I have a 1991 Cherokee 4.0 that has been giving me a really hard time to diagnose. It cranks over fine but occasionally will have no spark. I have changed the CPS-Crank Position Sensor three times, Pick up coil twice, Coil twice, Distributor Cap and Rotor once, the entire Distributor once, Plug wires, Battery and terminals, cleaned all the ground wires body and motor, cleaned and made sure all connections are making good contact. I have run all the ohm tests on the CPS, Coil and Pickup Coil and always within specs. What am I missing??

 

check the wire going to the coil itself. it could be rubbed/cracked and shorting out momentarily. My 88 xj did the same thing. also make sure you are not over turning the key when starting. i found that out the hard way.

 

Test your way to a solution. No more parts on a hunch.

Juice goes from the ignition switch to the ASD relay located in the power distribution center (black box) located under the hood, to the coil and the plugs.

Have you ever tried swapping the ASD (auto shut down) relay in that box with another relay? They are of the same part number so swapping can be done safely. Swap the ASD with a non-critical one and see what happens.

ASD can also be tested. Here is a link.

http://autorepair.about.com/library/faqs/bl561g.htm

 

These are both great ideas..The coil wire is new. I replaced it and relocated my coil to the passenger fender well. It was overheating and yes it has good ground.
The ASD I have not tried. I have unplugged it and cleaned contacts. But I have not swapped. I will give that a try the next time it happens. Thanks for the great tips!