How to reset the ECM/ECU - JeepForum.com
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post #1 of 14 Old 09-20-2019, 03:09 PM Thread Starter
Alientails
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Wj How to reset the ECM/ECU

I have been told that after replacing engine and electrical components (coil rail, plugs, IAC, MAP sensor, etc.) that I should "reset" the computer. Some videos show touching the (removed!) battery cables together; another says to remove the ECM fuse, turn the key on, then off, then drive it... a lot of generic stuff. I want to know how I can do this specifically for my 2002 WJ, 4.0L. Or, if it even really needs to be done!


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post #2 of 14 Old 09-20-2019, 03:48 PM
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If there were no codes for the components you should not need to - idle may be rough for a few minutes...

Why did you change those components?
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post #3 of 14 Old 09-20-2019, 08:46 PM
tebeve
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This is the procedure I use, hope you find it as helpful as I do.
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post #4 of 14 Old 09-21-2019, 01:51 AM
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I use the procedure that tebeve linked.
Be the first to tell you I don't get it though.
Turn the headlights on and off?
But you know what?
I do it anyway. LOL.
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post #5 of 14 Old 09-21-2019, 09:50 AM Thread Starter
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I will perform this procedure in the morning. To answer the question from jtec, I had a scan done at a local garage, and they replaced the pigtail on the #6 injector; they also recommended replacing the coil pack, and at least two plugs. But they wanted $9.00 each for the plugs, and $175 for the coil pack. I'm on Social Security. So I bought the coil and plugs on Amazon, and with a friend's assistance, did the work ourselves. I also replaced the IAC. I have yet to remove the throttle body and clean it or the MAP sensor. Still runs rough, except at speed. Occasional engine shakes, requiring stop and shut off engine; runs fine on restart.

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post #6 of 14 Old 09-21-2019, 12:47 PM
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Just FYI, there isn't a service procedure for the WJ known as "reset the PCM". You can search the factory service manual from end to end and you won't find one It's just a phrase that the DIY community has come up with for some reason.

I guess by "reset" they mean make the PCM wipe its RAM, and you do that by disconnecting the power. Just remove the negative battery cable for a second (that's all it needs) and you're done. The PCM memory has no battery backup, so the instant it loses power it loses all its learned parameters and is back to its default factory programming.

All that stuff about touching battery cables together or leaving the battery disconnected for 20 minutes, or anything else - it's all a waste of time, it does nothing. I know a lot of people insist it does actually do something because it's what their dad or their grandpappy taught them, and it might do stuff on other vehicles, but on the WJ, it does nothing more than momentarily disconnecting the negative battery cable does.
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post #7 of 14 Old 09-22-2019, 10:19 AM Thread Starter
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Quote:
Originally Posted by AVR2 View Post
All that stuff about touching battery cables together or leaving the battery disconnected for 20 minutes, or anything else - it's all a waste of time, it does nothing. I know a lot of people insist it does actually do something because it's what their dad or their grandpappy taught them, and it might do stuff on other vehicles, but on the WJ, it does nothing more than momentarily disconnecting the negative battery cable does.
I will try that first, since it is much simpler. I could always do the longer procedure later.

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post #8 of 14 Old 09-22-2019, 03:47 PM
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Quote:
Originally Posted by AVR2 View Post
Just FYI, there isn't a service procedure for the WJ known as "reset the PCM". You can search the factory service manual from end to end and you won't find one It's just a phrase that the DIY community has come up with for some reason.

I guess by "reset" they mean make the PCM wipe its RAM, and you do that by disconnecting the power. Just remove the negative battery cable for a second (that's all it needs) and you're done. The PCM memory has no battery backup, so the instant it loses power it loses all its learned parameters and is back to its default factory programming.

