My Jeep

CPS - Crankshaft Position Sensors

What's so special about this little piece of plastic with a single wire coming out of it? Only that without a proper signal from the CPS, the computer doesn't know how far the flywheel is turning and can't give a proper ignition signal...in other words you're STUCK.

The CPS on the 4.0L engine has long been known to cause erratic behavior. Anything from a plain ol' "no start" situation to randomly shutting off have all been attributed to the CPS. The CPS is located on the bellhousing between the engine and the transmission, and it monitors the speed the engine turns so the computer can open the fuel injectors and send an ignition spark at the right time.

Of course, it's not exactly easy to get to, so as a temporary measure I've been able to unplug the pigtail from the CPS and reconnect it. What often happens is that the connectors corrode or get a film of crud on them (using MgCl to clear roads will do that) and then signal stops. Disconnecting it can often clean off enough corroded material that the electrons get flowing again.

So where IS my CPS?
In about as difficult a place as it can be, of course. The electrical connector I mentioned earlier is at the rear of the engine head, where the wiring adds into the rest of the engine bundle. It is best accessed from the driver's side, and if you look down behind the intake manifold you can just see the CPS. For a better view of it, take a crawl underneath...

This view is from under the driver's seat. Picture yourself lying under the driver's seat, with your feet under the front axle. The CPS is at 11 o'clock on the bellhousing and is held in by two bolts.

In this image, the transmission crossmember and the front driveshaft have been removed, and the transmission has been dropped a couple of inches, tilting the engine back a bit.

Without dropping the transmission, it's a nightmare for a big guy to get at the two CPS bolts. When I did it in my '89 XJ I had someone stand on the wheel to hold the socket in place while I turned a motley assortment of extensions, wobbly joints and more extensions to get to the CPS. The helper is there to guide the socket and more importantly to keep the bolts from falling into the bellhousing.

Bare bellhousing with CPS mount at top leftThis picture of a bare bellhousing shows the CPS mounting position at the top left.

Take care removing the old CPS, and good luck installing the new one!

e-mail Jim
created: October 2, 2005