The original PCM on my 96 ZJ limited 5.2 V8 4X4 failed and could not be recognized by my scanners and was not outputting the 5 vols supply to the sensors so I bought a used one with the same part number on eBay. The replacement one does let the engine start. but it high idles at 2000 RPM, runs rough, and the gages do odd things. The PCM I purchased has the same part number of P56041 and 266 for the main part number, but has T0536 vs T0526 and JDB0BMXZ vs JBD0BJRR . Both have ZBYAB.

I've replaced the TPS and IAC and checked the other sensors (except incoming air which I don't find) and and they all seem to be to spec., so today I went to set my UltraGauge to report the long and short term fuel trims to try to see what was going on, and I found the UG had only the choice of fuel trims for bank 1 and nothing for bank 2. There were no bank 2 O2 sensor voltages either, and when I set up a gage page with bank 1 fuel trims they are reported to be 0 for both short and long term. Is it possible the PCM I bought Is for a 4.0 straight 6 and not a 5.2 V8?

 

I think that PCM will work, but needs to be programmed with your VIN. The other odd numbers have to do with the program code already installed in it. Check with All Computer Resources.com for more information. You'll probably need to call them.

 

Thanks for the reply, Dave

I looked over the Computer Resources.com web site and read the Tech Talk pdfs and I believe I have checked what they recommend on this replaced PCM. It does the bulb check and it will start the engine almost immediately from cold, but it has a 2000 RPM idle, surging engine, tach needle dipping, sooty exhaust and sometimes white or gray smoke from the exhaust when first started. The sensors are getting their 5 volt power and the outputs are in spec and I've not found any shorts that the web site said might have caused the no start, no fuel pump, no spark condition I had with the original PCM (the original one did do the bulb check but that was about it and two different scanners could not communicate with it)

The Computer Resources.com web site says they need the VIN and the mileage. I am curious about what must be programmed from the VIN. Engine perhaps? (My mileage display remained the same with the replacement PCM installed so that must be stored someplace other than the PCM). I note that their photo of a P56041266 PCM is a different type than the wrap around PCM in my ZJ and their photo looks like the one in the you tube video of how to remove one cover and re-solder poor solder joints. Mine is not like that one, so I also wonder why there are two different types for the same year and model. My 96 Cherokee Country with a 4.0 straight 6 also has a wrap around PCM so it is apparently not determined by engine type. I did finally pry up the connector side cover of my original PCM, but I don't see any obvious problems on the PC board on that side - definitely not like the one in the video.


Yesterday I ordered a second PCM from a different eBay seller that I had watched before, but which was sold when I clicked to buy it. All of its numbers matched mine except the top one was JBDOBJJP instead of JBDOBJRR. However, when I looked back at previously sold units I saw he had sold the same unit twice before and he did not answer my inquiry on whether this unit had been returned or if he was selling a different unit but using the same photos. I stipulated that I was buying it on the understanding that it was the unit in his photo with matching numbers. I'll see what happens, but so far no reply from him.

I also checked the bbb and Computer Resources.com does not have a very good record there; there are many complaints of their computers DOA or not working as promised and poor customer support. I hope this next one magically solves the engine problems so I don't have to find out for myself if a "reconditioned" PCM is the answer.

 

Is that your original PCM in the photo? Is that a shadow or burnt circuitry among the four shielded components in the lower left quadrant of the photo? ...looks like a hot spot to me. This appears to be repairable but definitely a difficult circuit repair...

 

Good eye. I hadn't noticed that before.

I spread it a little farther open and it does look like one device, a capacitor maybe?, may have leaked onto the PC board. (I think the gray blob is just some sealer that was misplaced when it was put together.) Not sure how to test the device unless it can be treated like any capacitor to see if it charges up.

This site says you can use something called an ESR meter, but I'd have to read more about it and not being an electronics expert chances are slim that I could do this with my limited tools at home.
https://www.yamanelectronics.com/tes...t-desoldering/

Thanks for noticing the discoloration, though. It is likely the cause of it not having a 5 volt output to the sensors.





Guess I need to crawl under the Jeep and make sure there are no potential shorts at the oxygen sensors before I try the "new" PCM that is due to arrive Friday.

 

It turns out that my problem was definitely the PCM and not any of the sensors. The original PCM died (probably due to one or more capacitor failures), the first PCM I bought on eBay was defective or not programmed for my Jeep even though it had the same part number, and the 2nd PCM from eBay with the same part number and the same T0526 number is working just fine with a 1000 RPM initial cold idle and clean exhaust, and an eventual 750 RPM warm idle. The fuel gage is still intermittent, but the check gages light stays off, the tachometer needle no longer dips, and the engine no longer surges or runs rich.

