This is my dad's 1999 ZJ with a 4.0. Ran perfectly yesterday, when he parked it in the garage.
Today it cranks & cranks, but doesn't fire.
There is no check engine light on.
I scanned it (think I did it right, virgin scanner) no codes.
I pulled off the spark plug rail & checked for spark at two of the boots, nothing.
Even though my dad knows nothing about cars, he's pretty sure I'm an idiot. That would make this so gratifying to fix...
I'm thinking Crank Position Sensor, any thoughts?
Is there a way to test the CPS prior to swapping it?
Yes there are resistance tests in the FSM. I don't have any info for a WJ (anyone else), but if you google it, they're out there... I understand the CPS usually gives some warning before failing though. Has this truck been getting harder and harder to start, particularly when warm?
It's really unlikely all 6 coils would fail at once, so you're looking for something in common for all of them. Check all the fuses and relays (does the WJ have an ASD?) are free of corrosion on their legs, confirm the alarm is off, make sure there's voltage at the wires TO the coil pack, etc...