How To Disconnect A Car Battery To Reset The Computer

ways to disconnect automobile battery

There are many reasons why people might want to remove the batteries of their cars. It could be that the batteries need replacement, or the terminals need cleaning. In any case, one of the most common reasons is that they want to reset the Engine Control Module — or ECM — which is on-board computer that controls most of its functions. This article contains instructions on how to disconnect a car battery to reset the computer.

Some general tips and guidelines on the do’s and don’ts of the procedure will also be discussed. There will also be a part explaining the potential risks of undertaking such a procedure by one’s self (instead of for example, by an experienced technician).

The General Approach To Disconnecting An Auto Battery

First, we will discuss how to remove the battery for general purposes. From there, we will present how to reset the car’s computer once the battery is disconnected. Every vehicle is unique, and it only follows that every make and model has a different setup, at least where battery connections are concerned. However, there are standard steps which are common and will apply to most automobiles.

1. Take Safety Precautions And Prepare Your Tools

Remember that while they may look harmless, car batteries still produce electricity and they pose a risk of electrocution when mishandled. Before executing to the following steps, one must sure that the car’s ignition is off.

You should also wear some protective gear for your safety. Goggles are a must for the eyes, as well as gloves for the hands. Aside from electricity, batteries have corrosive material that may leak when mishandled. Getting protection for your body will go a long way to ensure that you can proceed without damage to your skin.

As for the tools, the best to use would be a collection of sockets and wrenches of different sizes. You will never know what size is required exactly because, as mentioned earlier, different cars have different ways of component assembly. Unless you had done the research and know exactly what sizes of wrenches and sockets to use, a wide collection of this in a toolkit will come handy.

2. Identify Positive And Negative Terminals

Now, when you open the car’s hood, you can probably already see the top of the battery. Like a common “AA” size battery used for flashlights and other devices, a car’s battery also has positive and negative terminals. Identifying which terminal is positive (“+”) and which is negative (“-“) is important.

Usually, this is as simple as looking for the identifying marks on the terminal in question. A negative terminal will have the minus sign (“-“) as a label, and the positive will in turn have a plus sign (“+”).

In the rare cases where the manufacturer of the battery did not make clear which terminal is which, you can look for another sign. For example, there may be caps to protect the terminals, and the black cap usually is the negative one. The positive terminal almost always has a red cap.

3. Removing The Caps And Cables

Once you have determined which of the terminal is positive and which is negative, the real work begins. The instructions presented on this step apply for both terminals, but it is important to keep in mind that you should work on the negative terminal first.

With that said, inspect the negative terminal. You will most likely see that a nut holds the connection to a cable, which is then connected to the car. With your toolkit, find and approximate a socket size that is exactly or almost the same as the size of the nut. You can do this by taking a socket and placing it beside the nut to compare their sizes.

Once you are sure that you have the socket you need, attach it to your tool’s handle or a wrench extender, if you have one. Now, it is all a matter of fitting the socket into the nut and turning it counter-clockwise to loosen it (remember that going left is always to loosen, and right is to fasten tight).

With the nut on the negative terminal loosened, you will see that the cable will be disconnected. Take care that this cable will not touch the battery or any other metal part on the car, as residual current may damage circuits and thus other car components, as well (this applies for when you get to work on the positive terminal, too).

After work is done with the negative terminal, proceed to work with the positive terminal, being careful that the cables do not touch other metal parts of the car.

Sometimes, there are models in which the cables are seized to the battery post. In cases like these you might need special cable removal tools to remove the seized wirings. These tools are usually sold in mechanics stores or auto repair shops.

4. Proceed With The Required Work / Maintenance / Cleanup

With the battery disconnected, you can proceed to the work you need to be doing. You could perform additional maintenance work by using a baking soda solution and a wire brush to clean the terminals from residual dust or other dirt that may affect the connections.

For the solution, 1 tablespoon of baking soda dissolved in 1 cup of water should suffice. Also, the wire brush type is recommended over a reamer type brush. This is because reamers pose the risk of removing too much material on the terminals, and may thus cause loose connections.

If the battery components have been left without maintenance for too long, you may see some degree of corrosion developing on both the terminals and the ends of the cables themselves. In cases like these, some type of spray-on battery cleaner may be required to clean the corrosion off to optimize the connections.

Actual Resetting Of The Car’s On-Board Computer

Now that the car’s battery is out, we can proceed with actually trying to reset the car’s on-board computer module, or ECM. There are a lot of reasons why car owners might want to do this. It could be that they want to get rid of the diagnostic codes and the engine light warning that persists when the car is in operation.

Granted, it is not a good idea to forcibly remove these warning codes by resetting the computer. This is because the codes present a legitimate problem that needs to be checked, and it would be unwise to ignore the warnings by resetting the car’s ECM.

However, there are times when one already has repaired and checked the car’s systems, but the warnings still persists. In these cases, it may be necessary to manually reset the computer.

The actual purpose of the procedure is to momentarily ground out the power system, so that the computer will reset to its factory setting, of course with the active diagnostic codes removed. The steps may sound counter-intuitive, but it is one surefire way of resetting the computer.

First, as described above, you can proceed with removing the battery and the general maintenance and cleanup of the components. After this, you should take the positive and negative cables and connect them together.

Yes, bypass the battery altogether and connect the two wires. This is the “grounding out” procedure mentioned above. You can use electrical tape or clamps to keep the cables together, but it is important that they should not touch the battery terminals while they are connected.

After about five minutes of being connected like this, the computer will be rest and it will “forget” all the diagnostic codes that were activated. The next time the car runs, the codes will not pester the owner anymore.

Again, it is worth emphasizing that ignoring diagnostic codes is not a good idea. Be sure that you have performed all the unnecessary checks and maintenance, and only use manual computer rest procedures as a last resort.

Potential Risks

Given that this procedure makes the computer “forget” about ECM system details, it follows that it may also lose some settings that may have already been embedded into its memory.

For example, the ECM may forget the “released” position of the accelerator and the optimal transmission progressions. Most of the time this is an easy fix; one only needs to drive for a while so that the ECM will again be accustomed to the specific driving patterns it already learned.

However, in some cases, like on a 2002 Nissan Altima, the owner may have to go through manual procedures to “teach” the ECM again.

For example, in the aforementioned Altima, the way to teach the EXM the “released” position of the accelerator is by turning on the ignition, wait two seconds, turn the ignition off, wait ten seconds, and then repeat. This is done twice, with the accelerator pedal n the fully-released position.

(Watch if you are the 2002 Nissan Altima owner).


Of course, it still is an option for the owner to take the car to the mechanic. However, these directions on ways to disconnect a car battery to reset the computer may help when the owner feels the need to do some basic troubleshooting that may not require a mechanic’s assistance.


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