Why does everyone say remove battery cable for 10-15 minutes to reset computer?

[old mechanic]

My self and others have been removing the ground cable to reset the computer for 1 minute or so and I can't remember having a problem that I had to leave the cable off for longer than that. Is this with the newer vechiles? I know that the 1st gens only have to be removed for about 1 minute to reset the computer.
On most of the work I did I usually disconected the neg cable any way but I'm just curious!

 

[Ak LTD Xplorer]

just to make sure you cut ALL power to the truck

 

[Alec]

Various components in the truck can act as or contain capacitors which can store a little charge for a few minutes. This is enough to keep some of the ECU's memory.

 

[twomedicine]

I believe my 2nd gen X owner's manual states the 10-15min disconnect for resetting, or powering down the air bags when you plan on working around the sensors.

Then there are idle time requirements and also thereafter certain duration driving requirements for relearning after powering it back up (like city and open road)

 

[Charlie's_93EB]

Originally posted by twomedicine

Then there are idle time requirements and also thereafter certain duration driving requirements for relearning after powering it back up (like city and open road)

does anyone know what these times are?

 

[old mechanic]

Originally posted by a1993eddiebauer
does anyone know what these times are?

Not me! But we used to drive the vechile for a few miles after battery disconect to get things going properly. But like i said before that was on the older models.

 

[Alec]

Let it idle for 10 minutes with all accessories off.

Drive it hard for about 15-20 miles.

Drive it hard for the next few days in mixed conditions.

 

[410Fortune]

all the information I have ever come across says the battery needs to be disconected for 1 second to clear the PCM's memory

Everything else said here may be true, so it is just good practice to disco the battery before you begin whatever mod or test you are doing and reconnect when finished.

 

[twomedicine]

Originally posted by a1993eddiebauer
does anyone know what these times are?

Sorry to leave you in the dark. I have been having computer problems due to some sort of an unwanted program put on this thing .... been going round and round, but tomorrow night (Wed) I'll break out the manual and get those times.

I am fairly sure Alec posted the proper proceedure to the conservative side. Following his recipe will properly get you there/

 

[Charlie's_93EB]

i never thought about it before, i would just hook up the battery, start it up and go. have i done any thing bad? mpg?

 

[Alec]

It will just take longer for the computer to properly relearn idle and fuel curves. You probably get a bit worse mpg for a week or so.

 

[springtensi0n]

"drive it hard????"

i remember reading somewhere to let it idle, then take it nice and easy, slow starts and slow stops everytime you reset the ecu. not that i dont trust you alec...

 

[Alec]

Yeah, then the computer will think you always drive like a granny and may not optimize for power.

 

[springtensi0n]

yeah thats what i was thinking. my main concern is gas milage, but thats a good excuse to use on my friends when i lose a race...

 

[twomedicine]

By the book

Here’s what you’ve been waiting for, but it might not be all that clear. The following procedures are from the 1999 Owner’s Manual. Both are from within the Maint. and Care chapter of the book. The first is from the Battery service, or replacement section. The second is a subsection from within the EMISSION CONTROL SYSTEM section.


1)
Pg 216 of the Explorer Owner’s Guide for 1999 model yr says that when the battery has been disconnected or replaced the computer must relearn its idle and fuel trim strategy for optimum driveability and performance.

First set the parking brake

Place auto transmission in Park, or Manual Trans in neutral. Turn off all accessories, and then start the engine.

Let the engine idle for at least 1 minute

The relearning process will automatically complete as you drive the car. It may take 10 miles or more to relearn idle and fuel trim strategies.


2)
Now, on page 231 there’s a different story and it has to do with Inspection/Maintenance (I/M) testing. It’s for passing the on-board diagnostics systems test that some states require prior to licensing. “If the power train or battery has been serviced the system is not ready for I/M testing”. Allow vehicle to sit for at least 8 hours without starting the engine, (it doesn’t say whether or not the battery is disconnected during this period) then complete a driving cycle of 30 minutes of city/highway.

First, at least 10 min of driving on an expressway or highway is necessary.

Next, at least 20 min driving in stop and go traffic having at least 4 idle periods involved.

The engine must warm up and reach normal operating temperature. And once started, do not shut engine off until the above driving cycle is complete.

 

[Leyritz]

Would those requirements to re-set the computer be the same for a 96? I disconnected my battery last week to get rid of the Check Engine light to bring it through inspection (if the light is on it is an automatic failure). The next day I brought it in but I had 5 components reading "not ready". I've since driven it 150 miles, but I brought it in today and I still have 3 codes reading "not ready". I'm allowed to have 2 "not ready" readings, but the third aborts the state inspection computer. The guy doing the inspection ran a manual check on the output and tells me that all of the emission levels are passing, but the damn state computer won't run the automated test because of the third "not ready". Anybody know why this could be taking so long?

The Check Engine light has been on for 4 years and I've used this same tactic to get it through inspection the previous 3, but now I'm hoping there isn't something seriously wrong with the computer, it usually just takes a day or two to fully reset. I know that I just have a small window to get it through after the computer becomes ready but before the light comes back on again.

 

[Rhett]

For a 1st gen, the Manual states the procedure to disconnect the neg. bat cable for 1 minute to reset the PCM. To re-learn, the manual states to let the truck idle for 10 minutes. It could be different for 2nd/3rd gen as they are OBDII/EEC-V. I don't know.