Van With a Silent BCM on a Healthy CAN Bus
A 2018 light commercial van (2.0 diesel, 6-speed manual) is brought in with the MIL, ABS and battery lamps on, a dead speedometer and an engine that cranks but does not start. The workshop's scan tool communicates with the engine ECU and the ABS module but reports 'no communication' with the body control module (U0140). With the battery disconnected, resistance across DLC pins 6 and 14 is 60.4 Ω. With the ignition on and the BCM connected, CAN-H measures 2.51 V and CAN-L 2.49 V at the DLC. Fuse F12 (10 A, BCM battery supply) was checked by eye and with a test lamp and looks good; battery voltage is 12.5 V. The workshop has a two-channel scope, a DMM with duty cycle and min/max, back-probe pins, and no breakout box. The van is blocking the only ramp and the owner needs a diagnosis today, not a guess.
1. Interpret the 60.4 Ω and the 2.51 V / 2.49 V readings. Which faults do they already exclude?
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60.4 Ω across pins 6 and 14 (battery disconnected): the bus has two 120 Ω terminating resistors in parallel → 60 Ω. Reading 60.4 Ω means both terminators are present and the CAN-H/CAN-L wiring between them is continuous. It excludes:
- an open circuit in either data line (would read ~120 Ω if one terminator is isolated, or open-circuit),
- a short between CAN-H and CAN-L (would read near 0 Ω),
- a short of either line to ground or to battery (would read far from 60 Ω).
2.51 V / 2.49 V, ignition on: this is the recessive level - both lines biased at ~2.5 V with no dominant bits being driven. When any node transmits, CAN-H is driven to ~3.5 V and CAN-L to ~1.5 V (2 V differential). So the transceivers are powered and the bus is idle, not shorted or jammed dominant.
Conclusion so far: the network hardware is fine, but nothing is transmitting. The fault is local to the silent node - the BCM - i.e. its power supply, ground, wake-up input, connector, or the module itself. The engine ECU and ABS talking normally also shows the bus is not being dragged down by another module. Do not start cutting into the loom on the strength of a 'no communication' code.
2. Set out the test sequence that will separate a BCM supply/ground fault from a failed BCM, with the values you expect at each step.
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1. Supply, loaded. Back-probe every B+ pin at the BCM connector (permanent and ignition-switched) with the connector fitted. Expect within 0.3 V of battery voltage. A test lamp or load applied to the pin separates a 'ghost' voltage (high resistance upstream) from a real supply. Check the fuse by voltage drop across it (should be a few millivolts), not by eye - a hairline crack passes a test lamp and fails under load.
2. Ground. Measure from the BCM ground pin to battery negative, then to the chassis; expect <0.1 Ω, or better, do a loaded voltage-drop test (<0.2 V). Corroded body grounds behind kick panels are a common cause on vans.
3. Wake-up / ignition input. Confirm the ignition-switched and wake lines change state at the connector when the key is turned. A BCM with no wake signal is powered but asleep, and it will look 'dead' to the scan tool.
4. Connector and water. Inspect the BCM connectors for pushed-back terminals, green corrosion and water tracks in the footwell/A-pillar area; check terminal tension with a mating pin.
5. Only then, the module. If supply, ground, wake and the connector are all proven good, scope the CAN pins at the BCM itself. If the rest of the network is chattering and the BCM pin shows nothing, the module is not driving the bus - condemn the BCM, and prefer a known-good or bench-tested unit over a blind 'parts-cannon' replacement.
3. What would a scope show on a healthy bus, and what exactly would make you condemn the BCM?
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Healthy bus on the scope: probe CAN-H to chassis ground. You see a signal sitting at ~2.5 V with bursts of activity where it rises to ~3.5 V while CAN-L drops to ~1.5 V; a differential probe view shows 0 V recessive and ~2 V dominant. The bursts are present even with the ignition on and engine off, because the other modules are chattering.
Diagnostic use:
- Probe the DLC first to confirm the bus is alive (it is - the engine ECU and ABS communicate).
- Then probe at the BCM connector pins, referencing the same ground. If the bus is active at the DLC and dead at the BCM pins, the break is between the splice and the BCM - a connector or a chafed wire.
- If the BCM pins show activity but the module does not respond, it is receiving but not processing - internal.
Condemning the BCM - all must be true:
- B+ and ignition supplies present at the connector under load,
- ground proven by loaded voltage drop,
- wake/ignition signal present at the connector,
- connector and terminals good, no water damage,
- bus active at the DLC and at the BCM pins, yet the module neither transmits nor responds, and there is no other module jamming the bus.
Anything less and you have proved a wiring fault, not a module fault. Note that on this vehicle the no-start is consistent with a dead BCM (immobiliser and start-authorisation depend on it), so fixing the BCM should restore starting - verify by re-scanning all modules and clearing U0140 before releasing the van.