Rick's Free Auto Repair Advice

Replace a car battery

Follow this NEW procedure before you replace a car battery

You may have replaced a car battery in an older vehicle a dozen times before, but if the battery is dead on a late model car and you use the old procedure, you’ll be in for a BIG surprise. Late model cars and multiple computers and specialty modules that can “forget” their calibration settings is you simply disconnect the old battery and slap in a new one. If you’re going to replace a car battery on a late model car, you MUST follow the new procedure.

Disconnecting the old battery without backup power causes these problems:

• The PCM/ECM/ECU loses adaptive “keep alive” memory

These are all the adjustments the computer has made to keep your engine and transmission operating smoothly as it ages. When you disconnect the battery without providing a backup, the computer defaults to factory setting and the learning has to start all over again. Your engine may run terrible and the transmission may not shift properly until it relearns everything. That can take as long as 10 cold starts.

• Resets readiness monitors

If you have smog/emissions testing in your area, a “readiness monitors not ready” situation will be an automatic fail.

• Electronic throttle body loses its “home” position

The relearn process on some vehicles is complicated and some even require a factory or high end scan tool to force a relearn. See the electronic throttle body section below

• Ford Failure Mode Effect Management (FMEM) resets

The FMEM stores an adaptive “fail-safe” strategy that substitutes known good values for sensor data that’s missing.

• ABS module resets to factory default

On some makes and models you need  high end scan tool to reset

• Climate Control loses memory

Most climate control systems vary air temperature by opening and closes baffles and doors to regulate how much air passes over the heater core or evaporator coil. Those doors are operated by electric motors. When the HVAC controller loses power, it forgets what position the doors were in. Many late model vehicles require a scan tool re-calibration to return the motors to their home position.

• Body Control Module (BCM) may not work properly

Depending on the condition of the battery before you disconnect it, the BCM may scramble its memory and not operate properly. That will affect lighting, door locks, moon roof, power mirrors and windows.

• Anti-theft reset

Some anti-theft systems require a relearn if they lose power.

• Power accessory memory loss

If you’ve programmed memory seat positions or auto up/down features on your windows, those may be gone if you disconnect power.

• AGM start/stop reset

If you install a new battery on a vehicle equipped with start/stop, EVEN IF YOU DON”T USE THAT FEATURE and you don’t reprogram the computer to tell it you have a new battery installed, you will shorten the life of the new battery.

Keep the computers powered when you replace a car battery

If you don’t own a $3,000 factory scan tool and don’t want to have your car towed to the shop after you replace your car battery, you’d better be prepared to keep all the computers powered up during the swap. Here’s why. All the modules have an initialization point. When you disconnect power, they forget that point and that can cause a no-start, rough idle, sluggish acceleration, traction control or stability control malfunction, as well as loss of settings for your memory power seat, radio, mirrors and anti-pinch power windows.

Throttle body relearn is the biggest issue

Most late model engines use an electronic throttle body. A small motor controls the

electronic throttle body

The gas pedal sends electronic signals to the PCM which then tells the throttle body how far to open

throttle plate opening and closing. In order to open and close properly, the computer must know when the throttle plate is fully closed. Some car makers have a self learning throttle body re-learn procedure. In those vehicles, all you have to do is turn the key to ON and leave it there for 30-secs. without starting the engine. But other car makers have a complicated Hokey-Pokey procedure that requires multiple brake pedal applications, along with several key on/off cycles. Still others require a factory scan tool relearn.

Stability control relearn is also a big problem

Stability control systems compare inputs from the steering angle sensor to the readings from the yaw sensor to figure out if the vehicle is in a skid situation. If you disconnect the battery without providing backup power first, whichever position the wheels are in when you disconnected the power will be the new “straight-ahead” when you connect the new battery. As soon as you drive the car, the computer will detect a steering angle sensor problem and set the check engine light. How do you reset the steering angle? Yup, an expensive scan tool.

How to provide backup power when you replace a car battery

You’ll need three things: a jumper pack, a special cable, and electrician’s tape.

jumper pack

Get this ES5000 1,500 amp peak power jumper pack for about $125

Solar ESA30 OBDII Memory Saver Cable

Solar ESA30 OBDII Memory Saver Cable

Here’s the procedure:

1) Connect the D-shaped end of the cable to the OBDII port on your car.

2) With the engine off and keys out of the ignition cylinder, plug the opposite end into the jumper pack and turn it on.

3) Disconnect the negative battery terminal.

4) Disconnect the positive battery terminal and immediately wrap the entire terminal end with several layers of electrician’s tape. This terminal is powered and if it touches any metal component, it will blow out the fuse in the jumper pack and depower your car’s computer and modules. For extra safety, shove a disposal plastic cup over the end of the positive terminal and tape it to the cable.

5) Remove the old battery and clean the battery tray and negative terminal

6) Clean the posts on the new battery.

7) Replace the car battery with a new one

8) Remove the tape, clean the positive terminal and install on the positive post

9) Install the negative battery terminal.

©, 2016 Rick Muscoplat



Posted on by Rick Muscoplat


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