Sunday, February 19, 2012

2004 Jeep Grand Cherokee

Here is one 2004 Jeep Grand Cherokee with 88,656 miles on it. The issue here is no passenger side door functions such as power windows or power door locks. The other issue is no RKE (Remote Keyless Entry) functions as well from the key fobs. The drivers side operations work correctly. The Grand Cherokee platform is definitely one of the more popular vehicles I encounter. They have various issues both common and uncommon.
The shop has done some checking before I got there. Various panels have been removed in an effort to narrow down the search. The first thing that catches my eye is the aftermarket alarm module dangling. I am always wary of aftermarket alarms and their installations. I have seen many issues caused by either bad alarms or the installation. Let's have a look at the wiring diagram.

The system utilizes door modules in each door that communicate through a data buss called PCI ( Programmable Controller Interface) buss. It is very much like GM's Class 2 data buss. It is a peer to peer network meaning that any module on the network has the ability to communicate on it's own. Both the DDM ( Drivers door Module) and PDM ( Passenger Door Module) share a common power and ground feed as well. Looking through the service information I also come across this- I underlined the important tidbit of information.

The DDM and PDM each utilize integrated circuitry and information carried on the Programmable Communications Interface (PCI) data bus network along with many hard wired inputs to monitor many sensor and switch inputs throughout the vehicle. The PDM also receives inputs through an integral Radio Frequency (RF) Remote Keyless Entry (RKE) receiver. The DDM and PDM control and integrate many functions and features of the vehicle through both hard wired outputs and messages over the PCI data bus. 

We also know that at least our source for power and source for ground is good since we have proper drivers side window and lock operation. So what is next?
This is what I do. I reach for my DRB3 and go into this menu. Factory tools do so many functions that the aftermarket tools may not have. I hit the enter button.

This is the next menu. What the DRB3 is going to do is look for any modules that are communicating on the PCI data buss and report. Such a great feature.
This is the first page. As you can see it is in acronym form so you have to know your Chrysler modules. Let's check out the next page.

Well we see a Driver Door Module but no Passenger Door Module. So, we know that the PDM is not reporting on the PCI buss. Let's get right to the PDM.

Pulling the passenger dise door panel gives me access to the PDM. I want to check power, ground, and whether it is receiving PCI buss messages. Lets first check for PCI data messages. I grab my scope and backprobe the PDM.
We have communication. Like I said just like Class 2 on a GM vehicle. We have a 7 volt square wave slightly off of ground to help with noise. So we know we dont have an issue with the PCI wiring to the PDM. Let's check power and ground.
This is the wire that is supposed to have battery voltage. We have a problem. Checking the ground wire at the PDM and that is good. So we have a power feed issue no doubt. But, is that all?
So, backprobed at the PDM I run a fused jumper from battery positive to the PDM. Lets rerun the PCI buss check.
Hello there PDM. I check passenger side window and lock operation and all is well. Now, it is time to find where the loss of voltage is. I know where I am going right away.

There it is a broken feed wire in the passenger side bellows between door and A pillar. Checking the other wires in the harness and they were all good. A quick repair of the wire and reassembly and all is well. The RKE worked like a charm as well. The shop owner was more than happy. This car was fixed with no parts and quickly. It is a bit unusual to see this sort of failure on the passenger side. I see it all the time on the drivers side however. Think about how many more times you open the drivers side versus the passenger side.