GM Corvette 1984-1996 Repair Guide

Engine Control Module (ECM)

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OPERATION



Whenever the term Electronic Control Module (ECM) is used in this guide, it will refer to the engine control computer, whether it is a Powertrain Control Module (PCM) or Electronic Control Module (ECM).

The heart of the electronic control system, which is found on the vehicles covered by this guide, is a computer control module. The module gathers information from various sensors, then controls fuel supply and engine emission systems. Most early model vehicles are equipped with an Engine Control Module (ECM) which, as its name implies, controls the engine and related emissions systems. Later model vehicles may be equipped with a Powertrain Control Module (PCM). This is similar to the original ECMs, but is designed to control additional systems as well. The PCM may control the manual transmission shift lamp or the shift functions of the electronically controlled automatic transmission.

Regardless of the name, all computer control modules are serviced in a similar manner. Care must be taken when handling these expensive components in order to protect them from damage. Carefully follow all instructions included with the replacement part. Avoid touching pins or connectors to prevent damage from static electricity.

All of these computer control modules contain a Programmable Read Only Memory (PROM) chip, MEM-CAL or EEPROM that contains calibration information specific to the vehicle application. For all applications except those equipped with an EEPROM, this chip is not supplied with a replacement module, and must be transferred to the new module before installation. If equipped with an Electronically Erasable Programmable Read Only Memory (EEPROM), it must be reprogrammed after installation. Some later model vehicles have a Knock Sensor (KS) module, mounted in the PCM. The KS module contains the circuitry that allows the PCM to utilize the Knock Sensor signal to diagnose the circuitry.


WARNING
To prevent the possibility of permanent control module damage, the ignition switch MUST always be OFF when disconnecting power from or reconnecting power to the module. This includes unplugging the module connector, disconnecting the negative battery cable, removing the module fuse or even attempting to jump start your dead battery using jumper cables.

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.

REMOVAL & INSTALLATION



See Figures 1, 2, 3 and 4

  1. Make sure the ignition switch is turned OFF , then disconnect the negative battery cable.
  2.  


CAUTION
To prevent the possibility of permanent control module damage, the ignition switch MUST always be OFF when disconnecting power from or reconnecting power to the module. This includes unplugging the module connector, disconnecting the negative battery cable, removing the module fuse or even attempting to jump your dead battery using jumper cables.

  1. Locate the computer control module. On 1984-89 vehicles, it is located in the passenger compartment, under the right hand (passengers) side of the instrument panel. On 1990-96 vehicles, the module is located in the engine compartment, mounted against the left hand (drivers) side of the firewall, near the brake master cylinder and booster.
  2.  
  3. For 1984-89 vehicles, unfasten the retainers, then remove the right side kick/hush panel.
  4.  
  5. Remove the computer control module mounting hardware.
  6.  
  7. If equipped, remove the electrical connector cover.
  8.  
  9. Detach the electrical connectors from the control module.
  10.  
  11. Remove the computer control module from the passenger or engine compartment.
  12.  



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Fig. Fig. 1: Exploded view of the Engine Control Module (ECM) location-1984 vehicle shown



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Fig. Fig. 2: View of the ECM-1985-89 vehicles



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Fig. Fig. 3: Exploded view of the ECM with the cover and MEM-CAL-1990-95 vehicles, except 1994-95 VIN P



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Fig. Fig. 4: PCM location and mounting-1994-96 VIN P & 5 engines

To install:

If the control module is being replaced, the PROM, MEM-CAL or Knock Sensor (KS) module must be transferred to the new module, as outlined later in this section. Also, make sure the service number on the new module is the same as the defective module.

  1. Attach the electrical connectors to the computer control module.
  2.  
  3. Position the control module in its mounting location and secure with the mounting hardware.
  4.  
  5. If equipped, install the electrical connector cover.
  6.  
  7. For 1984-89 vehicles, install the right side kick-hush panel and secure with the retainers.
  8.  

On 1994-96 vehicles equipped with a VIN P or 5 engine, the replacement PCM is not supplied with an EEPROM program. The replacement PCM must be reprogrammed with a Tech 1®, or equivalent scan tool, with the proper software application for your vehicle. If the PCM is not reprogrammed, the car will not run.

  1. Check that the ignition switch is OFF , then connect the negative battery cable.
  2.  

PROM, MEM-CAL & KS MODule Replacement

See Figures 5, 6, 7, 8, 9, 10, 11 and 12

  1. For 1984 vehicles equipped with a PROM, remove it from the ECM as follows:
    1. Turn the ECM so the bottom cover is facing up.
    2.  
    3. Remove the slide off PROM access cover.

      Click image to see an enlarged view

      Fig. Fig. 5: The PROM cover slides off for access to the PROM carrier-1984 vehicles

    4.  
    5. Use a suitable PROM removal tool to carefully grasp the PROM carrier at the narrow ends, as shown in the accompanying figure. Gently rock the carrier from end to end while applying a firm upward force and remove the PROM from the ECM. Make sure to note the reference end of the PROM carrier, then carefully position the PROM carrier aside.

      Click image to see an enlarged view

      Fig. Fig. 6: You will need a suitable tool to rock the PROM carrier out of the ECM

      If installing a new PROM, make sure the notch in the PROM is matched to the small notch in the carrier.

      Click image to see an enlarged view

      Fig. Fig. 7: You must note the reference end of the PROM for proper installation

    6.  
    7. Take the PROM (which is mounted in the PROM carrier), and position the carrier squarely over the PROM socket with the small notched end of the carrier aligned with the small notch socket at the pin 1 end. Press on the PROM carrier until it is firmly seated in its socket.

      Click image to see an enlarged view

      Fig. Fig. 8: Press the PROM carrier squarely into the socket

      ***WARNING***If the PROM is installed backward and the ignition switch is turned ON, the PROM will be destroyed.
    8.  
    9. Reinstall the access cover on the ECM.
    10.  

  2.  
  3. For 1985-95 vehicles equipped (except 1994-95 VIN P engines) with a MEM-CAL or PROM, transfer it as follows:
    1. Remove the access cover.
    2.  
    3. Using 2 fingers, push both retaining clips back away from the MEM-CAL. At the same time, grasp the MEM-CAL at both ends then carefully lift it up out of the socket.

      Click image to see an enlarged view

      Fig. Fig. 9: Use 2 fingers to push the retaining clips away from the MEM-CAL or PROM to remove it

      ***WARNING***Do NOT remove the MEM-CAL cover.
    4.  
    5. Note the MEM-CAL alignment notches for installation.
    6.  
    7. Place the MEM-CAL in the socket, pressing on the ends of the MEM-CAL only. The small notches must be aligned.

      Click image to see an enlarged view

      Fig. Fig. 10: When installing the MEM-CAL or PROM, push it in on the ends, NOT the middle

    8.  
    9. Press on the ends of the MEM-CAL until the retaining clips snap into the ends of the MEM-CAL. Do NOT press on the middle of the MEM-CAL, only press on the ends.
    10.  
    11. Install the access cover, then install the ECM, as outlined earlier in this section.
    12.  

  4.  
  5. For 1994-96 VIN P or 5 engine, equipped with a Knock Sensor (KS) module, remove the control module access cover, then remove the KS module by pressing the clips in toward each other and lifting the module out.
  6.  



Click image to see an enlarged view

Fig. Fig. 11: Unfasten the retainers, then remove the KS module access cover



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Fig. Fig. 12: Push the KS module retaining tabs toward each other, then lift the module out of the PCM

 
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