99 cherokee with 4.0. Keep losing signal to crank at pcm. Ive replaced everything ignition wise and even purchased a new pcm programmed to the vin and mileage. When installed new pcm jeep would not stay running and threw trans codes. The current pcm is a year 97. Previous owner said he hired a mechanic to work on it and he doesnt know what hes done. Im starting to think it has the wrong trans or tourqe converter/flex plate combo, and thats the reason the crank sensor is losing signal. Current trans is the aw4 30-43le. But i thought that was the 2wd trans and the 4wd is the 40-30le. Any suggestions

 

wiring

 

Yeah ive checked all the wiring. Traced down and ohm'd out every wire to the pcm and checked for open/shorted wires. Itll run solid for about 30 mins and shut off. Sometimes it will crank back within 5 mins other times has to sit overnight or longer

 

Have you changed the crank sensor?

 

Open up the harness where it runs between the valve cover & firewall inspect for open/shorts.

 

ive opened up the entire harness and checked every wire. spent hours on it lol. ive replaced the crank sensor/cam sensor (igniter) ignition coil, spark plugs. pulled and checked distributor. when i put the new cranks sensor in it started knocking from the trans cause it was hitting the crank sensor.

 

What brand sensor? Mopar is always best. There should be a small gap between the crank sensor and the teeth. They should not be touching. Something isnt right.

I never swapped a PCM between 97 and 98-01, so i dont know if this is an issue, but wouldnt that cause some communication issues between the TCM and PCM regarding the speed sensors? The 97 used one, but the 98-01 used two. I would think that by using a 97 PCM in a 98-01, the signal from the 2nd sensor cant be processed, and the TCM gets lost because its not getting anything back from the PCM regarding the 2nd sensor, and throws a transmission code.

Still learning some of this stuff myself.

 

Keep losing signal to, or from the CPS? And, how do you know that? Are you placing an ammeter, or O-Scope between the source and/or signal of the CPS to monitor it? If so, what is that signal? The reason I ask is because you're not saying that the engine runs poorly. You're saying it simply dies and then has no pattern of time when it will restart. That's indicative of a bad CPS. Did you replace it with a quality product or go the cheapo route? A decent CPS will run about 70-80 bucks. My 45-dollar Rock Auto POS replacement died after 2.5 miles down the road. Replaced it with a 74-dollar sensor from Advance Auto and haven't had a problem in the 2 years it's been installed. Have you removed a spark plug, reattached the ignition lead and grounded it to witness the spark, or lack thereof, when cranking?

Also, are you sure you're not seeing a fuel problem, such as poor tank vacuum relief or even intermittent fuel delivery from the pump? You don't mention if there's fuel pressure on the rail, or even vacuum, when cranking.

 

Mopar crank sensors come with a paper wafer on the sensing end that sets the spacing from the flexplate and gets torn off in startup. It's only about 1/16" thick. If your new one was hitting the flexplate, it's probably junk now. If you hook up an oscilloscope to the output, you should see a square wave pulse while the engine is running. It should be at a 5v level. It may be a multi-pulse because there are multiple windows at each of 3 stations in the flexplate and the PCM uses those windows to set spark advance.

 

losing signal from cps to pcm. my snap on scanner told me lol. cant remember the code number exactly but it said no crank signal at pcm. and the engine runs very well no skip or studder until the crank loses signal and it starts to skip then shuts off. sometimes fires up sometimes doesnt until it sits overnight. and the crank sensor was from napa fleet

 

Just out of curiosity, do you have a leaking rear main seal?

 

dave1123 no the rear main isnt leaking. motor leaks no oil

 

These years the crank and cam sensors can throw each other's codes in my experience. Check or replace the cam sensor.

 

i have replaced both. both came from napa

 

i may have found the problem. pulled the trans and flex plate. the flex plate ring that the cps reads off of is bent in a couple spots. which would cause the cps to read abnormally and shut the motor off. im going to order a new flex plate and install it and see if that fixes the problem. and i noticed the cps sensor is chewed up from hitting the teeth on the flex plate. maybe i need to shim the cps sensor out more ?