Engine Control Computer


If you remove and repair a ground connection, be sure you reconnect it to the same place. Do not change the length of the wire.

  • People who work around sensitive components sometimes wear a ground strap attached to their wrist.
  • You can eliminate the static electricity by touching ground before you touch a computer.
  • Do not take the computer out of the container until you are already in the front seat.

The computer is rarely the cause of problems in the fuel system. When it is, the parts are not repaired. The entire unit is replaced.

  • Remanufactured computers are widely available at a reduced cost for popular makes of cars.
  • Some imported car computers are very costly because there are not enough cores available for exchange.

Be sure to check the power and ground circuits before replacing a computer.

Electronic instruments can be damaged by excessive voltage, heat, moisture, or vibration, so computers are usually mounted in the driver's compartment.

  • Sometimes, they are in the right-hand kick panel (in front of the passenger door) or near the glove box.
A computer loacted inside the right front kick panel. Courtesy of Mazda Motor Corporation.
  • Other times they are located under the passenger seat.
  • Newer computers are designed to dissipate heat more quickly. Some of these are installed in the engine compartment.
  • To be safe, pull the computer fuse before unhooking its connectors.

To remove the unit:

Disconnect the negative battery cable. Electronic Control Computer removal procedures vary according to vehicle make and model. Consult your service manual for the procedure.
  • If necessary, remove any plates or covers.
  • Disconnect the wiring harness.
  • Unbolt the module from its mounting.

Some computers have a replaceable element call a programmable read-only memory (PROM).

On some cars the PROM is replaceable. Courtesy of General Motors Corporation, Service Technology Group.

The old PROM is installed in the replacement computer. Other later model computers have electronically erasable (EE) flash PROMS. These are programmed electronically, either in the dealership or by telephone modem from the manufacturer.

To replace the unit:

  • Bolt the module to its mounting. Torque the bolts to manufacturer's specifications.
  • Connect the wiring harness.
  • Reinstall the kick plate or other cover.

After replacement of the computer or when a battery has been disconnected, poor driveability and performance can result until the computer relearns the best driveability settings. Hard starting, high idle, stumble, and stalling are possible symptoms.

Wiring Connections

When connections are replaced:

  • Use solder rather than crimp connectors.
  • In areas where the roads are salted in the winter, use shrink tubing over the soldered connection. Salt accelerates the corrosion process.