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Stock Grand Cherokee Tech. All ZJ/WJ/WK Non-modified/stock questions go here!ZJ (93-98), WJ (99-04), WK (05+)
All ZJ/WJ/WK specific tech questions asked here!
Working on the Jeep today the gauges will work fine as long as the big wire on the alternator is unhooked as soon as you hook it back up they go out all modules is disconnected except the pcm I have t found any short in the wiring another other thoughts
How is the voltage looking on the BUS? Is it still at 12 VOLTS?
Are you sure all of the modules have been disconnected? The Ignition Control Module is hidden pretty well. The fuel pump needs disconnected too, as it is a module. Audio Amplifier, A/C Control Head, Immobilizer Module, etc...
I will have to take a look again and try to see if I can find out exactly how many modules and what are their names. There should be somewhere around between 22 - 32 separate modules on the Grand Cherokee. It depends also on if your Jeep came equipped with extra options too. This information of the list of modules is near impossible to come by for some reason. The FSM is lacking it...
You may have a short to battery voltage with the 12 VOLTS, as this is generally what that means. It is impossible to know for sure unless every module gets unplugged first, and then you still have 12 VOLTS on the BUS wire... because having a corrupted module can also raise the voltage to 12 VOLTS too.
There are other steps to take looking at the OHMS on the BUS wire. Once all of the modules are removed and the battery is disconnected it should be open. This should help to rule out the PCM. If every module and the battery is disconnected, and the BUS wire does not read OL, then the BUS wire itself has a fault. If the wire is shorted to battery voltage, it may be necessary to trace it throughout its entire length.
I am a bit busy but, I will try to look things over when I get the opportunity. It is very important to have accurate and reliable information whenever searching for a problem in this system. I need to refresh myself on it, since I do not want to be supplying bad information to you. Doing so could lead to hours and hours of searching for nothing.
Okay the bus voltage is between .5 and the most I have seen it go up to was 3.0 even when the gauges go out the bus voltage is still going normal the gauges will only go out when the alternator is charging or the system is being charged we hooked another vehicle to it and with the voltage being up the gauges drop out take the charge off gauges come back on instantly I am pulling my hair out
After further checking to pci bus wire is shorted to voltage on the pcm wire. traced all the others back to the modules. I cut the wire at the pcm and that wire doesn’t have voltage on it unless you connect it back to the pcm, so I would assume that the pcm would be bad. what would your thoughts be now?
Engine: 4.0, new lifters valve job with new springs and exhaust valves, preload set with shims
Originally Posted by Rdytodive
After further checking to pci bus wire is shorted to voltage on the pcm wire traced all the others back to the modules I cut the wire at the pcm and that wire doesn’t have voltage on it unless you connect it back to the pcm so I would assume that the pcm would be back what would your thoughts be now
Separate your sentences with punctuation so we can understand what you are saying? Just a thought.
Not really sure what to make of it at this point. I am having trouble following along with what you are trying to say. I like to think I am usually decently adept at being able to decipher what it is people are trying to say. Aside from a time when I was only just messing around with another member on this site jokingly with what he said, I think this will be the first time to say it, that I am unable to make heads or tails out of your posts. I am happy to try and help if I think I am able to do so. The no punctuation and use of blended sentences running into each other like that are basically becoming incomprehensible to me though.
In order to help we have to be able and follow the imformation being relayed on both sides of the conversation. Misinterpretation of what is being said is about as useful as supplying erroneous information.
It is known to you what has been done at this point since you are the one doing it. The last I heard was 12 VOLTS had been found on the BUS. It is not clear wether this was actually ever the case or not? Maybe it was and it has changed to be 0.5 - 3 VOLTS? Maybe it never was at 12 VOLTS? Maybe besides the PCM most all the other modules have been unplugged at this point, and maybe not?
