No Communication After Transmission Install

Ric from Hooksett Owner/Technician Posted   Latest  
Network Communications
2004 Cadillac Escalade 6.0L (N) 4-spd (4L60-E)—3GYEK62N24G228007

Hello group. This vehicle is fixed but I wanted to run through my strategy with you to see if I'm on track or where I can improve. This vehicle was brought to me from another shop. They stated that shortly after a used transmission install, the cluster became inoperative, ABS, SRS and battery lights came on and the power windows stopped working. They also said they could not communicate with the PCM.

After a quick look at the transmission work, my first step was to try to communicate with the vehicle. No matter which module I tried, there was no communication. A quick check of the wiring diagram showed this to be a one wire, all modules on a single network setup. I'm not that familiar with this system; perhaps someone can shed some additional info. My second step was to scope the network for activity. I found the network stuck at just over 8 volts with no activity. I then proceeded to the network connector SP205 and unplugged it. Checking the open terminals, I found that static voltage on terminal A. Terminal A feeds the radio and the VCIM along with some other entertainment devices. Disconnecting SP207 eliminated everything except the radio (aftermarket Pioneer). With 207 unplugged, I still had the static voltage. I finally unplugged the radio and everything came back online.

Turns out the transmission had nothing to do with the failure. Is there a better approach to these problems, and could someone explain a little theory on this particular style network? I more accustomed to the 2 wire setups. TIA.


Michael from Holt



That is gm class 2 network and fortunately is one of the easiest to diagnose. It runs 0 to 7volts and has a comb where you can isolate the modules you can bring one module on at a time by jumpering the pins. Seems these trucks have problems with the radios and I think overall you did well as you went right to the splice pack. If it would of been shorted to power you would of had 12volts and ground it would of been at 0 volts. Since it was at 8 you can be confident that its a module causing a issue as they are the ones that control the 0 to 7 signal that you see. 


Martin from Burnaby



You did just fine Ric. Next time in a similar situation, I'll bet that you'll go straight to looking for and disconnecting those wonderful aftermarket accessories that people insist on installing. In the dealership, that type of failure on any GM vehicle would rapidly have us looking for non-OEM add-ins. At least this Class 2 system was a "Star" with splice packs rather than a loop as used on some models, where more extensive disassembly of trim to access connectors is necessary.

Since I had the distinct pleasure of working at a Chevrolet, Oldsmobile, Cadillac dealership, I also had the J42236 tool available that was inserted into the splice pack in place of the "comb" to isolate each node by rotating a rotary knob on Corvettes.

As for function, Class 2 toggles from ~0 volts to ~7 volts to "talk". Scoping, you might see as high as 7.25 v when there is activity on the bus, or maybe 6.5 v on a good day with a DMM in peak min max. On some of the Class 2 systems, especially those with few modules, it can sometimes be necessary to connect a scan tool to create activity on the bus and back probe terminal # 2 to obtain a signal.

FWIW, the Class 2 bus at terminal 2 has violet (purple) insulation to the splice pack on systems with Star configured networks and whatever colour was desired beyond the splice pack to the individual modules. On the older Class 2 systems that were usually on more of the luxury models, they used the loop system with violet wires in the loop, two wires to each module. There were also a few "hybrid" loop/star configurations such as the Olds Aurora, which used a loop with a splice and one node outside the loop connected on a single wire.

Class 2 works on a couple of principles. Periodic transmissions and send on change. Nodes on a Class 2 network might be inactive for significant periods, so a State Of Health message is transmitted. In comparison, "send on change" messages might be transmitted when a component changes state from off to on and vice versa, such as when a switch is activated. SOH messages transmit ~2 sec intervals with each "Hi, I'm here" message being ~5 milliseconds.

So, if nobody is active and suddenly a couple of modules transmit messages, they are prioritized for example, 0 through 7, with 000=0 (safety) 001 = 1, 010 = 2, 011 = 3, 100 = 4, 101 = 5, 101 = 6 and 111 = 7 (convenience). Thus, a message identified by "0", takes priority over a message identified as "7".

Specifications for Class 2 networks were total vehicle bus length 35 meters, scan tool cable maximum length 5 meters. FWIW, I do have a Tech 2 DLC cable that is ~5 meters in length, that is handy for single-handedly operating a component such as an evaporative emissions vent valve while standing at the back of a vehicle that is up on the lift in the shop.

Diagnosis could be straightforward or a bit challenging, depending on bus configuration. Using the star system as an example is easier, since it is a single wire system from splice pack to each node.

Think of a group of buddies Jim, Bob, Mike and Dave, who work together each day, arriving and "clock in" at work. Dave doesn't show up one day because he's still asleep. He didn't "call in" to let anyone know because his phone wasn't working (e.g. open module or bus to splice pack) Meanwhile, Jim, Bob and Mike who always work with Dave, tell the boss that Dave didn't show up and the boss makes a note of it. (flags a DTC)

To put this into context a few examples might be that an EBCM didn't send a SOH message and his co-workers with whom he always communicates, Messrs IPC and PCM, flag a U1041. Maybe the PCM didn't "show up for work" and the modules that normally communicate with this node flag a U1016. Using the DTCs for diagnosis can be handy.

However, consider if the bus is shorted to ground or voltage where the bus is "dead", that DTCs may be set, but not viewable until the short circuit is repaired. This is simply because in such an instance the scan tool will not communicate with the network. So, after restoring function to a shorted bus, you wouldn't be surprised to discover "U" DTCs set that couldn't be accessed prior to repair.

On a loop type Class 2 network, if there is an open circuit in the bus, communication usually still occurs because the data can be transmitted in the other direction around using the second wire from the module. So, if a single wire open circuit occurs on a loop style system, the bus could be somewhat fault tolerant. Short circuits require that the bus be accessed at two specified locations, while looking for restored network function.

Limitations of the Class 2 networks were when close to the maximum number of 32 nodes were on the bus on a fully-loaded model. It was very susceptible to voltage issues setting erroneous DTCs. An example of this I recall working on was a 2002 Silverado with Duramax engine. There was an issue with the fuel injection pump losing prime due to an internal failure. If cranked for about 20 seconds the truck would start, but a "U" (communication) DTC would flag. If the fuel manager primer was used, the truck would start and run within 2-3 seconds at most and no "U" DTC would set. So, it was common to experience "U" DTCs with no associated faults to be diagnosed at times, other than a low battery condition.

BTW, data transmission is ~10.4 Kbps on Class 2, or as high as 40 Kbps during programming. In contrast, Keyword 2000 was also ~10.4 Kbps, but 0-12v.

So, star systems are generally the easiest to diagnose and the splice packs are usually located a few inches from the DLC.