Ok. Let me start off by apologizing for not posting in awhile. I have been very busy thankfully. I will be updating more often. I have here a nice 2002 GMC Savana 6.5 liter diesel van with 99,859 on the odometer. I am called in after this shop put a motor in and wants to set injector pump timing. They hook up the shop scanner and get no communication. That is when they called me in. I get the same result with my Tech2 scantool. The vehicle starts and runs, so I know something is going on. I poll the modules with my Tech2 and can only communicate with the vehicle theft deterrent module. Lets look at how this system works.
This system utilizes what is called Class 2 communication. Think of it like a telephone line. This line is where modules communicate with each other and with a technician through pin #2 of the data link connector. Each module on Class 2 has the ability to communicate all on it's own. It is what is known as a peer to peer system. It doesn't need a "master" module to work.
Here, I circled the players involved on this vehicle. Since I can communicate with the theft module I know I have some Class 2 communication. I have been down this road before. There are two common modules I see that cause Class 2 issues. They are the ABS module, the SRS (Air Bag) module, and their related wiring. The quick and dirty on these problems is disconnect modules one at a time and see if the remaining modules start talking. So, I put the van up in the air and disconnect the ABS module on the frame rail and recheck Class 2 comunication-no dice. Still only the vehicle theft module reporting.
Now, the SRS module is under the drivers seat. Here is a shot from the passenger side. GM puts a one piece vinyl mat down throughout the front floor of these vehicles. This mat goes under the seats and over the the top of the SRS module. This vinyl mat is great for holding moisture underneath it. These vans all have water intrusion issues. If the fusebox and PCM isn't filling up with water, the windshield is leaking and causing water to collect especially under the driver's seat. So, we are left with taking the front seats out and peeling the vinyl mat back or slitting the mat around the SRS module with a razor to do some inspection. I get the ok to slice and dice and this is what I see.
Holy corrosion! I have seen these pretty bad, but this by far was the worst. The SDM (Sensing and Diagnostic Module) and it's connector are chia pets! Here is another shot.
I try to disconnect the connector from the SDM and will not budge. In fact, the shell comes off in my hand and the terminals are corroded to the module. Hard decisions need to be made here. I will have to cut this harness to see if we get communication back. I also give a sneak peek to the harness that is running back from the SDM to the front as they frequently corrode as well from trapped moisture. It doesn't look that bad, so I snip the SDM harness and now we have 5 modules reporting on Class 2. SDM, PCM, Radio, Vehicle Theft, and ABS. Now, that I am able to communicate with the PCM we check and adjust injector pump timing as per GM specifications.
Here is a scope pattern of Class 2 after we snipped trhe SDM harness and got the other modules back on line. I should have taken a capture before snipping the SDM harness. I guarantee you that it didn't look anything like this. This is a nice healthy Class 2 pattern. Nice well defined square waves that are 7 volts in amplitude and are off ground approximately 200mv for less noise. The square waves come in "data packets" as modules send signals to one another. What is funny is that the week before this job I was at another shop with a similiar problem. Someone had an aftermarket radio shoved in it and I was very suspicious that the shoddy radio install was causing issues. I pulled the radio out and was shining a light in there and noticed strong evidence of rodents-seeds, acorns, droppings, and chewed wires. Still shining my light in this area I was also greeted by roaches! At this point I fled the vehicle and informed the shop owner he needed an exterminator for the vermin and roach problem his customers van had. The next day another shop had van issues and lifting up the vinyl floor mat revealed a rotted harness and black stringy mold. At that point I informed the shop owner he had a EPA issue with that van. You never know what you are going to find.