The BUS voltage is a very dynamic value. Data BUS issues are many times intermittent in how they present, and/or with observed symptoms. Many times the modules do not outright fail, rather they instead make for what is a 'noisy' BUS to cause the problems on it. Thousands of bits and bytes of information are happening along this wire at any given point in time, and this changes from one second to the next. The multimeter is useful. It definitely has its limitations to truly see the issue. This is why a high-normal value has to be suspect. The average of voltage seen on it may be skewed into the upper normal range by the primary fault, and possible to only be seen when looking at abnormal spikes of an electrical waveform with an oscilloscope. This is something to keep in mind when communications are lost, and being unable to use a scanner.
Little things may sometimes get easily overlooked. Just to be sure.. Have you looked into all of the fuses? I believe there are two or three different fuses which help supply the power going to the cluster. One or two of them would be obvious to you with other symptoms happening. I think there should be one specific fuse to possibly be able and cut off the power going to the cluster gauges how it is happening on your Jeep. The Jeep would still run. And even the Jeep could still swing through pre-checks to move the needle gauges during initial start-up, and then kill the gauges.. if the power is not totally disrupted as it may only become limited by corrosion of the fuse and/or its terminals, or with the junction block itself how it distributes the electrical power to the systems. I think it would be worth it to check on the junction block and look over all of the fuses very closely.
One thing is for certain though.. if 12 VOLTS never existed on the BUS, this changes things.
I typed this up fairly quickly. It is only a few thoughts on it for at the moment on what came to mind. I do think you can get this resolved. Your efforts show that you believe the same thing too. I would like to help, and will continue to do so if I think I am able...
I am not exactly very good with grammar myself. I try though. If you can communicate more effectively it would help.
Sorry for the lack of punctuation. The last that I was working on the Jeep, I took the terminal plug off of the data bus wires under the steering column. I then traced each one to the perspective module. The one that had 12 volts on it went to the pcm. I then cut the wire at the pcm and checked it for voltage. It did not have any voltage on it . When I put the wire back together and checked for voltage again the voltage was gone and it was doing the normal fluctuating voltage. So I put the terminal plug back on the other wires and cranked it up. The voltage was acting normal even at the instrument cluster. The instrument cluster was reading everything just fine, but when I plug the alternater back up the gauges drop out. The bus wires are still doing the normal fluctuating voltage. The gauges will work just fine as long as the system is not charging the battery. We put a new alternator on it and it will still do the same thing. I hope this is a little clearer. Please any advise is appreciated.
The whole 12 VOLTS ordeal has me perplexed. I do not understand how it could do this and then fix itself like that? The fact of the matter is the bigger part of the whole picture does make more sense without the 12 VOLTS being on it now though. Everything I ever read says having a 12 VOLT BUS problem is an extreme rarity. It is almost always seen as an abnormally high fluctuating voltage.
The whole alternator thing seems to point at having an electrically noisy BUS, and not an all out obvious failure of any part of the system. It might be when the Jeep is charging, the entire BUS voltage essentially shrinks. I am not sure how to describe it?
If you have an oscilloscope to try? I would guess something like this you might see happening on it;
(The normal fluctuating 0 - 7.5 VOLTS seen as a square wave).
This is an oscilloscope reading of another persons Jeep Grand Cherokee who had the exact same type of issue as yours is having. It had a BUS problem with no gauges working. The gauges worked fine whenever he disconnected the alternator too. Several mechanics charged him a lot of money for a lot of different reasons.. including having his alternator and PCM replaced at one point. In the end, none of anything the mechanics ever did helped him to fix the issue. A family member of his with a scope hooked it up to the BUS wire and started to take readings.
In the above picture you can see how the waveform changed, becoming abnormal when the alternator was connected up on the Jeep. The lower part of the wave is nearly 2 - 3 VOLTS above zero when having the alternator working to charge for some reason. I am not really sure why? I would think maybe you would find something similar as this too. With the multimeter everything looks like it is within normal parameters. However, the wave above definitely shows the full voltage is not there.. which is what causes the communications to stop from being able to go through.
