GM Cutlass RWD 1970-1987 Repair Guide

Electronic Control Module (ECM)


See Figure 1


The ignition must be OFF whenever disconnecting or connecting the ECM electrical harness. It is possible to install a PROM backwards during service. Exercise care when replacing the PROM that it is installed correctly, or the PROM will be destroyed when the ignition is switched ON.

The Electronic Control Module (ECM) is located under the instrument panel. To allow one model of ECM to be used on different vehicles, a device called a calibrator or Programmable Read Only Memory (PROM) chip is installed inside the ECM which contains information on vehicle weight, engine, transmission, axle ratio, etc. The PROM is specific to the exact model and replacement part numbers must be checked carefully to make sure the correct PROM is being installed during service. Replacement ECM units (called Controllers) are supplied WITHOUT a PROM. The PROM from the old ECM must be carefully removed and installed in the replacement unit during service. Another device called a CALPAK is used to allow fuel delivery if other parts of the ECM are damaged the ("limp home") mode. The CALPAK is similar in appearance to the PROM and is located in the same place in the ECM, under an access cover. Like the PROM, the CALPAK must be removed and transferred to the new ECM unit being installed.

If the diagnosis indicates a faulty ECM unit, the PROM should be checked to see if is the correct part. Trouble code 51 indicates that the PROM is installed incorrectly. When replacing the production ECM with a new part, it is important to transfer the Broadcast code and production ECM number to the new part label. Do not record on the ECM cover.

  1. Disconnect the battery cables.
  3. Remove the wiring harness and mounting hardware.
  5. Remove the ECM from the passenger compartment.
  7. The PROM and CALPAK are located under the access cover on the top of the control unit. Using the rocker type PROM removal tool, or equivalent, engage one end of the PROM carrier with the hook end of the tool.
  9. Press on the vertical bar end of the tool and rock the engaged end of the PROM carrier up as far as possible.
  11. Engage the opposite end of the PROM carrier in the same manner and rock this end up as far as possible.
  13. Repeat this process until the PROM carrier and PROM are free of the socket. The PROM carrier should only be removed with the removal tool or damage to the PROM or PROM socket may occur.

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Fig. Fig. 1: Computer Controlled Catalytic Converter (C-4) system schematic