Grand Cherokee [“ZJ”] Project
Click on boxes below to view different phases of the build.
New/remanned/used factory computer. Can be found in wrecking yards, online on sites like Ebay and others, or from the dealer for various price ranges.
Socket wrenches, dielectric grease
30 minutes to 1 hour of time
The reason we decided to replace the computer (PCM) was to address a really random stalling problem that the Jeep had for quite some time. After replacing numerous sensors under the hood (IAC, Crank-Position Sensor, and TPS) and not fixing the problem, we decided to give the computer a try. The first re-manned computer we got from the dealer was for the wrong engine (the 4.0L), and the second computer had an issue with the voltage regulator, which we found out AFTER replacing the alternator. Finally, we got to our third computer and fixed a few wires in the wiring loom and we fixed our problem. All of this can be seen in Part 3 of our video series on Youtube.
STEP 1 - Remove coolant overflow tank: The computer is tucked up against the firewall on the passenger side of the Jeep. Use a socket wrench to remove the coolant overflow tank to allow access to the computer.
STEP 2 - Remove computer: First, disconnect the battery. It is always a good idea to remove the connection to the battery when working on anything electrical under the hood. Then, remove the bolt that holds the wire plug into the computer and unplug it. There are three bolts that hold in the computer to the firewall. Remove those and remove the computer
STEP 3 - Swap out new computer: Place the new computer into place. Use the three bolts to secure the computer back to the firewall. Use dielectric grease on the plug and plug into the computer. Tighten the bolt that holds in the plug.
STEP 4 - Inspect wire loom (if needed): This isn't a necessary step, but will help to make sure everything is in good order. There is a lot going on in the loom and it is a mess to take apart (see right). We found a few wires that were potentially problematic, but added some electrical tape on exposed areas to prevent any shorting out.
STEP 5 - Put everything back, run Jeep: Re-install the coolant overflow tank, then reconnect the battery. Start and run the Jeep for several minutes to let the new computer learn its parameters. The Jeep may take longer to crank and may run rough at first, but things should smooth out within a few minutes. Test drive the Jeep to make sure there are no issues. The PCM has an adaptive memory that learns as you go and bases engine outputs as needed.