The case of the automotive Chia Pet

July 1, 2016
This Hummer is one vehicle that really stands out in my collection of automotive oddities

I was called to a shop on a 2003 Hummer H2 with a 6.0 L engine with about 120,000 miles on it (Figure1). The complaint was a no start condition. The owner of the vehicle said that he had parked the truck to go into a store and when he came out about an hour later the truck would start up and then immediately cut out. The shop could not communicate with the Engine Control Module but they were able to run the truck with carburetor cleaner. The truck had spark & fuel pressure but was losing fuel pulse. This led the shop to believe that the vehicle was immobilized preventing the vehicle from being started. The shop knew that this vehicle had the Pass Lock 2 system on board and that the Body Control Module played a role in starting the vehicle.

Figure 1

The Pass Lock 2 systems are used on many GM vehicles from 1995-2005 and can be identified by a security light on the dash and by a key with a "PK2" stamped on the metal part of the key where it meets the plastic end. There are some keys that may not have the "PK2" stamped on them and are also used for this system. These keys do not incorporate an internal chip but only function to turn a lock cylinder that houses a magnet in the center of the key cylinder assembly. When the lock cylinder is rotated in the crank mode it will travel far enough for the magnet to go past a 3-Wire Hall Effect sensing device located in the lock housing allowing the sensor to pull the 5 Volt Pass Lock signal from the Body Control Module to a certain predetermined level. It is this specific value that is learned within the BCM's memory. If the vehicle battery was to go dead or a new Pass Lock sensor installed in the vehicle you would have to perform a 30 minute learn procedure to allow the vehicle to run.

The shop was unable to talk to the Body Control Module (BCM) as well so they removed part of the left side dash to gain access to the Pass Lock sensor and BCM for further testing. They validated the power and ground feeds to the BCM and Pass-Lock sensor and were not too quick to condemn any unwanted parts. Since both the Engine and Body Control Modules were unresponsive it only made sense that the truck would be in a theft mode. The engine did start and die so that indicated that the Engine Control Module was working but just was unable to communicate. The shop at this point decided to call me in to get a second opinion.

Enter autotech on wheels

When I arrived I validated the shop’s findings by trying to communicate with both the Engine and Body Control Modules and had the same results with a no communication issue. It would be less than likely that two modules were both bad or that they shared a problem with the exact same power and ground feeds. What they definitely did share was the same data line. Validating communications with just two modules does an injustice in diagnostics. You need to go into every control module that your scan tool supports. This way you are actually validating your scan tool and the entire network so you can dot the "I" s and cross the "T"s. In doing so I discovered that I could not communicate with ANY control modules on board. This vehicle had a major network problem. It is not uncommon to have a network that is shorted to ground or power by a bad controller on a network or due to faulty wiring. The hardest part here was to narrow down where the problem was exactly located between the front and rear bumpers. I had to put a game plan of attack together by printing out a network diagram and flowing the wiring schematic to see the players involved (Figure 2).

Figure 2

The network was a single wire Class 2 Serial Data system that branched out into a parallel circuit. There were about fourteen control modules on the network with some modules being optional. I highlighted the grounds in green at the Diagnostic Link Connector (pins #4 & #5) and the Power Feed in red (pin #16). These are standard power and ground feed pin locations on most vehicles with OBD II diagnostic connectors. I also highlighted the Class 2 Serial data line in purple at pin #2 of the DLC connector. This vehicle used three network splices to serve as central connecting terminals for multiple controllers so I highlighted these in yellow. These splices were incorporated specifically by GM to aid in network diagnostics so that a technician could narrow down a bad control module without tearing the whole vehicle apart to access each control module to pull them off the network one by one. The main 12-pin network splice was conveniently located just above the DLC diagnostic connector taped to a harness conduit (Figure 3).

Figure 3

When aftermarket is a bad thing

As I was inspecting the connector my eye came in quick contact with an aftermarket alarm wire tapped into the main purple Class 2 data wire at the diagnostic connector (Figure 4). I suddenly had a chill run down my spine thinking "I found the culprit!" and it was payday! It is not uncommon for an aftermarket alarm to be placed within a network causing a problem in taking down the entire system. Alarm installers will tap into certain networks so that the alarm module they install in the vehicle can go online with the existing controllers and interact with them. There is no need for the bulk of wires needed on earlier alarm systems when everything can be done by network commands. An alarm module today can take a simple command from a user's remote to unlock a door by sending a message out on the network to command a Door Control Module to unlock the doors. This function is only a fraction of what is capable in today's technical advances in aftermarket electronics. At this point I had no choice but to remove the alarm wire, but to my surprise the problem was still there. It was well worth a shot because I have fixed many cars in the field using visual inspections but this Hummer was going to give me a run for my money. It was all good because I was up for the challenge.

