Toyota Pick-ups, Land Cruiser, and 4 Runner 1997-00

Electronic Control Module (ECM)



The Electronic Control Module (ECM) is the vehicle's on-board computer, the center of the vehicle's electronic control system. The ECM receives signals from various sensors on the engine. The ECM constantly looks at the information from these sensors, processes this information and calculates the correct air/fuel mixture, ignition spark timing and controls the systems that affect vehicle performance.

The ECM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the MIL (Malfunction Indicator Lamp, also known as the "Service Engine Soon" light), and store Diagnostic Trouble Codes (DTCs) which identify the problem areas to aid the technician in making repairs.

Current systems, including the Toyota trucks covered by this guide, use a next-generation system called On Board Diagnostics II (OBD II). This makes available a great deal of information to the service technician. A specialized piece of equipment called a Scan Tool is connected to vehicle's Diagnostic Connector. The scan tool can then read the data stream from the ECM, interrogate the computer for stored DTCs, even obtain a freeze-frame (snapshot) of the vehicle's operating conditions when a DTC is set. While the computer can't tell a technician which component to replace, it can narrow down which system or circuit may require attention, and read real-time operating temperatures, voltages and resistance values of various sensors. A scan tool connected to the vehicle's on-board computer can even take over control of some systems during troubleshooting (fuel pump control, for example).

While an OBD II compliant system may make more information available that can be very helpful when troubleshooting, it also means that traditional methods of troubleshooting have changed. In almost every case, an expensive scan tool is required along with the experience and expertise to interpret the data being presented. This takes most all electronic engine control diagnostics and troubleshooting out of the hands of the non-professional.

The inexperienced and ill equipped can cause a great deal of damage to any automotive computer controlled system. Due to the very small voltages and resistances involved with the computer and its related sensors and controls, using traditional testing methods such as test lights and voltmeters can cause such a relatively large electrical draw that costly electronic components can and will be damaged. On the other hand, a good circuit producing its designed output voltage (for example, very small voltages, usually in the millivolt range) may not be producing enough voltage to light a test light or move an analog meter needle, giving a false indication of circuit or component failure when it actually is within its operating parameters.

The ECM itself is a relatively fragile and expensive component. Always follow these precautions when servicing the electronic control system.


To prevent possible Electrostatic Discharge damage to the ECM, DO NOT touch the connector pins or the soldered components on any circuit board.
To prevent internal ECM damage, make sure the ignition switch is in the OFF position when installing or removing the PCM connectors and disconnecting or reconnecting the power to the ECM (battery cable, ECM wiring, jumper cables, etc.).
Do NOT permit parts to receive a severe impact during removal or installation. Always handle all fuel injection parts with care, especially the ECM. DO NOT open the ECM cover!
Before removing the fuel injected wiring connectors, terminals, or any other electric component, first disconnect the power by either disconnecting the negative battery cable or turning the ignition switch OFF .
Do NOT be careless during troubleshooting. There are numerous transistorized circuits. Even a slight terminal contact can induce a problem into the system.
Use care when detaching and attaching electrical connectors. Release the lock and pull on the connectors. At assembly, fully insert the connector and check that it is locked.


  1. Disconnect the negative battery cable.

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Fig. The ECM is usually found behind the glove box2000 Tundra shown, others similar

  1. Disconnect the negative battery cable.
  3. Remove the glove compartment door.
  5. Remove the glove compartment frame (Toyota calls it the "Lower No. 2 Finish Panel").
  7. Some vehicles may have a heater duct or other component that should be removed.
  9. Unbolt the ECM from its brackets.
  11. Carefully detach the electrical connectors.

To install:

  1. Carefully attach the electrical connectors.
  3. Install the ECM mounting brackets, as required.
  5. Mount the ECM on the vehicle in the proper location.
  7. Install the remaining components including the finish panel and glove compartment door.
  9. Connect the negative battery cable.