Inop cluster and power windows

Luis from Lake Forest Technician Posted   Latest  
Question
Driveability
1998 Jeep Grand Cherokee Laredo 4.0L (S) 4-spd (42RE)
Cluster Inop
Power Windows Inop

This Jeep starts and runs at all times. The problem is the instrument cluster and body control module do not come on line when the key is turned to run or start. This is not a power or ground issue, got power and good grounds, as the ABS light comes on in the cluster because its on a different circuit in the cluster. So when the condition is present, the cluster is completely inop and the power windows are also inop. So I found that when I hooked the snap on scanner up to the DLC, the cluster came alive and the windows started working. Now here is where it gets weird! If I disconnect the scantool the windows stop working, but the cluster stays alive. When the cluster is alive you can cycle the key on/off and the cluster will come back on repeatedly, but the windows stay inop until the scantool communicates on the network. This condition is completely repeatable. So I have surmised that introducing another module i.e. the scantool on the network brings the cluster and BCM online or awakened so to speak. Well not quite, simply plugging in the scantool doesn't wake them up, it's only when actual communication occurs. There are no communication codes in any modules and disconnecting any of the modules on this network which is called the ccd bus has no affect on the condition. Disconnecting the battery or cluster puts the cluster back into the inop condition until scantool communication is attempted again. I can't find any information regarding which module is the master or who sends the wake up ping on this system. My suspicion is that one of these modules is not doing its job but can't determine which as they all work when the scantool is communicating on the network. It would be helpful to know if the pcm is supposed to wake up the cluster and bcm or vice versa. I appreciate any input. verified for voltage at dlc connector white/black and violet/brown both have 2.5v when is communication on the network they toggle as I assume it should do that. unplugging the cluster, window master switch, abs control unit and airbag control unit don't make any difference

+1

Rick from Tucson

 

Instructor
 

My first thought was that you have unstable network traffic that is stabilized by the addition of the scan tool. Option 2 is a module that won't wake up on it's own, as you suggested. Either way I would hook up a labscope and monitor traffic with and without a scan tool hooked up to see if it looks dramatically different. I'm not sure a DVOM will help much as CCD networks only toggle about .5 volts from bias and won't change your readings much. As a backup plan if you do not have a labscope try min/max on your DVOM. 

One final thing, I would caution against giving the powers and grounds a clean bill of health just from the ABS light coming on. You might want to double check those with a voltage drop.

I am attaching a screen cap from ATG's Vehicle Network Diagnostic Strategies that shows a CCD bus labscope capture while traffic is present.

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Luis from Lake Forest

 

Technician
 

power and grounds checked all good

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Luis from Lake Forest

 

Technician
 

the screen shot is very similar to what I got thanx

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Chris from Commack

 

Diagnostician
 

Hello Luis. What happens when you unplug the pcm? do the windows work?

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Luis from Lake Forest

 

Technician
 

ecm unplugged no difference no windows and cluster don't work

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Michael from Cartersville

 

Owner/Technician
 

I think it sound more like no wake up. Not very familiar with Chrysler CCD but looking at a diagram it appears wake up is from ignition switch. By entering the network through the DLC your creating wake up for the cluster and BCM.

The thing that is inconsistent with that theory is that the windows should work as long as the module is awake.

I think you should confirm ignition switch voltage at pins 15 and 18 of the BCM. That is probably wake up.

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Luis from Lake Forest

 

Technician
 

Thanx Michael I will check voltage at pin 15 and 18 once I get the car back, customer took it. also I notice when I cycle the power door locks from master switch all windows start working normally until I cycle the ignition switch

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Mike from Mount Pleasant

 

Mobile Technician
 

Longshot but I was thinking that there may have only been only one good ground at the DLC and with the scan tool plugged in it was shorting the 2 grounds together providing a ground back to a module. From what I can tell the grounds for the DLC go back to G102 and the only other module on that circuit, the ABS module, is spliced in at that ground so I don't think that's the case. I feel like it may be an issue caused by a weak power or ground. I was taught to check powers and ground with a sealed beam headlamp. If the circuit can flow enough amperage to light up a sealed beam brightly it should be able to power a module/ign switch. I'm curious to hear what ends up fixing this truck!

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Luis from Lake Forest

 

Technician
 

Thanx for all the help the fix was a (body control unit), got a used good unit replaced it all came alive, All power and grounds and voltage drops had been check prior.

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Luis from Lake Forest

 

Technician
 

The solution was a BCM replacement got a used good unit everything came alive

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