will a pcm off older models work on my 01

im having problem toubleshooting my xj. it will turn over but not start. it is a 2001 automatic with np231. only 70000 miles. i parked it and it was fine 2 days ago and went out to leave yesterday and it would turn over but not start. i alread check the fuel system and its good the cam sensor looks to be good the coil pack is only a few months old. the crank sensor idk about it i know its still magnetized and clean. if the ecm i bad will one off of an older modle work? like 98 and newer maybe? or if u think it may be something else im am ears open to it. i already checked all fuses and relays and they are all good.

Before you waste you money on a pcm, do some troubleshooting. Is it getting spark? Is there fuel pressure at the rail? Pull a plug and make sure it isn't wet fouled.

it has fuel pressure at the rail and its got the coil on plug system. idk where to even begin to troubleshot that. i am gonna put a diagnostic scanner on it before i buy a ecm. f icant find out 100% what it is for sure im just gonna go to pull a part and get all the sensors and ecm coil and all that stuff and pray i fid the problem. also it has no leaks at all.

Ya, definitely don't just go out and by a new ECM, that is completely a waste since you haven't really done any diagnostics. There really is no reason to jump to the conclusion that your ecu is bad.

Remove the coil packs, and remove a plug and put it back in the coil. Ground out the plug, and have someone turn the engine over while you see if the plug has spark. If not, it is likely a CPS issue. Look at all your sensors and do diagnostics on them by checking resistances with a multimeter and compare them to the standard values. Just because your CPS is still magnetized and clean doesn't mean anything, its not suddenly going to become demagnetized if it stops working.

Just plugging in a scanner into the OBDII port won't really tell you much unless you have a CEL on, unless its one of the really expensive chrylser ones that repair shops have.

To answer your original question, no the ecu will not work from on older vehicle because '99 and older had a distributor ignition system, where as '00 and '01 used the coil packs. There is also the California exhaust system with the 4 o2 sensors and cats in it, so unless you get an ecu from an '00 or '01, it won't be able to read that.

If you do not have spark, the #1 suspect for the Jeep 4.0 engine is ALWAYS the crankshaft position sensor.

Located on the bellhousing, it can be tested with a meter. I would highly recommend that you do so.

Here is a thread you should read about the crank sensor.

http://www.naxja.org/forum/showthread.php?t=1044073

BTW, replacing a computer should be the last thing you do. It should be a "diagnosis of exclusion", meaning that every other possibility has been ruled out. Computers are often blamed for no-starts, but they are generally pretty hardy. Sure they can fail, but the other components on your XJ are more suspect than the computer.

Good luck!

thas alot guys i didnt want to waste money on parts i couldnt get money back on if it wasnt the fix. and i am hooking it up to one of the expensive scanners this afternoon and i hope it gives me an idea. but the cel has never came on.

eddieusmc said:

thas alot guys i didnt want to waste money on parts i couldnt get money back on if it wasnt the fix. and i am hooking it up to one of the expensive scanners this afternoon and i hope it gives me an idea. but the cel has never came on.

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Good idea to scan for stored codes, but a high percentage of the time, a failed crankshaft position sensor will NOT store a code.