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Last night we got a text from Aaron of AJ's Automotive of Minford Ohio, one our best customers when it comes to advanced diagnostics. Aaron always calls me with fun ones, and today's project certainly did not disappoint. Today's patient was a 2017 Chevy Equinox that had been wrecked and put back together. The IPC had multiple lights on, and there was no power steering in the vehicle. It also displayed "AWD Problem, and "Service Steering System" on the display.
Preliminary inspection yielded little results. This vehicle being a salvage that was put back together, I expected to find a broken wire somewhere. So I spent a few minutes just combing over the vehicle looking for something obvious. You could tell that the vehicle was hit pretty hard in both the front and driver's side, but nothing really stuck out as a problem.
Having come up with nothing during my visual inspection, I decided to whip out the scanner. I chose the Verus Edge for the task. I find that the Verus Edge works very well with most domestic vehicles, and is my goto for a scan tool most of the time. I pulled codes from the vehicle and had two CAN communication faults in the the Body Control Module that stuck out; a U0121 (loss of communication with Electronic Brake Control Module) and a U1814 (Powertain Wake Up Circuit fault). The subtype for this code was a 02 which is a short to ground condition.
Anytime I deal with CAN communication codes the first thing I like to do is take a tally of who is actually communicating on the network. This vehicle had only the primary modules currently on the network. I was able to talk to the PCM, TCM, BCM, Antitheft, and SRS system. Everything else was offline. I was missing the PSCM, ABS, AWD module, TPMS, etc.... Clearly we were going to have to delve into the communication system on this vehicle.
After spending some time doing some research on the communications circuit, I found that the BCM sends out a 12v wake up signal to each of the modules that were not talking on the circuit. The DTC detection logic of the U1814 states that the BCM will not send out the wake up signal when the code is set, so I figured this is certaily the code I need to chase.
As with everything electrical, you can always reduce any problem to the most basic electrical circuit. The BCM receives a communication signal from the PCM when the ignition is turned on. The code indicated that the BCM saw a short to ground on that circuit. This in mind I disconnected the connector at the BCM and using my PowerProbe I saw path to ground on the circuit. Now this didn't really mean a whole lot because the circuit was not loaded at this point, so I figured I was likely just seeing path to ground back through the other modules. If I saw a dead open I might have been worried, but this was expected.
I decided the easiest course of action to test my theory was a simple resistance test. My thoughts was that if I had a short to ground that was a problem, I would likely have a pretty low resistance to ground in it. The meter showed 1k Ohm resistance, which although is a little low for a communication circuit, it didn't seem like a short to ground for me. Nevertheless I decided to start pulling modules off the line by disconnecting them.
According the wiring diagram the only modules on the wake up circuit was the Fuel Pump Control Module, The ABS Module, PCM, and the Transmission control module. After disconnecting all of them one at a time I had infinite resistance, and as I disconnected each one I maintained my 1k ohm resistance. This was certainly looking more like a bad module than a shorted wire at this point.
The only conclusion I could come to was that the BCM was somehow shorted internally to ground on the sense circuit. However, I really wanted to prove this to myself. I decided that I would try to do a bypass test and send voltage to the modules in place of the BCM using a test light attached to battery positive. I did this as I started the vehicle, and low and behold all my modules come back on line. I advised Aaron that we needed to get the customer an estimate for a BCM and was on my way.
Note: I uploaded a snip of the wiring diagram. The BCM monitored Pin 22 of Connector 4 for the short. The modules I unplugged were on this circuit. The BCM flags a code for a short on this circuit that doesnt exist. Pin 23 of the same connector is the wake up circuit I used a testlight to send voltage down and wake up all the modules.
Eddie Trapp