It sounds like you traced the wires from the PCI bus diagnostic junction port under the column. It ties all of the PCI bus lines together in groups.
The other guy started adding in signal ground circuits and obtained further readings. You can see it shows electrical noise on it. There is a group of data seen above having an odd shaped shark fin pattern, and it spikes higher than all the rest of the data occuring on it. The abnormal pattern and the overall shrunken PCI data line voltage was invisible to be seen on a multimeter, and his BUS line was at times also not communicating with scanners.
At the PCI BUS diagnostic junction port he began disconnecting the lines one at a time. He discovered that the one allowing the PCI data line to return to normal when disconnected was grouped with the PCM, the Immobilizer, and the right (passenger side) door module. Even when there is a normally working PCI Grand Cherokee BUS, disconnecting this particular line will still cause the cluster gauges to drop.. even though their BUS line remains connected. (The passenger door in particular)
There is no easier way I know of to find the reason for these types of problems, besides for physically locating every last module to remove them all one by one, and continually rechecking the PCI line voltage if it is abnormal or not (or high-normal... it should not stay much past 2.5 VOLTS very often at all, from what I have read about it). Or, watching for the electrical noise/shrunken waveform when using a scope on it.
If in any way the passenger door is still connected at all on your Jeep?? Definitely get that disconnected (both doors actually)... The Grand Cherokee door module and door switches seem like they are the reason and cause of soo many electrical problems. The guy who supplied those scope graphs.. it was the passenger side door module was the reason for his BUS problem, and why his gauges stopped working. He disconnected the door wiring and then everything worked normal.
Maybe it could still be that easy for on yours.. if it has not been done yet?
Yes I have disconnected both doors. Now it makes sense to me. When I check the data bus wire on the passenger door it is an intermittent signal when checking continuity. So I need to check that wire a bit more. Thank you for the information. I will check further and let you know.
If you have access to use an oscilloscope? I think this would really help a lot. It could help in verification of an abnormality as it is happening specifically on the data line. I think it would help to provide reassurance that you are truly looking in the right location of things, to know for sure that there is a fault occurring on the BUS, you know... and not something else. It would help to see what is happening for comparing different things while you are working on it, like what it looks like with and without the alternator connected. Or, anything... since the scanner is not an option, and the multimeter alone may not be a good enough tool to see it.
So I did some more tracing and found the pcm wire was shorted. Instead of taking the dash out to fix I ran a new wire to the pci bus test port. I also ran a new one from the passenger door. Now everything is working as it should be. Thank you for all your help and advice.
So I did some more tracing and found the pcm wire was shorted. Instead of taking the dash out to fix I ran a new wire to the pci bus test port. I also ran a new one from the passenger door. Now everything is working as it should be. Thank you for all your help and advice.
I am soo happy for you man!
Was it the PCM wire which was shorted to give the 12 VOLTS on the data line or, do you think it was originating intermittently from the door to do that how you saw it?
It is really impressive to get this problem fixed. If it was brought to a dealer, or most mechanics, I am willing to bet there would be a new alternator, a new PCM, a new BCM, etc...and the problem never having been resolved.
All was fine last night but now the issue has returned. I am in the process of trying to find an oscilloscope to borrow. Hopefully I will be able to find the problem. Do you know if anyone has a diagram of how the bus wires route and what modules are on each one? This Jeep is very basic. It doesn’t have the Immobilizer, heated seats, power seats, or tcm. The only components that I can find are the air bag, instrument cluster, ddm, Pdm, pcm, Bcm abs, and the radio has been replaced so the pci bus wire behind is just there not hooked up and the air conditioning controls does not have a pci bus wire to it. Also talking to the guy I bought it from says the gauges worked until the motor was replaced. That’s when the issue started. When I hook it up to a code reader it shows no codes, but it’s not an expensive one like from snap on.
02-17-2020, 12:14 PM
