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XJ Wrong PCM replacement? No spark

XJIron

NAXJA Forum User
Location
Maine
New to the forum and the world of Jeeps. I recently purchased a 2000 XJ, 4.0, Auto Trans that was missing the ECU/PCM. I purchased a used ECU (P56041 638AD) online and plugged it in. Where the original one is missing I am not sure how to figure out which one I should have in there and this may be the cause for all of this. If anyone knows a way that I can figure that out please do. When I turn the key the fuel pump primes, the starter will crank, but engine does not turn over. I pulled the coil rail off and checked for spark by putting a spark plug in and grounding the spark plug. I have no spark. IT seems the CPS is a common point of failure so I checked pins "B" and "C" i get minimal resistance (4.5 I think) so I think it is good. The cluster back lights work but none of the gauges move. I have pushed and held the trip reset while turning key to on and the gauges all do there happy dance. I am also getting the dreaded "No Bus" error on the odometer after 30 seconds. The Engine only has 132,000 so I think this old girl should have plenty of life left in her.....
 
It's definitely a GC computer, will it work I don't know. The GC also doesn't use the AW4 trans so who knows what else is different.
 
Okay thank you. Any idea how to tell if I need an "AD","AF,
Or "??" computer without having the original one? I think I am going to pick up a CPS as well. From the forum it sounds like I should keep a spare in the glove compartment.

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That PCM isn't going to work for you, you're getting the no bus error because the Grand Cherokee has a PCI network and no CCD network, the gauge data is bused over the CCD bus.

Since you already mentioned it's an automatic, you have two choices for a PCM. P56041634AG which is for a Federal emissions and a P56041635AJ which is for CA emissions. As for the last two letters of the part number, that just signifies the version of software running on the PCM, the higher letter, the newer the software.
 
As stated already, you just need the correct ECU/PCM part number, the last two letters are the software revision code, and can be ignored.
 
I have 2 2000 XJ's , they are both identical "Federal" emissions automatics. I have four ECU's 2-634AE's, 1-634AF, and one that was custom tuned so I don't know what it started life as.
 
Having the VIN flashed into a replacement ECU is only necessary if local Smog Inspections check for it. An ECU with the correct part number will run properly without being re-flashed.
 
Having the VIN flashed into a replacement ECU is only necessary if local Smog Inspections check for it. An ECU with the correct part number will run properly without being re-flashed.

Unless it is equipped with the Sentry Key Immobilizer System(SKIM), then if the VIN isn't set the vehicle will only run for ~3 seconds.
 
Pretty sure this started out in the great North East so don't think it would have CA emissions. Time to order some parts. I will update as thing progress. thanks for the help everyone. I hate to see the old girl just sitting like this. Not gonna give up.

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where in maine are you ? i have an extra 2k computer kicking around . i am in central maine. i am not sure if it is a cali or federal, it has the 2 pre cats. i think that makes it a cali, but not sure. also have extra odometers if you need it, but most of the time the nobus can be fixed by cleaning the contact points on yours.
 
I should have checked here first. I am in the Portland area. I picked up a replacement PCM online. Should be here soon.

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Well I got the new PCM and CPS installed and cleaned up the pins on the gauge cluster. No more BUS error and the gauge work, but it still does not start. I can hear the pump prime and smell the gas. I pulled the air box off and sprayed some starter spray hoping it would turn over for a few seconds but no go. I am guessing still not spark. To top it all off my wife washed the pants I had the title in and I have not transferred it yet. FML. Two steps forward one step back. Any other common things that go and cause no spark?

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Genuine Jeep CPS or a cheap crappy Chinese clone part ?

It is likely that the PCM has no adaptive data stored in memory due to extended storage and you will have to feather the gas pedal to start the engine, and continue until the idle speed becomes steady. If you do not have success fairly quickly you will need to follow the troubleshooting below.


For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem. A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will relearn the idle settings after a short period of driving and the engine will sometimes start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.
 
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Ok wow. Lot of info there thank you. I purchased a used OEM PCM from the same year and I do not have an I mobilizer on it as far as I can tell. When I bought it the negative terminal was corodid so I should check the other grounds and the CTS. Thanks again

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Search on how to check coil rail -- easier if you watch a video

Better check your fuel pump pressure -- Might be able to get a loaner gauge at one of the auto parts stores -- I believe its 49 PSI at the rail you are looking for

Have you pulled any spark plugs to see what condition they are in?
 
Okay so I cleaned all the grounds and checked fuel pressure at the rail and it is about 45 PSI. I did also get a Bluetooth OBD2 and installed torque. I get the two error codes in the photo. I see one may be the TPS sensor so I cleaned it's ground with no luck. Maybe the sensor? I am not sure what the other code means but I still don't have spark. Not sure if it is the rail or something before the ignition rail.
7151116c07e67a9d46f408b78b5a065a.jpg


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