Multiple U-codes and a 121 Ω bus reading
A 2019 heavy truck arrives with the ABS lamp on, the transmission keypad dead and a tachometer that drops to zero. The dash shows U0100 (lost communication with engine ECU), U0121 (lost communication with ABS ECU) and U0155 (lost communication with instrument cluster). A previous workshop replaced the instrument cluster with no change. With the ignition off and both battery negative leads disconnected, you measure 121.4 Ω between pins 6 and 14 of the diagnostic connector. With the ignition on and engine off, pin 6 sits at 2.52 V and pin 14 at 2.48 V. The network terminates at the engine ECU and at the ABS ECU. An aftermarket telematics unit was fitted six months ago.
1. Interpret the 121.4 Ω and the two bus voltages. What are the possible causes, and which test separates them?
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121.4 Ω means one terminator, not two. Two 120 Ω terminators in parallel would read about 60 Ω. So the bus is intact enough to reach exactly one terminator.
Possible causes:
- One terminator missing or failed (module replaced with a non-terminated unit, or an internal resistor failed).
- An open circuit in CAN H or CAN L between the diagnostic connector and whichever module holds the second terminator - the resistance path runs H → terminator → L, so breaking either wire hides that terminator and leaves ~120 Ω.
- A module carrying a terminator unplugged or dead.
The voltages (2.52 V / 2.48 V, summing to ~5 V) show a correctly biased recessive bus with no short to ground, no short to battery and no H-to-L short. That fits a physically split or half-terminated bus, not a shorted one.
Test that separates them: keep the batteries disconnected. Measure continuity, end to end, from diagnostic pin 6 to the engine ECU CAN H terminal, and pin 14 to the engine ECU CAN L terminal; then the same to the ABS ECU. One wire will read open (or hundreds of ohms) - that localises the break. Then unplug the ECU and measure directly across its own CAN pins: ~120 Ω confirms its terminator is present. A plain DMM continuity test can pass through a corroded joint that cannot carry bus current, so confirm with wiggle tests and a loaded check (a 21 W bulb in series, or resistance while flexing the harness at the suspected splice).
Given a telematics unit fitted six months ago, inspect that tap-in first: back-probed terminals, a scotch-lock, a corroded splice or a chafed wire under the dash.
2. A colleague says 'U0121 names the ABS module, so swap it'. Explain what the U-codes actually tell you and critique the earlier cluster replacement.
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U-codes identify victims, not culprits. Each code is stored by the module that expected a message and did not get it: U0100, U0121 and U0155 say 'I lost the engine ECU / ABS / cluster'. The module that is named is the one that went missing, not the one that broke.
Three modules going missing at once points to a shared cause, not three simultaneous module failures: bus wiring, a power or ground feed shared by those nodes, or a split bus. Random module swapping is exactly what produces a repeat visit.
The cluster replacement was unjustified. U0155 is set by other modules about the cluster - but if the cluster were the only problem, the engine ECU and ABS would still talk and the transmission keypad would still work. Replacing it changed nothing because the cluster was a victim of the same loss of communication, not its source.
Correct approach: read codes from every module on the network, note which nodes are actually present, then check powers and grounds to the missing nodes before touching the bus. Only then measure the bus itself - resistance, then point-to-point continuity.
3. You find a corroded, moisture-contaminated splice where the telematics unit was tapped into CAN L. Describe the repair, the verification, and why fitting an extra 120 Ω resistor is not acceptable.
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Repair:
- Cut out the corroded section and any scotch-lock/insulation-displacement joint.
- Re-terminate with a proper crimped terminal and a sealed, adhesive-lined heat-shrink splice; maintain the twist of the CAN pair up to the joint so the pair stays balanced; support the harness so the joint carries no vibration load.
- Refit the telematics connection through a proper mating connector or a factory body-builder take-off, with a drip loop and sealed entry. No solder-only repairs in a wet, vibrating chassis area.
Verification:
- Batteries reconnected, key off: measure ~60 Ω between pins 6 and 14 at the diagnostic connector.
- Key on: CAN H and CAN L each near 2.5 V (sum ~5 V), no fault.
- Clear all codes, then confirm every module reports present - engine, ABS, transmission, cluster.
- Operate all affected systems: tachometer live, ABS lamp off after self-test, keypad working.
- Road test and re-scan; check no U-codes return and no new ones appear.
- Re-check key-off parasitic drain, since the telematics unit is now a permanent load.
Why not add a 120 Ω resistor:
- It raises no fault - it masks an open circuit. The bus may work on the bench and at low data rates but be marginal against electrical noise, and the real fault (moisture, corrosion, vibration) keeps corroding until it fails intermittently again.
- It changes the bus loading and bias network away from the manufacturer's design, and it will confuse every future technician who measures 60 Ω and concludes the harness is healthy.
- It addresses the symptom that a meter can see, not the condition that actually stops the truck.