Make sure the outlet remains “hot” when the key is off, otherwise it won’t feed voltage from the auxiliary battery back into the system. This could be another battery, a battery charger or a “memory saver” that plugs into a power outlet. We’ve even heard of 4×4 modules on some GM SUVs never waking up when the battery is disconnected (fixing the fault requires installing a new module).įor these reasons, never disconnect a battery on a late-model car without first attaching a backup power source to the vehicle’s electrical system. On late-model vehicles with controller area network (CAN) electrical systems, some modules may go into a default mode, standby mode or sleep mode, and may require pinging or reprogramming with a scan tool to restore normal function. Some climate control systems can forget the position of flow control doors and actuators. The PCM can forget fault codes and other diagnostic information (long-term fuel trim adjustments, the readiness status of OBD II system monitors, electronic transmission shift adjustments, etc.). Radio presets and seat/mirror settings are lost, and the adaptive and learned memories in the PCM and other modules disappear, as well.
When a battery is disconnected or goes completely dead, the loss of voltage can cause a variety of problems. And when the key is off, the battery has to provide a steady supply of voltage to any onboard modules or other accessories that need power. When the engine is running, the charging system can usually produce enough current to meet all of the vehicle’s electrical needs, but when the engine is idling and the lights, heater, defroster, radio or other accessories are placing high loads on the system, additional power may be drawn from the battery if the alternator can’t produce enough juice. The battery must also be capable of providing backup power for the vehicle’s electrical accessories and electronic modules. That’s why batteries in hot climates usually don’t last as many years as those in more moderate or colder climates. While cold weather can be hard on batteries, so can hot weather because it increases water loss from the electrolyte inside the battery. A weak battery, or one that is rundown, may not deliver enough amps to crank the engine when temperatures plunge and the oil thickens.
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