Roadmaster 1992

Electronic Control Module



Click image to see an enlarged view

Fig. ECM location-1991-1993


The Computer Command Control System (CCC) is an electronically controlled exhaust emission system that can monitor and control a large number of interrelated emission control systems. It can monitor up to 15 various engine/vehicle operating conditions and then use this information to control as many as 9 engine related systems. The system is thereby making constant adjustments to maintain good vehicle performance under all normal driving conditions while at the same time allowing the catalytic converter to effectively control the emissions of HC, CO and NOx.

The 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.

Inputs are received from many sources to form a complete picture of engine operating conditions. Some inputs are simply Yes or No messages, such as that from the Park/Neutral switch; the vehicle is either in gear or in Park/Neutral; there are no other choices. Other data is sent in quantitative input, such as engine RPM or coolant temperature. The ECM is pre-programmed to recognize acceptable ranges or combinations of signals and control the outputs to control emissions while providing good drivability and economy. The ECM also monitors some output circuits, making sure that the components function as commanded. For proper engine operation, it is essential that all input and output components function properly and communicate properly with the ECM.

Since the control module is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The dashboard warning lamp-CHECK ENGINE or SERVICE ENGINE SOON-will illuminate to advise the operator that the system has detected a fault.

More than one code may be stored. Although not every engine uses every code, possible codes range from 12 to 999. Additionally, the same code may carry different meanings relative to each engine or engine family. For example, on the 3.3L (VIN N), code 46 indicates a fault found in the power steering pressure switch circuit. The same code on the 5.7L (VIN F) engine indicates a fault in the VATS anti-theft system.

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 possibly at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Drivability is almost always affected when the ECM enters this mode.

Removal & Installation

  1. Before servicing the vehicle, refer to the Precautions Section.
    Disconnecting the negative battery cable on some vehicles may interfere with the functions of the on-board computer systems and may require the computer to undergo a relearning process, once the negative battery cable is reconnected.

    If working near and/or around the SRS system and components, be sure to properly disable the SRS system. See disarming/arming the SRS system.

  3. Disconnect the negative battery cable.
  5. Remove the right side hush panel.
  7. Disconnect the electrical connectors.
  9. Remove the ECM from its mounting.
  11. As required, service the PROM, using a PROM removal tool.

    Click image to see an enlarged view

    Fig. ECM and related components-1991-1993

    Click image to see an enlarged view

    Fig. ECM-PROM location and related components-1991-1993

    Click image to see an enlarged view

    Fig. ECM-PROM removal-1991-1993


To install:

Be sure to use new fasteners, as required.

Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread sealer are identified. Do not use paints, lubricants or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners.

  1. Installation is the reverse of the removal procedure.
    A code 51 indicates bent PROM pins or incorrect installation. Do not install the PROM backward or it will be damaged once the ignition key is positioned in the ON position.

  3. Using the GM diagnostic scan tool or aftermarket equivalent reprogram the necessary systems and components. Be sure to follow the scan tool manufacturer-s directions.