Engine Control Module (ECM)


The heart of the electronic control system which is found on all 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). This is similar to the original ECM, 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, CALPAK or MEM-CAL 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. Some late model vehicles equipped with a 3.1L engine utilize both a PROM chip and an Erasable Programmable Read Only Memory (EPROM), which must be programmed with a scan tool after installation. On these models, it is recommended for this procedure to be performed at a properly equipped, reputable shop.

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.

Engine Control Module Functional Check

  • Turn the ignition switch  ON.
  • Enter diagnostics, by grounding the appropriate ALDL terminals. Refer to diagnostics procedures in this section.
  • Allow Code 12 to flash 4 times to verify that no other codes are present. This indicates the PROM or MEM-CAL is installed properly.
  • If trouble Codes 42, 43 or 51 are present or if the SERVICE ENGINE SOON light is on constantly with no codes, the PROM/MEM-CAL is not fully seated, installed backward, has bent pins or is defective.

    NOTE: Anytime the calibration unit is installed backward and the ignition switch is turned ON, the unit is destroyed.

  • If it is not fully seated, press firmly on the ends of the MEM-CAL.

ECM - Engine Control Module Diagnosis and Testing: Troubleshooting

Diagnosis of a driveablility and/or emissions problems requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to perform any repairs before performing the preliminary diagnostic steps. In many cases this will shorten diagnostic time and often cure the problem without electronic testing.

The proper troubleshooting procedure for these vehicles is as follows:

Visual/Physical Underhood Inspection

This is possibly the most critical step of diagnosis. A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other component. Wiring should be checked carefully for any sign of strain, burning, crimping, or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit.


If a fault occurs intermittently, such as a loose connector pin breaking contact as the vehicle hits a bump, the computer control module (ECM or PCM depending upon application) will note the fault as it occurs and energize the dash warning lamp. If the problem self-corrects, as with the terminal pin again making contact, the dash lamp will extinguish after 10 seconds but a code will remain stored in the computer control module's memory.

When an unexpected code appears during diagnostics, it may have been set during an intermittent failure that self-corrected; the codes are still useful in diagnosis and should not be discounted.

Circuit/Component Diagnosis and Repair

The fault codes and the scan tool data will lead to diagnosis and checking of a particular circuit. It is important to note that the fault code indicates a fault or loss of signal in an ECM-controlled system, not necessarily in the specific component.

Refer to the appropriate Diagnostic Code chart to determine the codes meaning. The component may then be tested following the appropriate component test procedures found in this section. If the component is OK, check the wiring for shorts or opens. Further diagnoses should be left to an experienced driveability technician.

If a code indicates the ECM to be faulty and the ECM is replaced, but does not correct the problem, one of the following may be the reason:

  • There is a problem with the ECM terminal connections: The terminals may have to be removed from the connector in order to check them properly.
  • The ECM or PROM is not correct for the application: The incorrect ECM or PROM may cause a malfunction and may or may not set a code.
  • The problem is intermittent: This means that the problem is not present at the time the system is being checked. In this case, make a careful physical inspection of all portions of the system involved.
  • Shorted solenoid, relay coil or harness: Solenoids and relays are turned "ON" and "OFF" by the ECM using internal electronic switches called "drivers." Each driver is part of a group of four called "Quad-Drivers."A shorted solenoid, relay coil or harness may cause an ECM to fail, and a replacement ECM to fail when it is installed. Use a short tester, J 34696, BT 8405, or equivalent, as a fast, accurate means of checking for a short circuit.
  • The Programmable Read Only Memory (PROM) may be faulty: Although the PROM rarely fails, it operates as part of the ECM. Therefore, it could be the cause of the problem. Substitute a known good PROM.
  • The replacement ECM may be faulty: After the ECM is replaced, the system should be rechecked for proper operation. If the diagnostic code again indicates the ECM is the problem, substitute a known good ECM. Although this is a rare condition, it could happen.

Engine Control Module Related Symptoms

  • No Crank
  • No Start condition
  • No communication with scan tools
  • No communication with other modules
  • MIL lamp illumination
  • Intermittent component functions
  • Poor fuel economy
  • High emissions
  • Drivability concerns (stalling, bucking, stumble etc.)
  • Charging system malfunctions

Engine Control Module Replacement

  1. Turn the ignition switch  OFF.
  2. Disconnect the negative battery cable.
  3. Remove the right side hush panel, glove box or interior access panel, as required for access to the control module.
  4. Detach the electrical harness connectors from the computer control module.
  5. Unfasten the module-to-bracket retaining screws, then remove the ECM or PCM as applicable.
  6. If replacement of the calibration unit is required, unfasten the access cover retaining screws, then remove the cover from the computer control module. Carefully remove the calibration unit from the ECM/PCM, as follows:
    1. If the ECM contains a PROM carrier, use the rocker type PROM removal tool.
    2. If the ECM contains a CAL-PAK, grasp the CAL-PAK carrier (at the narrow end only), using the removal tool. Remove the CAL-PAK carrier.
    3. If the ECM/PCM contains a MEM-CAL, EPROM or KS Module, push both retaining clips back away from the MEM-CAL/EPROM. At the same time, grasp it at both ends and lift it up out of the socket. Do not remove the cover of the MEM-CAL/EPROM/KS module.

      NOTE: Before replacement of a defective computer control module, first check the resistance of each ECM/PCM controlled solenoid. This can be done at the module connector, using an ohmmeter and the ECM or PCM connector wiring diagram. Any computer control module controlled device with low resistance will damage the replacement ECM/PCM due to high current flow through the internal circuits.

      To install: 

  7. Fit the replacement calibration unit into the socket.

    NOTE: The small notch of the carrier should be aligned with the small notch in the socket. Press on the ends of the carrier until it is firmly seated in the socket. Do not press on the calibration unit, only the carrier.

  8. Install the access cover, then secure using the retaining screws.
  9. Position the computer control module in the vehicle, then install the module-to-bracket retaining screws.
  10. Attach the module electrical harness connectors.
  11. Install the right side hush panel, glove box or interior access panel as required.
  12. Check that the ignition switch is  OFF, then connect the negative battery cable.
  13. If your vehicle has the 3.1L engine which is equipped with a PCM with EPROM, the EPROM must be reprogrammed using a scan tool 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.
  14. Enter the self-diagnostic system and check for trouble codes to be sure the module and calibration unit are properly installed. For details, refer to the procedure for checking trouble codes, later in this section.

ECM - Engine Control Module Diagram