What does unplugging your battery do to your car's computer?

Discussion in 'SVT Tech Forum' started by Miklowcic, Jun 17, 2004.

  1. I have been told by several mechanics that when the battery is unplugged everything resets and that the computer has to relearn things about the car.

    How much truth is there to this/how much could the computer have to relearn?

    How long does it take before your computer relearns?

    If you have a chip in your computer, how does that effect things after the battery has been unplugged?

    I have heard that some 03-04 cobras actually dynoed higher with a K+N air filter after they had been driven for some time compared to what they dynoed right after getting the filter installed...the reason given was that the computer had to relearn or adapt to the new air flow....


  2. All of the info in your post is true man! The ECM in our Cobra's has sort of an adaptive memory. As you drive normally the ECM is constantly checking different sensors and adjusting the A/F and spark advance. For example, it's smart enough to know when you are climbing a mountain to adjust the A/F ratio as the air thins out. It knows to richen up the mixture when the air temps are cold or to lean it out when warm. It constantly saves this data to a table in its memory. Of course when you go WOT the ECM defaults to a set of hard coded tables in the memory (in your case when you go WOT it looks at the data on your aftermarket chip), but when just cruising it's always monitoring, adapting, and learning. When you disconnect the negative battery terminal you remove power from the ECM and that portion of it where the adaptive memory resides is cleared. Just like any info in the RAM memory of a PC. Once the PC is powered off everything in RAM is lost. It normally takes 3 to 5 driving cycles before the ECM can learn all this all over again. So yes, you could install a CAI right there on the dyno, yet not get the results you were looking for immediately until after a few driving cycles. If you have a chip in the ECM it does not affect the adaptive memory from what I'm told, and disconnecting the negative battery terminal to clear the memory will not affect your aftermarket chip.

  3. Yes to all above, but my last 1998 GT used to run a .15 sec quicker quarter right after dumping the memory. The trans had a quirky shift and it was probably just running lean until it re-learned, but it was faster. (I kept resetting mine to turn off the rear O2 monitor "check engine" light, until I went to see my tuner)

    My Cobra idles erratically and hesitates until it re-learns after the battery is disconnected.

    I've heard that a performance tip is to drive a car very hard during the re-learning phase and it'll "learn" to be faster. I don't know about that rumour for sure but I'll try anything for power.
  4. thanks


    My car has been running funny since exhaust install...i remember my GT doing the same thing and then tripping the Engine light for o2 sensors. Got Mils for that. This time around the chip was supposedly reburned to turn off the o2 sensors...

    What is three driving cycles? Is that three starts or is a cycle like 30 starts or something?

    Thought I heard that after like 30 starts the computer has recycled.....thats why it was explained to me anyway...your Engine light doesnt come on right away after changing the exhaust and not doing anything about the o2 sensors...although some people's cars the same exhaust I had didnt trip the sensor....
  5. A driving cycle would be starting the vehicle and taking it for a spin for 5 minutes or so at varrying speeds.

    That 30 start deal you heard about probably had to do with new Cobra's I believe. I've read, and have been told that brand new Cobra's had some sort of temporary boost restriction set in the ECM from the factory that cleared itself after 25 key cycles. This was to prevent vehicle abuse prior to the owner taking delivery.