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Jeep Grand Cherokee PCM Module Replacement



  • I just got doing testing voltage on my auto shutdown relay and fuel relay. I know the power side of my relay and the ground side on the electro magnetic coil. with the asd relay out and fuel relay in (both work off same power and ground circuits) my voltage is showing correct on the ground side (with my neg lead off of meter in the ground side of relay and pos lead from meter on battery power) showing 12 volts but when I put my asd relay in and pull fuel relay its testing through the fuel relay holes that i have to put pos lead from meter to ground side of relay and neg lead on a ground I pull both relays and everything tests right again could i have the wrong asd relay the numbers that napa gave showed 2 diffferent numbers but napa said they were the same? I think its my relay
  • also from what my scematics show is that the fuel relay isnt spose to chatter once power is sent through the fuel and asd relays once running they are a solid circuit when you turn your vehicle on they click on and if you dont start it they click off but that is all the clicking they should do i beleive: my 92 jeep dont click
    So pretty much whatever one is clicking with the other out has a problem somewhere.
    On my 93 once its running with fuel relay out it stops clicking its just a solid circuit.
    So i guess i have 2 look for shorts coming off the fuel relay

    the pcm clicks the injectors and ign coil off and on by controlling thier ground circuit
  • Yeah, I had replaced my O2 sensor as well. Another interesting thing is that when I disconnect the MAP sensor everthing seems to work as normal except it will not start when cranking over. The check engine light comes on when key is turned to on position and the fuel pump will run for a second and the pressure will stay up around 40lbs and the relays do not chatter!. So I know my fuel pump is working, something is shutting it off though. I did go ahead and replace the MAP sensor just in case, but that did not change anything. It wil still not start.

    So..I am wondering if the rebuilt PCM is my real problem? Maybe not providing enough current to some of the sensors? Or maybe a broke wire I can't find?

    Wish I knew, I'm already into this beast for a lot.
  • have u traced your map sensor wiring to make sure there is no shorts
    it sounds like we both have a short somewhere in the harness
    I dont beleive it is the pcm even though there is alot that runs through it a short in one little wire will make it look like it could be the pcm

    mine seems like the power feeds are alright but it seems like the pcm is not putting out enough ground to my relays which could very easily be a short in the wiring
  • Also I do beleive if it was the pcm our jeeps would not fire at all
  • right above the pcm there is a connector that has several wires running through it I just happen to see that my fuel pump - oxy sensor wire was melted going into this connector so i started tracing the wires back and its on the fuseblock side of the connector so i disconnected the connector out of the 100 way connection inside the vehicle my ohm meter is reading that the short is towards the fuel pump somewhere (have yet to find) but this explains why my fuel relay would be tripping out
  • Yeah, I noticed that wiring block and thought about looking into it. The problem I have is I do not have a schematic to check this out. Is there one on the web that is available? Also, have you tried to pull the connector to your MAP and see if you have the same conditions I have?

    This all started few backs when I had my exhaust manifold changed out. The mechanic did not re-connect my O2 sensor and my check engine light came on after I started driving around the next day. The JCG ran without it connected for awhile and then started giving me issues. I found the problem and reconnected the O2 and it seemed to work ok. THEN, I had my catalytic convertor swappped out because mine had a horrific rattle (been like this for a couple years) and I just got around to having it fixed. It wasn't long after I had the catalytic conver changed out that my JCG died. I wonder if the guys may have shorted out my O2 when they were welding? not sure..I did replace the O2 after my first round of problems but that didnt seem to change anything.

    Just hope the current O2 is good. Is there a way to test these without being powered up?
  • go to they have everything you need wire schematics they pretty much have all the paper work the dealer does
  • and yes you test the oxy without being fired up all you do is backprobe the power and ground (not the signal wires) have your meter ready and have someone turn the vehicle on you should get 12 volts for about a second then it goes to 6 volts then none
  • refresh: i started off with my engine fuse blew once - i replaced it ran good for a while then it blew again (knew there had to be something wrong) replaced it a second time and tryed to start the vehicle wouldnt start the only way i could get it to start was by pulling the fuel relay out. so of course its on the fuel relay somewhere. my fuel relay feeds my fuel pump and oxy sensor - fuel pump works fine till you try to start the vehicle. once i tryed starting the vehicle the fuel relay would start chattering (fuel and asd relays should click twice at most) checked voltage at oxy and fuel pump voltage is good. went through testing and replacing alot of sensors didnt help.

    Any how I just happen to see a melted wire going into my inj connector above my pcm started tracing that wire back to the pdc (fuseblock) and found a bare spot on the wire with burn marks right out of the pdc. I disconnected that wire from the pdc left fuel wire in and it works so I guess I have to replace this wire all the way to the oxy sensor.

