How to Replace a Mercury PCM

by Cayden Conor

The powertrain control module, or PCM, in the Mercury Sable reads information sent via voltage from certain sensors to figure the proper fuel flow rate required to maintain the proper air-to-fuel ratio. It uses this information to control relays, solenoids and actuators, via voltage signals. The PCM also sends a command to the fuel injectors, telling them how long to stay open. The longer the injectors are open, the more fuel the cylinders receive. The PCM also senses changes in altitude, and automatically compensates for those changes.

Step 1

Disconnect the negative battery cable, using the appropriate wrench, and set it aside, ensuring that it does not touch metal. Remove the cowl deflector, using the appropriate socket. Remove the screw that secures the ground cable for the engine control harness to the dash panel.

Step 2

Ground yourself by touching metal. Even a small discharge of static electricity from your fingers can damage the PCM. Loosen the bolt holding the engine control wiring harness onto the PCM. You don't need to remove the bolt, just unscrew it about halfway. If the plug does not easily pull off the PCM, loosen the screw a few more threads. Pull the connector off the PCM, ensuring you pull it straight off, so you don't bend the pins.

Step 3

Remove the PCM insulator retaining nuts, using the appropriate socket, then remove the insulator. Lift the PCM out of the vehicle.

Step 4

Remove the programmable, read-only chip from the PCM, if your year, make, model and engine uses this type of PCM. It is located on the underside of the PCM's PROM cover, which is attached by two screws. Pull it straight out, then transfer it to the new PCM. Reinstall the back cover on both PCMs.

Step 5

Install the PCM into its bracket, then reinstall the insulator and tighten the nuts snugly. Position the connector against the back of the PCM, ensuring that the pins line up properly. Push the connector straight on, ensuring that the pins do not bend. Tighten the bolt snugly.

Step 6

Reinstall the ground cable for the engine control harness onto the dash panel. Reinstall the cowl deflector. Reinstall the negative battery cable.

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