I just put a new (but used, with less miles) 4.0 in my 99 XJ. everything is hooked back up and i went to go crank it and it does not crank. It just spins over...and i get a code: No reading coming from the Crankshaft Position Sensor. so I replaced the sensor, and still, wont crank. engine was running before put in this one. it was a direct swap. Basicly just unbolted flaywheel from the tranny, engine from the frame and harness from the engine. distributor wasnt touched, coil wasnt ,essed with or anything. ANY help would be AWESOME. I need to get this jeep back on the road before I push it off a cliff

 

what year engine did you swap in?

Just to clarify (i'm confused by your use of the term "crank"), does the engine turn over (crank) and doesn't catch and run?

I would check first to make sure that the power feed wire running from the PCM to the crank sensor isn't chafed and grounding out on the body, which would cause a "no signal" condition.

It's also possible you didn't get the sensor positioned correctly, resulting in a low signal strength. Make certain that the CPS is completely bottomed out on the machined lip on the bellhousing and is as close as possible to the flywheel sensing ring without contacting it.

I'd also like to point out that it is completely possible that your "new" CPS is faulty. Try another one. I recommend ones from a jeep dealer.

I would also investigate the ASD (auto shutdown) relay