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.
 

2002 Saturn L300

Here is a 2002 Saturn L300 with 78,865 miles on it. The complaint here is no air conditioner compressor operation. The shop already did some testing and came up empty. They needed answers fast. I start the car and turn on the air conditioning. I do not hear the click of the AC compressor turning on. At this point I break out the Tech2 knowing full well that the AC compressor control is PCM controlled. Here is some data that I captured.

So here is the data capture. I am really only interested in 3 items at this point.

Here they are. From these 3 pids I can determine the following; It has or thinks it has freon, the PCM is allowing AC operation, and the PCM wants to turn on the AC relay that controls clutch operation on the AC compressor. So now I can concentrate on checking the "muscle" of the system because the "brains" seem to be working properly.

  
Looking at the actual AC compressor. The compressor clutch looks a bit worn and rusty. At this point I would like to get an available voltage reading at the clutch. Unfortunately, due to the placement in the vehicle I will have to go downstream to get my readings.

Here, I have the compressor clutch power and ground backprobed at the most accesible area closest to the actual compressor. Well, we need to investigate the wiring a bit more. After doing a voltage drop on the ground side of the circuit, I know that our issues are with the power side. Time to get out and review a wiring diagram.

Ok, here we go. We have the typical GM compressor control circuit. The main players are the 10amp A/C fuse and the A/C relay. I didn't go after the control side of the relay because there were no circuit codes that typically set when the control side of the relay is not intact. So let's get to the underhood fusebox.

Checking the A/C fuse and low and behold it was blown. This is going to be easy. Inserting a fresh fuse and it immediately blows when the compressor is supposed to engage. Alright, I was suspicious of the ac compressor clutch all along. So I reach underneath and disconnect the connector right at the compressor clutch and try a fresh fuse and guess what it blows as well. Now, I am down 2 fuses. I have a rule with checking shorts. If I blow 3 fuses it is time for me to put a circuit breaker in place of the fuse. I hate having to do this. I look at the wiring harness and inspect closely. It does not appear to have any obvious damage, such as from a front end hit. My mind starts to wonder is there some type of hidden gremlin on this car and I am going to be pulling harnesses apart for the afternoon? When this happens I take a breath and go back to the wiring diagram. I try to KISS (Keep it simple stupid).

Looking back at the diagram I see there is a diode on the power side of the relay. This diode is there to suppress voltage spikes when the compressor clutch releases. One side of the diode is on the power side and the other is right to ground. I know others were in here testing before me. Could I have a biased diode? Basically, a direct short to ground. Let's test.

I remove the diode and hook up my meter leads to the diode terminals. Set the meter to diode test. The diode test will do a voltage drop test across the diode. With the meter leads one way I should see approximately a half of volt (0.50v) then reverseing the "polarity" of the leads on the diode should net us an open circuit. Let's see what we got.

Yikes! Let's reverse the leads.

Double Yikes! Basically this diode is now a direct short to ground. It is now a paperclip instead of a semiconductor. Luckily, this shop owner had a GM car that was going to the boneyard. I quickly stole the diode out of the fusebox. Let's test that one.

Ok. Let's reverse the leads.

Nice. Textbook. I install the good used ac diode. I turn on the ac and listen for that familiar click from the compressor. I hear nothing. What now? I recheck the fuse-good. Rehooking up my meter leads at the compressor clutch wires I am greeted to this.

So, I now have correct voltage feed. But, still no operation. Remember, that compressor clutch I suspected all the while.

I instruct the shop to drop the compressor. Here, is the clutch coil resistance 4.69M ohms basically an open circuit. Normal clutch coil resistance is 3.22 ohms. There is a thermal fuse incorporated in the clutch coil that will open at 417F. Once this opens up there is no going back-replacement is necessary. My guess here is the compressor clutch overheated. I think the diode failure was previous tester error. Someone got happy with the power probe. The lesson here is keep cool when there are multiple issues.