Car Battery – Know What Can Go Wrong, Before You Disconnect It

This may cause the engine to run poorly. Because, the air/fuel mixture, is too rich or too lean, until the (PCM) can relearn, the fuel trim adjustments. The transmission may not feel the same, until the (PCM) or (TCM), relearns the shift adjustments. Furthermore, the affected module, may prevent the (ABS) or airbag systems from working.

Air Bag Warning Light
Air Bag Warning Light

It also, resets the climate control module. So, on some vehicles, that means the A/C will not work. Until, the module is programmed, with the correct instructions. It also, resets the (BCM). This can mess up the operation of power accessories. Such as, power windows, memory seats, power sunroof or electronic suspension settings.

It may, reset or disable the anti-theft system as well. The engine may, crank but not start. Because, the anti-theft system thinks, somebody is trying to steal the vehicle.

This is necessary, because the vehicle’s charging system is programmed, to gradually increase the charging rate, as the battery ages. Finally, it causes a loss of channel settings, on an electronic radio and clock.

Learn How To Safely, Disconnect Your Car Battery

A Handy Tool Everyone Could Use, Is A Memory Saver.

(OBD)-II Vehicle Memory Saver
(OBD)-II Vehicle Memory Saver

On any vehicle that has computerized engine controls, the loss of voltage to the (PCM), causes it to forget its, adaptive memory settings. To clarify, adaptive memory, contains the adjustments, the (PCM) has learned over a period of time.

For example, the fuel mixture, transmission shift points and other control functions. The keep alive memory, also includes all the results, for the diagnostic self-tests, the (PCM) runs on itself.

Disconnecting A Car Battery, May Cause A Whole List Of Issues:

  • Starting problems
  • Drivability issues
  • Loss of air conditioning
  • Power accessory functions (door windows, seats, sunroof)
  • False warning lights
  • Damage to electronic modules
Removing Car Battery
Removing Car Battery

And, never, disconnect the car battery, while the engine is running or the ignition key is turned on.

Loss of power window and/or power sunroof position settings. Loss of steering angle sensor settings. Because, the steering angle sensor, will have to undergo a relearn procedure, following the battery disconnect. Replacing the car battery on some vehicles, requires entering the new battery info, into the (PCM) with a scan tool.

Abbreviations, that will be used in this Blog Post will include:

  • (ABS) = Antilock Braking System
  • (BCM) = Body Control Module
  • (ECM) = Engine Control Module
  • (LTFT) = Long Term Fuel Trim
  • (OBD) = On-board Diagnostics
  • (PCM) = Powertrain Control Module
  • (TCM) = Transmission or Transaxle Control Module

In addition, disconnecting the car battery, can create a high voltage spike in the electrical system. Consequently, damaging electronic modules.

Disconnecting Car Battery
Disconnecting Car Battery

So, Loss Of Memory, Is Where Things Start To Go Wrong

Car Battery - Know What Can Go Wrong, Before You Disconnect It
Car Battery – Know What Can Go Wrong, Before You Disconnect It

Disconnecting a car battery for any reason, may cause unexpected things to happen.

So, before you disconnect a car battery, there are things you need to know.

Above all, battery voltage, must be maintained to the (PCM) and other modules, while the battery is disconnected.

So, it takes approximately 10 cold starts, with a running time, of approximately 15-30 minutes, for the (PCM)/(ECM) to completely relearn, (LTFT) values. During this time, the engine will run, progressively better, with each cold start.

The steps to use a memory saver are quite easy:

  • Always turn off the engine, before disconnecting the battery.
  • Then, plug in the memory saver, with its small battery attached, into the cigarette lighter or the (OBD) connector.
  • The (OBD) connector is located, under the driver’s side of the dashboard, on the steering column.
  • Next, remove the old battery, making sure that the positive battery wire is not, touching the car’s body.
  • Then, install the new battery and attach the connectors.
  • Finally, unplug the memory saver from the car. It’s that easy.


So, a memory saver, is simply a connector with a small battery. It has just enough voltage and current, to keep all of the computer memory alive. Until, the cars main battery is reconnected. And, the connector just plugs into, the car’s cigarette lighter or the (OBD) connector.

The memory saver tool, incorporates a diode on the positive side. A diode, is essentially a one-way valve for electricity. Consequently, it stops the electricity from flowing backwards causing damage.

Using A Memory Saver When Disconnecting Your Car Battery
Using A Memory Saver When Disconnecting Your Car Battery

Thank You !


So, disconnecting the car battery, to reset the (PCM) on late model vehicles, can do more than just erase, the keep alive memory. It can also erase vital learned information, that is absolutely necessary, for other modules to function normally.

Learn What Can Go Wrong, Before You Disconnect Your Car Battery

So, It all depends On, The Year, Make And Model Of Your Vehicle.