
     

    Related Chevy Questions

    Chat Now With A Chevy Mechanic
    Mike S.
    Mike S.
    Chevy Mechanic
    5261 Satisfied Customers
    ASE Certified Master Technician