One thing I did learn when inspecting the O2 sensor wiring is that my Jeep has only two O2 sensors and they are on the single exhaust pipe on the right side and down stream of where the two bank's down pipes come together. I don't know if this is normal or someone's previous change, but the working PCM has only bank 1 data for O2 sensor and fuel trim readings which apparently affects fueling on both banks equally. In any case it is back to its previous normal now with the advantage of a nice clean throttle body, fresh Champion spark plugs, and the vacuum connection to the cruise control restored.

I still don't know what the vacuum device under the battery is nor where the capped off T in the cruise control vacuum line that the manual says is for the climate control is supposed to connect, but for now I am just happy to have a starting/driving Grand Cherokee Limited again..

 

Okay, normal vacuum routeing is manifold to heater supply T at firewall, then to cruise control T, and on to vacuum reservoir under the battery. The reservoir's function is to store vacuum and smooth out vacuum pulses for the cruise control. The vacuum storage part is used to close heater doors to outside air on shutdown. There should be a check valve at the manifold source and at the heater T.

 

Thanks for that info, Dave. So the thing under the battery tray is a vacuum reservoir. I still wonder what is the function of the plunger-like thing on top of it that the battery pushes down. Some kind of warning device that signals someone has stolen your battery?





As for the Climate control vacuum connection, I did find this comment on line, "Later, we learned that it enters the passenger side alongside the main air conditioning lines." so I'll be looking for that between rain showers today. It has been disconnected since I've owned the Jeep, but the climate control has seemed to work OK and even now I can hear something moving behind the dash for several seconds when the engine is started.

EDIT: I looked and felt all around where the metal AC lines go through the firewall and neither saw nor felt anything like a vacuum line there. (It doesn't help that the big stiff wiring harness is tight against the AC lines).

Is it one of those tiny brittle plastic vacuum lines like the one through the firewall on my 90 Cherokee limited? The one that so easily breaks off? Anyone have a photo of the Grand Cherokee climate control vacuum line?

 

Yes, it's a brittle black plastic tube that tends to break off right where it goes thru the firewall. There should be a check valve in the T fitting before it disappears.

The black plastic plunger-looking thing under the battery is the battery temperature sensor that tells the PCM to cut back the charge rate when the battery gets hot. It's just a variable resistor that changes value when it's temp changes. Without it, the battery could get hot enough to boil the electrolyte.

 

Thanks, Dave,
I wondered about a battery temp sensor, but I did not see any wires going to it - just the vacuum line going under the battery tray (which I could not remove because that bottom bolt is too rusted away to turn it with a socket.) I saw a you tube video of a 98 6 cylinder GC that had what appeared to be a rubber vac hose through the firewall just above the AC line that has the square keeper on it, but mine has nothing there. The video was about his vacuum connection at the T having broken and the climate system being permanently on defrost,

You Tube

I'll check again, but I don't think mine has that problem.

 

On my WJ the battery sensor just lifts out and the harness has enough slack in it to put the connector up enough so you can disconnect it, but don't let it slip back down or you'll have to remove the tray. It's wired into the harness for the headlights and the fan control relay on mine. My fan control relay and vacuum reservoir are mounted under the headlight support bulkhead and front fascia (Bumper cover). Your reservoir may be mounted underneath the front wheelwell splash shield and be ball-shaped whereas mine is a weird piece-of-pie shaped box.

 

I believe my vacuum reservoir is the pie shaped one under the battery tray, too. I could feel the vacuum line going to it but did not feel any wiring there and thought the plunger like device was part of it.

I checked my climate system this morning and it seems to do everything it is supposed to do switching between defrost and vents and various other modes changing temperatures. The AC clutch was going on and off as it should but does not have enough R134a in it to actually cool.

Starting the engine this time took some throttle before it caught (cranked twice without fire first) and then all seemed to work OK except the fuel gage couldn't decide where to be and vascillated between 1/8, 1/4, E, and back again (and I know the tank is half full). Fuel trims and O2 sensor voltages were changing so the PCM is doing at least part of its job.

 

Yours is a 1996? Mine is a 1997 ZJ. Although, my ZJ is not a V8 model. Mine has the little black vacuum line at the firewall with the T-connector thing. It was broke when I got it. I guess your ZJ does not have this same small vacuum hose piece though, and everything works so that is good.

 

Yes, Noah, it seems to be working so it must be getting a vacuum source from somewhere. I wouldn't have thought that the climate systems of a V8 and a straight 6 would be different and mine does have a capped T in the vacuum line where yours would connect. It is a mystery.

A new mystery is that after a difficult start this morning my CG is back to crank/no start. I thought my troubles were over with the 2nd replacement PCM, but apparently after starting well for a week it has developed a new gremlin. So far I know it has 50 PSI fuel pressure and a 5.12 Volt supply at the MAP sensor and the Crank sensor when cranking, but it does not fire. My frustration has reached its danger level for today so I will look further another day.

 

What is the reason?

Fuses.. relays.. battery, cables, and grounds.. These are all good?

Spark?