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Old 04-20-2018, 05:22 AM
Meineke1808 Meineke1808 is offline
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Default Help with Scope Signal (Jeep Communications)

Dear friends,
I have a car in the shop that is kicking my butt (I'm sure you know the feeling) and am at my wits end about where to proceed with the diagnostics. My main question centers around a scope signal for a communications network and how to interpret the signal that I see. Thank you for taking the time to read this message.

First, a little background. The vehicle is a 2000 Jeep Grand Cherokee with the 4.7 V8. The problem centers around crazy electrical problems mainly manifested in the gauge cluster area. This car is possessed. Symptoms include: all the gauges drop to zero, all the warning lights come on, the overhead display gives no information...just dash lines, the AC stops working, the power windows stop working, there is often no communication to any module on the car except the PCM while this problem is occurring, and sometimes the vehicle stalls (probably because the security module, SKIM, is going offline). Unfortunately, this is a fairly intermittent condition, so it is very difficult to get the car to consistently act up while it's in the shop. You may get one chance for the event to happen within a week and even then it sometimes only lasts for 5-20 minutes or so. There is a code p1687 for communications with the instrument cluster that resets after it has been cleared within a few minutes. After the event occurs, there are normally also codes p1686 and p1698 for communication problems to other modules. As I'm sure many of you know, the communication network on these Jeeps is prone to be brought down if any of the numerous modules on the vehicle has some sort of internal short. One good way to try to diagnose this issue is to unplug each of the numerous modules (I think there are 13 in all, or something like that) while the problem is occurring. However, due to the intermittent nature of this condition, it makes it difficult to unplug each module WHILE the problem is occuring. In my experience, most of the time, there is an issue with the door modules on the driver's or passenger side (which are mainly glorified window switches) or with the wiring thereto. However, I have unplugged both modules, driven the car for a time and confirmed that the problem still occurs with the door modules unplugged, leading me to the conclusion that this is NOT a problem with the door modules. Also, I have pulled apart the wiring harnesses from the door to the body of the vehicle and found no signs of wires broken. I have also replaced the PCM on this vehicle in order to try to amend this condition, but there is no change.

The best information that I have to go on is a scope signal. I am not sure however how to interpret the information that I have. On the Jeeps during this year, there is a diagnostics junction port for the communication network (which is separate from the data link connector/OBD port). I have been scoping the communication signal at pin 2 in the DLC/OBD while removing modules one at a time from the separate diagnostics junction port connector. The signal is supposed to be a square wave going from 0-7.5 volts. One abnormality I notice is that the bottom line at 0 volts is sometimes popping up (briefly, but fairly consistently observed) to around 2-4 volts while monitoring the signal with the vehicle running (please note, this is when the event is NOT occurring, since it happens so intermittently). I have noticed that when I eliminate pin 6 from the DJP (which goes to the Instrument cluster module), this popping up of the bottom line on the scope signal goes away and it seems to be a much cleaner 0-7.5 volt signal (this would perhaps explain why code p1687--an instrument cluster communication code--comes back so quickly after being reset, even outside of this main electrical event occurring). This led me to believe that there is something wrong with the instrument cluster itself or with the grounds to the instrument cluster, and that this may be the source of the communication network collapsing under certain circumstances. However, just yesterday, through some kind of divine intervention, a 2001 Jeep Grand Cherokee appeared in the shop while I was working on this 2000 model. The owner was nice enough to let me experiment with his instrument cluster. I quickly took it out of his car and installed it in my 2000 model and then checked the scope signal, expecting that the scope signal would now not pop up at the bottom edge if this was a problem with the cluster itself. Unfortunately, the signal still popped up with a different instrument cluster installed--so it must not be that. (I also scope checked the signal on the 2001 before I removed his instrument cluster and found that the scope signal there was NOT popping up at the bottom edge, but looked clean from 0-7.5 volts.) I next checked the grounds to the instrument cluster and found no abnormalities there. The resistance was less than 1 ohm, even when I tried tugging on the wiring loom.

So, basically my main question is this: when a scope signal is supposed to have a range between, e.g., 0-7.5 volts and the bottom line is not staying consistently at 0 volts, but is often popping up to 2-4 volts (albeit just briefly), was I correct to assume that this is some kind of GROUND issue somewhere, not allowing the pulsed signal to return completely to 0 volts, or is that an incorrect assumption? Does any body have experience with a scope signal like that?

Thanks again for listening and I hope that some one has some input.

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Old 04-20-2018, 05:58 AM
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Steve6911 Steve6911 is offline
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This is how I would go about it. Below is a diagram of your PCI bus. I would make jumper wires up at the PCI diagnostic connector and when the problem is happening (from your post I know this will be difficult) start disconnecting the wires one at a time until the problem stops. This will give you an idea which module group is causing the problem. I recently had a similar one 2001 Jeep G/C 4.0 which caused gauge and lighting issues and it was the PCM. I was able to get the problem to happen KOEO and doing it this way I was able to determine the PCM was taking down the Bus. And with the problem happening I did have Comm with the PCM and TCM but no other modules.

PCI buss.JPG
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