Just had a very similar problem in my 1996 cherokee with 4.0 inline 6 - turned out to be the crankshaft position sensor. Jeep would crank but not start, no spark, no pressure in the fuel rail, only lights that would come on in the dash were the brake and seatbelt lights. All other electrical devices would work - power doors, windows, radio, headlights, etc.
The onboard computer would not spit out self-diagnostic codes, and would not interface with an OBD-II code reader.
I unplugged the CPS sensor connector in the engine compartment (right side of engine as seen from the front, at the back near the last fuel injector, oval shaped with three pins). After this, the check engine light came back, I had pressure in the fuel rail and could hear the fuel pump, and the computer would generate the self-test codes telling me that I had no CPS input. The CPS is located on the driver's side of the transmission bellhousing near the top, has two bolts holding it in place, and can only be reached from underneath. Size 11mm socket and several different extension sizes are necessary depending on the size of your hands. Mine wasn't too horrible to replace.
I used an ohm meter - holding the cps sensor connector with the plastic prong on top I measured the old connector from the left pin to the middle (approx. Zero ohms), and the right pin to the middle (infinity). On the NEW CPS sensor connector I measured the pins the same way and read infinity on both readings. It seemed to me that my old CPS sensor was shorted internally, plus disconnecting it from the circuit brought my jeep partially back to life, so it seemed like I was on the right track.
I hope this info helps, my jeep started first try and no problems so far.
Friday, July 27th, 2007 AT 6:38 PM