Intermittent CAN Bus Collapse After a Dash Camera Install
A 2017 van has an intermittent no-crank and multiple warning messages, often after driving over bumps. The scan tool communicates with the PCM but not with the BCM, ABS, or instrument cluster. With the battery disconnected, resistance between DLC pins 6 and 14 is 60.2 ohm. With key on, engine off, CAN H measures 2.65 V and CAN L measures 2.35 V. An oscilloscope at the DLC shows a normal-looking differential signal most of the time. When the harness near the BCM is wiggled, both CAN lines drop to 0.0 V for about 150 ms and then recover. The only aftermarket item is a dash camera spliced into the OBD power pin. The BCM powers and grounds have not yet been checked. The customer authorizes a BCM replacement if needed.
1. Interpret the resistance and voltage measurements. Does a 60.2 ohm reading prove the bus is healthy?
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The 60.2 ohm reading is close to the expected 60 ohm for two 120 ohm terminating resistors in parallel, so it suggests the termination is present at the moment of the test. It does not prove the bus is healthy under all conditions. A break or short that appears only when the harness is flexed can be invisible with the vehicle stationary. The key-on voltages of 2.65 V on CAN H and 2.35 V on CAN L are plausible biased-recessive values, but a DMM averages the signal and can hide short glitches. The oscilloscope is more useful because it shows the 0.0 V collapse on both lines during the wiggle test. That collapse is the important clue: it indicates a temporary loss of bus bias or a short to ground, not a normal communication event.
2. Explain why the scan tool can talk to the PCM but not the BCM, ABS, or cluster. What should be tested first?
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The vehicle likely uses more than one CAN bus or a gateway. The PCM may be on a different bus or may still communicate through a gateway path that remains alive, while the BCM, ABS, and cluster are on the faulted bus or downstream of the BCM. A loss of BCM power or ground can also take down modules that depend on it. Before replacing the BCM, check its battery power, ignition power, and grounds under load, and inspect the harness near the BCM for chafing. Then use a breakout box at the DLC or disconnect modules one at a time to see which one causes the 0.0 V collapse. The dash camera splice should also be removed and the OBD power pin inspected, because an aftermarket splice can disturb power or ground pins even if it is not on the CAN pair.
3. The customer authorizes a BCM replacement. Should the technician proceed? Give a reasoned recommendation.
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No, not yet. The wiggle test changes the symptom, and both CAN lines drop to 0.0 V when the harness near the BCM is moved. That points to a wiring or connection fault, not a confirmed internal BCM failure. A new BCM may not fix a chafed CAN wire and may introduce programming and configuration complications. The technician should first inspect and flex-test the harness, check BCM power and grounds, remove the dash camera splice, and isolate the bus by disconnecting modules. A BCM should be condemned only after its power, ground, and bus wiring are proven good and the fault follows the module itself. If the BCM is replaced, follow the manufacturer's programming and configuration procedure.