    It happens to be that the dealership had spliced a wire in right here and from the looks of it they ripped insulation off this wire at the same time creating a month and a half worth of problems. never again will i allow jeep dealerships to touch my jeeps.
  • Was the shorted wire obvious or did you have to peel back the protective cover around the bundle?

    "Any how I just happen to see a melted wire going into my inj connector " What is inj connector?
  • first did you go to and get the info needed for you vehicle they have all the wire diag. plus they have info on how the pcm works inputs & outputs and info on your relay and fuse boxes

    ok above the pcm in my 93 (95 should be the same) there is a wire connector or plug on the firewall my wire going to the oxy sensor was melted i had to pull the harness apart all the way to the fuse block the wire was melted all the way back with other wires

    if yourjeep is doing the same as mine which is not starting till you pull the fuel relay and the fuel relay is causing the chatter and not the asd relay

    then you have the same problem a short on a wire that feeds out of the relay either the ground wire dark blue and yellow stripe wire from pcm slot 51 pcm to ground side of electro magnetic coil in relay or the actual power feed to the fuel pump dark green with black stripe
  • Thanks for this great info, my Jeep is a ZJ (Grand Cherokee) so I'll look for that schematic. I have not gone back to the jeep yet, kinda cold in the garage right now so I will be going to get a space heater so I can work out there this evening.

    I have not had any blown fuses and based on what you found I should see something similar (shorted) which I would supsect would cause a blown fuse somewhere. I'll keep looking in and around the harness for a short or broken wire.
  • I just happened to re look over your original messsage.

    yes when you pull the fuel pump relay and it starts fuel pump doesnt prime - you just took the power from your pump pulling the relay.

    look over the schematics on the link i gave you it sounds like you have a short or melted wire too.

    Even though a month ago i found a short on my ign coil trying to fix this problem im on now. the power wire to the coil was bare about 2 inches and bouncing off the frame. Im wondering if this had anything to do with my melted wire. I cant find a real bad short except for the bare spot the dealership obviously made and it only very rarely could actually ground out. so im wondering if a short could backfeed through the relays.

    Any how no matter what you probably have a short going to your fuel pump somewhere. My readings on my meter showed contunity and voltage but there was still a problem there.
  • Yeah. I'm fairly certain I have a "short" somewhere and it's most likey under the hood somewhere.

    I'm confident that the short is not in the fuel pump wiring as it will run if I force the fuel realy closed manually. Then I can start the vehicle if I pull the fuel something is shutting the fuel relay off and I assume that voltage comes from the PCM based on a sensor input. I don't think there is a Inertia / rollover switch on these models so that should be counted out. I'll keep digging...If I find something I will come back and post.

  • Found a short in my O2 Sensor harness, close to the connector. The wires were melted together. I couldn't see why it would have happened, the harness didn't look crushed or pinched. Guess the O2 could have shorted out the the wires just got hotter at this particlar place, which caused the meltdown. I'm worried that there could be other shorts in the harness so I'm going to pull the bundles apart and take a look. It's troubling that there are no fuses that could have saved this from happening.
  • ya and I thought I had mine fixed but i didnt

    the short going to my oxy sensor probably the same as yours I couldnt find what could of caused it but I thought i had it running but at the time my fuel pump was not plugged in so when it died it was because i was out of gas

    so when i plugged my fuel pump in by the gas tank my fuel relay started acting up again unplug it and my relay doesnt chatter so i am in the process of pulling my fuel pump out

    i know that my fuel pump works but there is a problem in the fuel pump module you could have the same problem

    it looks like something in my pump went wrong sent a surge back to my fusebox and instead of blowing the engine fuse it melted the wire going to the oxy sensor
  • Please let me know if you find an issue with the fuel pump. I did the same thing, unplugged the fuel pump to see if the relay would stopping chattering. I have a replacement PCM coming tomorrow and definitely dont want to plug it in if the fuel pump is bad. I have thought about replacing the fuel pump, before I connect the new PCM, just to be sure my current fuel pump does not short out my new PCM or some other sensor. What stinks is that the tank was recently filled and it's a pain in the rear to drain..I did it 2 years ago. Not looking forward to that but it is probably the best thing to do before I go and plug in my new PCM.
  • I finally got my JCG running. I believe the shorted out O2 sensor wires took out my PCM. Once I fixed the shorted wires and got another PCM it started up and is still running fine (knock on wood!). But, the ordigianl problem was hard to put my finger on although I believe it was a problem with the O2 sensor being shorted which most liskey caused the PCM failure?? Im still not 100% sure. I would like to know why there is not a fuse somewhere that could have save the PCM from getting fried.
  • ya on the scensors there is really no fuse for protection i have been thinking about throwing my own inline fuses in so i dont go through this again the only fuse for protection is the engine control fuse but it feeds so much stuff and miles of wiring