Figure 4
Figure 5

I pulled the 12-Pin splice comb (Figure 5) from the main splice connector in order to isolate all the onboard controllers from the diagnostic connector Class 2 data feed line located at position “D” of the 12-pin splice connector leading back to the DLC connector pin #2. I hooked up my OBD II breakout box to the DLC connector. This is a very handy tool to have especially if you don't desire to lay twisted under a dash. The OBD II breakout box gives you quick access to all 16 diagnostic connector pins and incorporates LED's for each port to show ground, power and data transmission activity. It also incorporates a male OBD II port so you can hook up your scan tool while testing the ALDL ports. I used a jumper wire from the #2 DLC pin and connected it to each of the twelve individual splice connector pins one at a time to validate each controller for communication (Figure 6). I was able to talk to all the controllers that were built for this vehicle except for the Air Bag and Driver Memory Seat. These modules shared a common blue/white wire on pin "H" of the main splice connector. This same wire ran to a second 6-Pin splice connector that was located in the left kick panel. It was here that the Air Bag Control Module data line and the Driver Memory Seat Module data line tied into the blue/white wire. I removed the splice comb from the 6-Pin splice connector at the left kick panel to completely isolate both the Air Bag and Driver Seat Memory control modules from the network (Figure 7). I cycled the key to reset the vehicle network and attempted to start the vehicle. To my surprise the Hummer started right up immediately. I was on the right track and now it was time to move in for the kill.

Figure 6
Figure 7

Honing in on the problem

I next used a jumper wire to jump out the brown/white data line coming from the Driver Seat Control module to the blue/white wire feeding back to the main splice connector pin “H”. I again attempted to start the Hummer and it started without a problem. Then I jumped out the blue data wire feeding back to the Air Bag Control module and jumped it to the blue/white wire leading back to the main splice connector pin “H”. I attempted to start the Hummer once again and it would not start. Okay so now I found the culprit. The problem was in the Air Bag Control Module or wiring.

Just out of curiosity I used my Power Probe just to see if this wire was shorted to ground and I was very surprised to see 13 volts on the wire (Figure 8). How can this be? Did the air bag module have a bad ground causing feedback voltage on the data line or was it simply a bad internal Air Bag Control module board causing the same result? It was time to take a closer look. I carefully pulled back the carpet under the driver seat so that I could validate the Air Bag Control Module wiring. I was suddenly shocked at what I saw! The module looked like an "Automotive Chia Pet" and it was caked with some nasty white milky powder (Figure 9). I wasn't about to touch the mess for fear of the unknown substance. It seemed like moisture had built up under the carpet and created a corrosive breakdown of the aluminum housing of the Air Bag Module. This in turn worked its way into the harness connector causing cross voltage shorting into the data line. At this point I immediately disconnected the battery as a safety precaution to prevent an unintentional Air Bag deployment. I also instructed the shop technician to handle the “Chia Pet” with latex gloves. I did not want him to get the corrosive powder in his skin. I even gave him the option of using a hazmat outfit before he attempted to extract this Chia Pet from the vehicle.

Figure 8
Figure 9

What a humdinger of a diagnostic adventure this was for me! You diagnose cars for over 25 years in business and you come across something that so unimaginable to believe. This is the kind of stuff that keeps me in the business. It's the daily challenges and the rewards we reap that make it all worthwhile. I probably could have used a scope or meter and placed it on the data line to check for a short to ground or power and validated the data line for a known good communication pattern. This would also involve knowing high and low signal thresholds and possibly frequency stats but I decided to take the low road to keep it basic. I know that a lot of guys out there may not be a network expert or may not have GM experience. What I do know is that the average guy is able to follow a diagram and flow it mentally so he can build a game plan of attack without getting too technical.

My basic goal was to keep it simple and just break down the system piece by piece until I found that needle in the haystack. There are many networks that are much more complex but it seems to me that GM simplified this system by using network splices to aid in isolating a circuit fault in the system. Sometimes these splices are hidden and taped in inaccessible areas of the vehicle but having good information systems can help you pinpoint their locations in the vehicle. In the end it's all about knowing how to utilize your equipment, information systems and building a game plan of attack that makes you a winner. Hope you enjoyed the article and that this one will hit home with a lot of my fellow technicians out there.

About the Author

John Anello | Owner and operator of Auto Tech on Wheels

John Anello is the owner and operator of Auto Tech on Wheels, established in 1991 in Northern New Jersey. He provides technical assistance and remote reprograming with 21 factory PC-based scan tools. Driven by a passion for cars, John's business now services roughly 1,700 shops.

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