The CPS, also known as a TDC (Top-Dead-Center) Sensor is sensor that is a root of many problems people experience with their XJ. All this sensor does is tell the computer when the motor is at Top Dead Center. It sounds like a pretty trivial thing, but without this signal, your precious XJ is pretty much dead in the water. If you do any four wheeling into areas where it would be impractical or dangerous for you to be stranded, it might be a good idea to buy one of these sensors ahead of time, and keep it in your XJ (with tools), just like people used to carry spare spark plugs, etc. ;-)
A bad CPS can be the cause of an engine that turns over but will not start, or rough idle and missing. Keep in mind that other things can cause this also, but the CPS is one area to check.
On late model XJ's, the CPS is wired directly into the wiring harness, and you need to splice wires to replace it. On earlier models, the CPS has a plug on the wire harness.
One method that may help you determine if your CPS is the cause of your trouble is to simply UNPLUG it, and then plug it back in. If the XJ starts after doing this, you can be pretty well assured that your CPS is bad. This sounds stupid, but many people have said that this is a way to verify it, including mechanics at the Jeep dealership. This is also a field fix that MIGHT help you get back home.
The CPS is located on the edge of the transmission bellhousing, on the left side. If you crawl underneath the driver's side of the vehicle, with your head approximately underneath the rear of the transmission, and look forward and up between the left of the transmission and firewall, you will see the sensor at about the 10-11 o'clock postion of the bellhousing.
The cost of a replacement CPS seems to vary from year to year, but falls in the $30 to $50 range. I've heard rumors of a "kit" from Jeep with the CPS sensor that is actually cheaper than buying the sensor itself. Also, there seems to be two types of CPS's. One for high altitude driving and one for "normal" altitudes. I haven't investigated these bits of information on my own. It's worth grilling the parts folks at the dealership to see whats on their computers.
To remove the CPS, first disconnect the battery's positive terminal. (Always a safe thing to do whenever working on the engine...)
Getting at the bolts on the sensor can be a challenge. It's possible, with a ratchet and a long extension. You can reach the bolts from below, once you figure out a twisted postion for your arms and body.
WARNING! DO NOT DROP the two bolts that hold the CPS in place. If you happen to drop either of these bolts INTO the transmission, you will need to REMOVE the transmission in order to retrieve them. DO NOT DROP THEM.
It would probably be safest to remove the top bolt first. That way, if you drop it, the CPS will still be in place, and hopefully will prevent the bolt from dropping into the transmission. When you remove the bottom bolt, it is at the lower edge of the hole in the transmission, and will hopefully just fall to the ground if you drop it.
After the bolts are out, you will need to disconnect the wiring from the harness. On some models, that's just a matter of unplugging it. On others, you will have to cut the wires and splice in the new sensor.
Plug in or splice in the new sensor, and install the bolts in the reverse order, to help minimize the chance of dropping a bolt in the transmission.
That's it! Connect your battery, and start the vehicle back up! Wasn't that fun?!