Truck Battery Dies in Two Days - 420 mA Key-Off Draw
A 2015 SUV comes in with a complaint that the battery goes dead in two to three days if the vehicle sits. The battery passes a conductance test at 82 percent of its rating, and the charging system holds 14.2 V. You connect an ammeter in series at the negative post, close all doors, and wait 45 minutes for the modules to sleep. The meter stabilizes at 0.42 A. The OEM specification is 50 mA or less after 30 minutes. When you pull the 15 A fuse labeled "BCM / interior lamps," the draw drops to 0.03 A. When you reinstall the fuse, the draw stays at 0.03 A and does not return to 0.42 A. The customer also mentions that the dome lamp has not worked for about a month. All doors were shut during the test.
1. Does the measured draw explain the dead battery? Support your answer with a calculation, and explain why the draw did not return when the fuse was reinstalled.
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Yes, the draw is large enough to matter.
- 0.42 A x 24 h = about 10 Ah per day
- Over three days of sitting: about 30 Ah consumed
- A typical 70 Ah battery with maybe 50-55 Ah of usable capacity below the point where cranking fails has now lost more than half of it, before any cold-weather margin.
Why the draw vanished on fuse reinstall: pulling the fuse removed power from the module that was latched awake. When power came back, the module booted normally and went to sleep on schedule. The fuse-pull identified the circuit, not the cause - the fault is a module (or its input) that intermittently fails to go to sleep. This is why a fuse-pull that "fixes" the draw is a clue to keep investigating rather than a repair.
2. The dome lamp is inoperative. How does that clue change your diagnosis, and what would you check next?
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It points toward the BCM believing a door is open.
- Many BCMs keep interior lamps on, and keep themselves awake, whenever any door-ajar input reads open.
- A latch ajar switch stuck closed, a chafed latch wire, or a poor latch ground will produce exactly this pair of symptoms: no dome lamp and a module that never sleeps.
Next steps:
- Pull up the door-ajar status PIDs for all four doors and the liftgate in the scan tool datastream, and open/close each door while watching.
- On the door that misbehaves, backprobe the latch switch connector and check the signal as the latch is cycled by hand.
- Verify the lamp bulb and the BCM's interior lamp output before condemning the latch - a burned-out bulb is a red herring that can coexist with the real fault.
3. Describe how to isolate this draw without pulling fuses, and why fuse-pulling is misleading on this platform.
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Better method:
- Use a high-resolution meter to measure millivolt drop across the fuses in place, then convert with the fuse mV-to-amps chart for that fuse type and rating. The fuse with meaningful mV is the loaded circuit.
- A low-current clamp (mA resolution) around individual harness branches does the same job with no power interruption.
- Keep a battery maintainer connected so system voltage stays stable and modules do not brown out and re-boot.
Why fuse-pulling misleads:
- Removing a fuse resets the module behind it, which can mask an intermittent "stuck awake" fault - exactly what happened here.
- On some networks, pulling a fuse wakes the bus and other modules, producing false readings.
- You also lose radio/clock/seat/steering-angle memory and can set communication codes, which complicates the rest of the diagnosis.