GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Computer Control Module (ECM/PCM/VCM)

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The heart of the electronic control system which is found on most vehicles covered by this manual is a computer control module. The module gathers information from various sensors, then controls fuel supply and engine emission systems. Most vehicles are equipped with an Engine Control Module (ECM) which, as its name implies, controls the engine and related emissions systems. Some ECMs may also control the Torque Converter Clutch (TCC) on automatic transmission vehicles or the manual upshift light on manual transmission vehicles. Later model vehicles may be equipped with a Powertrain Control Module (PCM) or a Vehicle Control Module (VCM). Both of these are similar to the original ECMs, but are 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. The VCM is used to control the manual shift lamp and electronic brake control functions.

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 chip or CALPAK that contains calibration information which is particular to the vehicle application. This chip is not supplied with a replacement module and must be transferred to the new module before installation. Late model vehicles equipped with a VCM utilize both a PROM chip and an Electronically Erasable Programmable Read Only Memory (EEPROM) which must be reprogrammed after installation.


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.

REMOVAL & INSTALLATION



See Figures 1, 2 and 3

For most applications, the control module is located inside the passenger compartment, on the right side of the vehicle. For these vehicles it is usually found either under/behind the glove box or behind the kick panel. Some late model applications equipped with a VCM have the control module located in the engine compartment mounted on the right side, on/near the fender and wheel well. But not all VCMs are mounted in the engine compartment.

  1. Make sure the ignition is turned OFF , then disconnect the negative battery cable.
  2.  
  3. Locate the computer control module. If not readily visible on the right side of the engine compartment, it is probably mounted under the dash.
  4.  



Click image to see an enlarged view

Fig. Fig. 1: Common control module mounting on early model vehicles

  1. If equipped with a passenger compartment mounted module, remove the dash panel located below the glove box and/or passenger side kick panel, as necessary.
  2.  
  3. Carefully disengage the connectors from the control module.
  4.  

On some modules, it may be easier to unfasten and rotate the module for easier access to the wiring. If necessary, do this before attempting to disengage the connectors.

  1. Loosen and remove the mounting hardware, then remove the module from the vehicle.
  2.  

To install:
  1. If the module is being replaced, remove the access covers and CAREFULLY replace the PROM chip or CALPAK. refer to the procedure later in this section for details and cautions.
  2.  
  3. Make sure the access cover is properly installed.
  4.  


CAUTION
Remember, the ignition MUST be OFF when connecting the control module wiring.

  1. Position the computer control module to the vehicle and secure using the hardware. If it was necessary to pivot the module for wire removal, then make sure the connectors are engaged before positioning the module.
  2.  
  3. Engage the wiring connectors to the module.
  4.  
  5. If removed for access, install the dash and/or kick panel.
  6.  
  7. Make sure the ignition is still OFF , then connect the negative battery cable.
  8.  
  9. If equipped with a VCM, the EEPROM must be reprogrammed using a suitable service system and the latest available software. In all likelihood, the vehicle must be towed to a dealer or repair shop containing the suitable equipment for this service.
  10.  
  11. Enter the self-diagnostic system and check for trouble codes to be sure the module and PROM/CALPACK are properly installed. For details, refer to the procedures for checking trouble codes, later in this section.
  12.  



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Fig. Fig. 2: Common instrument panel control module mounting on late model vehicles



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Fig. Fig. 3: Late model VCM mounting in the engine compartment

PROM/CALPAK REPLACEMENT



See Figures 4 through 8

As stated earlier, all computer control modules contain information regarding the correct parameters for engine and system operation based on vehicle applications. In most modules this information takes the form of a PROM chip or CALPAK, though some modules also store this information in an EEPROM. Since replacement modules are normally not equipped with this PROM/CALPAK, you must transfer the chip from the old component.


CAUTION
The PROM/CALPAK chip and computer control module are EXTREMELY sensitive to electrical or mechanical damage. NEVER touch the connector pins or soldered components on the circuit board in order to prevent possible electrostatic discharge damage to the components.



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Fig. Fig. 4: Retaining tabs must be pushed back to release the PROM/CALPAK unit on some late model vehicles



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Fig. Fig. 5: The PROM unit for the VCM



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Fig. Fig. 6: PROM replacement on late model ECM and PCMs



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Fig. Fig. 7: Replacing the PROM unit on VCMs



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Fig. Fig. 8: Removing the PROM/CALPAK and carrier using a removal tool

  1. Make sure the ignition is OFF , then remove the computer control module from the vehicle.
  2.  
  3. Remove the access cover from the module.
  4.  

Never force a PROM/CALPAK into or out of position. The component and connector pins are easily damaged.

  1. Remove the PROM chip or CALPAK from the module:
    1. If a removal tool was supplied with the replacement module it should be used to prevent damage to the components.
    2.  
    3. Some early model units utilize a PROM and carrier assembly. On these components, the carrier and PROM should be removed together.
    4.  
    5. Some late model units are equipped with retaining tabs which must be pushed back to release the chip. Other models, such as the VCM, utilize a PROM unit with retaining tabs on either end, these must be gently squeezed as the unit is pulled upward.
    6.  

  2.  

To install:

Use extra care to properly align the PROM during installation. In most cases, if the PROM is installed backwards and the ignition is turned ON, the PROM will be instantly destroyed.

  1. If equipped with a PROM carrier, position the chip at the top, flush with the top surface of the carrier.
  2.  

If equipped with a PROM carrier, the carrier and chip (positioned at the top) should be carefully inserted into the module. Once the carrier is in place use a pencil eraser to gently push down on the top of the chip until the pins are properly in contact with the module.

  1. Check the PROM/CALPAK chip or unit and carrier (if equipped) for alignment marks or tabs. Make sure the pins are properly aligned and carefully install the PROM/CALPAK to the module.
  2.  

Except for VCMs equipped with a PROM unit with attached tabs, DO NOT press the PROM/CALPACK in until it clicks. The retaining tabs on ECM and PCM modules should be gently pushed over the edges of the unit after installation. Pressing on these PROM/CALPAKs until the tabs snap into place could cause circuit board or clip damage.

  1. DO NOT force the PROM/CALPAK into position. Carefully press until the unit is seated. For VCMs equipped with a PROM unit, press until a light click is heard and the tabs are in place. For other late model units equipped with retaining tabs, the tabs must be gently positioned over the ends of the PROM/CALPAK.
  2.  
  3. Install the access cover to the module.
  4.  
  5. Make sure the ignition is OFF , then install the computer control module to the vehicle.
  6.  

 
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