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 hazards 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 realize that disconnecting or replacing a battery may cause a problem until after they have done so.

What kind of problems am I talking about? Loss of memory is one. When you disconnect the battery on any vehicle that has computerized engine controls (which is virtually every car and truck that has been built since 1981), the loss of voltage to the Powertrain Control Module (PCM) causes it to forget is adaptive memory settings in its Keep Alive Memory (KAM) chip. Adaptive memory contains the adjustments the PCM has learned over time for 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, 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 turning 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 keep alive memory 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 of temporarily and return 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 late model (2003 and newer) vehicles can do more than erase the Keep Alive memory. It can also erase vital learned information that is absolutely necessary for other modules to function normally.

Also Note: Disconnecting the battery will NOT allow a vehicle to pass an OBD II plug-in emissions test. It may temporarily turn off the Check Engine light, but this won't fool the computer at the test lane. When the test computer is plugged into the vehicle diagnostic connector, it will check the PCM to see if all of the OBD II self-diagnostic monitors have run. If all of the monitors have not completed, the vehicle will be rejected at the test lane. You will then have to continue driving your vehicle until all of the self-checks have completed (which may take up to several days). If the Check Engine light does not come back on after all of the self-checks have completed, the vehicle will pass the test. But if there is a problem, the Check Engine light will come on again and the vehicle will fail the test.

So what happens when the battery is disconnected? It depends on the year, make and model of the 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 model Ford vehicles. 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 SIR (Supplemental Inflation Restraint) 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 reprogram 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 mess up the operation of 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 recognize 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.

SPECIFIC VEHICLE PROBLEMS

• Chevy Tahoe Loss of voltage to the vehicle electrical system causes the 4WD module to go to sleep permanently. The module never wakes back 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 a hundred dollars!).
• 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 key 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.
• Saturn L-Series The body control module may forget the odometer display reading. Nice if you are selling a car and want a 0 mileage reading, but expensive to fix because it requires replacing the BCM (at a cost of $300 to $400) and reprogramming the odometer reading.
• Honda (various models) Disconnecting the battery will set a code and turn on the air bag 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 key is on. Doing so can create a high voltage spike in the electrical system that may damage electronic modules and/or the charging system.

HOW TO SAFELY DISCONNECT OR REPLACE A BATTERY

If a battery has to be disconnected prior to doing electrical work on a vehicle (which is highly recommended to prevent an accidental short that might damage wiring or modules), or if an old or weak battery is being replaced, voltage MUST be maintained to the PCM and other modules while the battery is disconnected.

Voltage to the PCM and other modules can be provided by connecting a 12v backup power supply to the battery cables, using jumpers to attach another battery to the battery cables, or plugging a small 9 volt battery Keep Alive Memory (KAM) saver into the cigarette lighter or a 12v power receptacle. NOTE: If using a plug-in battery memory saver, make sure the cigarette lighter or a power receptacle is always on and has power when the ignition key is off. A plug-in battery memory saver can be purchased at most auto parts stores, and usually sells for less than $20.

If using a plug-in battery saver, use a new 9v alkaline battery.
These devices can keep memory alive while a battery is disconnected, but not for long
(maybe only 15 to 30 minutes depending on the current drain on the 9v battery).
Why not use a battery charger to keep the memory alive? Some professional grade battery chargers have a built-in battery backup mode, and can be used for this purpose. But an ordinary home battery charger is designed to charge batteries, and not to serve as a battery backup. If the battery is disconnected, it may attempt to send too much charging current through the battery cables and damage a module. A low amperage trickle charger (1 to 3 amps) is less apt to overload the circuit than say a 10 or 20 amp charger, but it's still a risk. So unless the charger has a battery backup mode, or the charger manufacturer says it is okay to use their charger for this purpose, do not use an ordinary battery charger as a battery backup when disconnecting a car battery.

KEY-OFF CURRENT DRAIN

If replacing a battery (new or used), make sure the battery is FULLY CHARGED before installing it. This will assure proper voltage for all of the modules when you start the vehicle, and will reduce the load on the charging system. A low battery can affect the operation of the onboard electronics, including the PCM, anti-theft system, ignition system, fuel injectors and fuel pump.

Connect the POSITIVE (+) battery cable first (which may be color coded RED), then the NEGATIVE (-) GROUND cable last (which may be color coded BLACK). Make sure BOTH cable connections are clean and tight. Clean the battery posts and cable terminals if they are corroded. Replace the battery cables if they are damaged or do not fit the battery posts tightly (and remember to use a battery backup power supply while doing this!).