what do they do when they go bad?

 

by pcms you mean the computer, well my friend has had 2 change of cmputers on his ZJ.. and normaly the simtoms were, it try to shot down while crusing or it did shot down, all lighst start poping like crazy,, IDK what else..

 

Usually they die and the engines will not restart. The PCM or ECU will not have the correct voltages at the pins or no voltage at all. HTH wicked.

 

Had to replace the PCM in the wife's 5.2L Limited. As the weather turned colder it would take longer for the computer to warm up (something internal) which would trip the fuel pump relay and pressurize the fuel rail allowing it to start. This is not uncommon i hear.

Reman unit from ebay fixed it right up.

 

with a bad computer

 

I've had mine go bad in my ZJ...

 

Under my hood theirs a sticker that says it been replaced, but it happened to the po, not me

 

Yes.

Typically a crank with no start can be attributed to the crank/cam position sensor(s), coil, fuel pump, plug wires. Don't be too quick to condemn the PCM.

 

Most of the XJs that have a 4.0L have great ECUs and they last forever as long as they don't get wet. The ZJs had a different less reliable ECUs and they crapped out early.

 

frankz i put in a new cps tested that a whole coil pack from a running jeep and still no spark any were wtfffffff!!!!!!! i wanna bring my xj sunday to October truck fest at k4wd

 

Have you checked the input voltage to the coil? What is it?

Have you checked the input voltage for the Crank and Cam position sensors?

 

cranks new tested at 200

 

Coil input voltage should be around 12VDC.

Input voltage for the crank and cam sensors should be approx. 5VDC.