Posted on April 23, 2018
Disconnecting or replacing a battery on a computer-equipped vehicle may cause starting, driveability and a number of other problems, including loss of air conditioning, power accessory functions (door windows, seats, sunroof), false warning lights, even damage to certain electronic modules! So, if you are replacing or disconnecting a battery for any reason, proceed with caution.
One of the things I wanted to do with the article was list all of the vehicle applications that have known battery-disconnect issues. Unfortunately, I have never found such a list. Most vehicle owners’ manuals don’t offer any precautions about disconnecting or replacing the battery. There may be some cautions in the factory service literature, but it is often hard to find, even if you know where to look for it.
On vehicles with known battery-disconnect issues, there should be a big red warning label on or near the battery to warn people. But except for shock hazard’s warnings on high-voltage hybrid batteries, no such warnings are posted anywhere. None of the battery manufacturers warn consumers or technicians about the possible risks of disconnecting or replacing a battery on their websites. Consequently, most people do not realise that disconnecting or replacing a battery may cause a problem until after they have done so.
Disconnecting your car battery can cause memory loss: What kind of problems am I talking about? Loss of memory is one. When you disconnect the battery on any vehicle that has computerised engine controls (which is virtually every car and truck that has been built since 1996), the loss of voltage to the Powertrain Control Module (PCM, that’s commonly called “brainbox”) causes it to forget its adaptive memory settings in its Keep Alive Memory chip.
Adaptive memory contains the adjustments the PCM has learned over time for the fuel mixture, transmission shift points and other control functions. The KAM also includes all the results for the diagnostic self-tests the PCM runs, its sensors and emission control components, plus any fault codes that may have been set (including history codes and freeze frame data that may be needed for diagnosis).
An old mechanic’s trick for temporarily eliminating certain kinds of driveability problems is to disconnect the battery ground cable for 10 seconds to “reset” the computer. Essentially, this trick erases the stored fault codes in the PCM’s KAM and turns the check engine light off.
But this is not a fix for a check engine light. All it does is turn the light off temporarily and returns the PCM to its initial base settings for the air/fuel mixture, idle speed and other control functions. If the vehicle has a problem, sooner or later, the same fault code(s) will reset and the same driveability problems will return because the cause has not been diagnosed and repaired. It may take a few days, but the problem will be back.
Warning: Disconnecting the battery to reset the PCM on a growing number of later models (2003 and newer) can do more than erase the KAM. It can also erase vital learned information that is absolutely necessary for other modules to function normally.
Problems that may occur if you disconnect your vehicle’s battery: What happens when the battery is disconnected? It depends on the year, make and model of your vehicle, but any of the following may happen: It erases all the learned values in the PCM’s Keep Alive adaptive memory. This may cause the engine to run poorly because the air/fuel mixture is too rich or too lean for a period of time until the PCM can relearn the fuel trim adjustments. This may take up to several days and 50 to 100 miles of driving until engine operation returns to “normal.”
Erasing the PCM’s adaptive memory may also affect the way the transmission shifts and feels. The transmission may not feel the same until the PCM or transmission control module relearns the shift adjustments. This may take 50 to 75 miles of driving.
It resets the FMEM (Failure Mode Effects Management) module on certain late Ford models. This module provides an adaptive fail-safe strategy that substitutes estimated or fixed data for missing sensor data. Normally, this should not cause a problem, unless the vehicle has a bad sensor and has been substituting data from the FMEM module for a missing input.
It resets the ABS (Antilock Brake System) and SRS (Supplemental Restraint System) or airbag modules. This should not be a problem, unless one of these modules requires a special relearn or reprogramming procedure after power has been lost. In that case, the affected module may prevent the ABS or airbag systems from working.
It resets the climate control module. On some vehicles, the module will not start working again until a special relearn procedure or reprogramme procedure is performed with a factory scan tool. That means no A/C until the module is programmed with the correct instructions.
It resets the Body Control Module (BCM). Like the climate control module, the BCM may not resume normal operation until it has undergone a special relearn procedure or is reprogrammed with a factory scan tool. This can negatively affect the operation of the power accessories such as power windows, memory seats, power sunroof, or electronic suspension settings.
Worse yet, the BCM is the “gate keeper” module on many 2003 and newer vehicles that have a CAN (Controller Area Network) system. If the BCM cannot communicate properly with all of the other modules, or it does not recognise the addresses of other modules, it can cause all kinds of problems.
It may reset or disable the anti-theft system. The engine may crank but not start because the anti-theft system thinks somebody is trying to steal the vehicle. Again, it may require a special relearn procedure or reprogramming the anti-theft system with a factory scan tool to resolve the problem.
It causes a loss of channel settings on an electronic radio and clock. This is more of an annoyance than a problem, and can be fixed by resetting the radio channels and the time.
Here is a short list of some of the problems that can occur when disconnecting or replacing the battery on the following vehicle applications (refer to the OEM service literature for specific model and year applications and cautions):
Loss of voltage to the vehicle electrical system causes the 4WD module to go to sleep permanently. The module never wakes up when power is restored, and the only way to restore normal 4WD operation is to replace the module with a new one (a repair that may cost you over N36,000).
Mercedes (various models)
Loss of voltage to the vehicle electrical system will prevent the A/C from working. The climate control module must be reset to restore normal operation. It may also disable the Stability Control System. The ABS module has to undergo a relearn procedure for the steering angle sensor to restore normal operation.
Toyota (various models)
If the battery is disconnected while the ignition is on, it can set a fault code for the airbag system and turn on the airbag warning light (which deactivates the airbag system until the fault is cleared with a scan tool).
Subaru (various models)
Disconnecting the battery can trigger the anti-theft system, preventing the vehicle from starting when the battery is reconnected.
The body control module may forget the odometer display reading. Nice if you are selling a car and want a zero-mileage reading, but expensive to fix because it requires replacing the BCM (at a cost of N180,000 to N240,000) and reprogramming the odometer reading.
Honda (various models)
Disconnecting the battery will set a code and turn on the airbag light (which also disables the airbag system). The dealer must reset the system with a scan tool to restore normal operation.
BMW, Audi & VW (various models)
Disconnecting the battery requires numerous module relearn procedures, which can take up to several hours with a factory scan tool.
Warning: Regardless of the year, make or model of vehicle, never disconnect the battery while the engine is running or the ignition is on. Doing so can create a high-voltage spike in the electrical system that may damage electronic modules and/or the charging system.