    but mine is still not running i pulled my fuel pump and thats it it got cold and started snowing so im trying to get enough room in my garage to keep trying

    but good job im happy to hear its fixed
  • Man it sounds like there is a lot of problems with your jeeps. OK now I have one for you. I own a 2001 Jeep Grand Cherokee Laredo. The battery and alt. or both good. The jeep turns over just fine but wont start all of the time. If I keep playing with the key as in turning it on and off a bunch of times finally i hear the fuel pump kick on and it will start. All of my gauge lights come on but the gauges are dead unless the fuel pump comes on then the gauges read fine and it will start. I also notice that when the fuel pump turns on the light with the key and a line through it goes out but when it don't the light stays on. When i get the jeep to start it runs just fine. I don't think its a pcm problem or it wouldn't run at all would it? If it was the fuel pump then it would shut off while im driving it wouldn't it? Any help would be greatly appreciated. I see that some of you had similar problems but I didnt see what the fix was. Sorry so long but I wanted to let you know all of the info that I have. Thanks.
  • Ya it sounds like you have a short somewhere off your ignition wiring. On my 1992 jeep i had a short in the park/neutral switch wiring and it did exactly what your talking about. my 93 is doing almost the same thing and i am still trying to locate the short
  • Hey
    I had a 1998 JGC Laredo that I loved vary little problems, sold it and I have a 2001 JGC laredo V8 auto, that I just got,here's the problem, It starts great when cold but when it runs for a while and I shut it off I have to crank it longer then I did when it was cold sometimes longer then others, and it lopes (studders) when I stop at red lights sometimes. I changed the plugs, no distribitor or rotor. I get from reading in the forums it sounds like it might be the Crankshaft Position Sensor? How do I locate it an is a pain to do ? And what about the Oxygen Sensor? Thanks
  • I would test my oxy sensor first then camshaft and crankshaft sensor
  • Thanks I will. Do you know where the Crankshaft Position Sensor is located? And is a pain to change. Do I have to jack up the front end?
  • on my 93 jeep the crank sensor is right behind the engine on the flywheel and yes it is a pain to get to but you can test it with out pulling it. just need to find the wires that go to it and make sure you are getting proper signal from it
  • Hey
    I found the crankshaft position sensor location on my 2001 JGCLV8, right behind the starter, had to jack up the front end and pull the starter, bingo, there it was. changed it, seems a little better on the idle side.dosent seem to be as rough, time will tell. Maybe it dosen't seem to crank so much when I go to start it when it's hot !!!!!!! Will post later when and if it is at all better If any other ideas on what I should try per original post please advise.
    Thanks for the replys
  • I have a 2001 JGC laredo V8 auto, that I just got,here's the problem, It starts great when cold but when it runs for a while and I shut it off and wait about 45min to a hour it starts hard ( Have to crank for around 10 12 seconds is this normal ) have to crank it longer then I did when it was cold sometimes longer then others, I changed the plugs, no distribitor or rotor. I get from reading in the forums it sounds like it might be the Crankshaft Position Sensor? Changed the CPS seems to run smoother but still starts hard when hot What if the battriey is smaller the the orignal? Thanks
  • 03fxdwg03fxdwg Posts: 2
    4.0 L with about 150,000 had distributor bushing make screeching nose. Had distributor assembly, wires, cap, rotor and plugs replaced and by independent shop and vehicle began to die while running without any warning. Just seemed as though someone cut off the fuel or there was water in fuel. Live in SC and checked with stations where I fill and used many cans of gas dry without any change. Engine starts back up and runs as though nothing happened for seconds, minutes, hours or days. Had distributor replaced again but problem persisted. Took to Jeep dealer and and had error code for crank position sensor throttle position sensor. Replaced them and then had error for bad PCM. Replaced that with Mopar part and dealer went on to have injectors cleaned, coil replaced after error code showed, Replaced battery and cleaned cable connections. Dealer replaced fuel pump and distributor primary field assembly and problem still persists. Will backfire and miss at speed, stalls at idle or under acceleration or while coasting. Sometimes engine will fire back up and other times I have to coast out of traffic. Sometime when cranking, then engine speed will increase like it wants to run, but isn't getting enough fuel or the spark timing is off. It will usually start right up if you turn off ignition and wait for 15 to 30 seconds, but that is had to do if you are sitting in the middle of an intersection with cars coming at all directions. Dealer is at a loss (no new error codes even though I had a solid check engine light for a day a week ago and then it just went out the next time I started the car. Dealer is replacing PCM under warranty as I have spent over $2500 in the past 9 months.
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