All that stuff about touching battery cables together or leaving the battery disconnected for 20 minutes, or anything else - it's all a waste of time, it does nothing. I know a lot of people insist it does actually do something because it's what their dad or their grandpappy taught them, and it might do stuff on other vehicles, but on the WJ, it does nothing more than momentarily disconnecting the negative battery cable does.
I'm not doubting this at all. I just always remove my negative battery connection and go find something else to do for ten minutes......I think, for me, it's more superstition than anything..I don't do all the cable end touching and voodoo crap, either.

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post #9 of 14 Old 09-23-2019, 03:15 AM
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Originally Posted by Mattyjm View Post
I think, for me, it's more superstition than anything..
For sure. As said, the PCM's memory is wiped the instant it loses battery power (but you definitely need to wait at least two minutes for the airbag capacitor to discharge).

I do sometimes wonder where all the "voodoo" about touching the cables together or removing a fuse and starting the engine, etc, came from. Other vehicles from other manufacturers, perhaps? Whatever, some people absolutely swear by it and insist that it does actually do something over and above simply disconnecting the negative battery cable for one second... I guess you just have to let them get on with it
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post #10 of 14 Old 09-23-2019, 09:28 AM Thread Starter
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Originally Posted by AVR2 View Post
As said, the PCM's memory is wiped the instant it loses battery power (but you definitely need to wait at least two minutes for the airbag capacitor to discharge).
That reminds me, since the problems with the rough-running began, the Airbag warning light comes on periodically. Not always. Odd thing...

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post #11 of 14 Old 09-24-2019, 06:29 PM
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Maybe that waiting period came from the computer world. Some years ago I worked in corporate IT, PC and LAN support, and I did a lot of building and repairing computers and servers.
Often just unplugging the computer for a second wouldn't actually clear the RAM completely because there was still a trace of residual charge in the motherboard or other components.
If you reconnected the power too soon, some of the memory registers could remain marked in use; they wouldn't really be holding useful data but the system wouldn't use them.
Or sometimes there was corrupted data there and the system would try to use it which resulted in system errors.


Anyhow, we used to unplug the computer, remove the back-up battery for the clock, count to five and then reinsert the battery, plug in and restart.


It could be that something like this got translated over to cars because "it's a computer" or something.



So, disconnect the ground, count to five and reattach - that should be adequate to clear the PCM or whatever memory is at issue.

"Let all the laws be clear, uniform and precise:
To interpret laws is almost always to corrupt them."

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post #12 of 14 Old 09-24-2019, 08:39 PM
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We used to use terms such as a "warm restart", which was just restarting the operating system to give it a new look at recent changes. We also used to use a term "cold restart", or "hard restart", meaning you let the PC power down and reboot to adjust to major changes or upgrades.

Our WJ PCMs aren't $3500 units from the factory at time of manufacture. They're vehicle PCMs, at best....We can get into an open and closed loop discussion real quick on here, and maybe we will. Resetting your vehicle's PCM before AND after ANY major changes could bring you a variance of results, and a variance of different codes.

I used to HATE defragging my desktop computers. Even more, I hated installing a new version of Windows on them. BUT, I used to love the results of an old machine with a brain and facelift. It was like Christmas morning all over again....Same old machine with a new way of thinking and operating.

Your PCM works, thinks, and keeps the WJ running to what is supposed to be the manufacturer's expectations, whether they knew the WJ would last this long, or not. I'm telling you, and all the doubters, that SOMEONE designed and built these vehicles to last longer than the "throw-away" mentality that we witness today.

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post #13 of 14 Old 09-25-2019, 10:58 AM Thread Starter
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Originally Posted by Mattyjm View Post
SOMEONE designed and built these vehicles to last longer than the "throw-away" mentality that we witness today.
Amen to that! My WJ has 304,000 miles on it, and in spite of that returning P0172 code, and a pesky airbag light, it runs well enough and may last another 30,000 miles. I can't afford the clock spring to fix the airbag light, but fix what I can with my limited Social Security income. Cars today, especially, are highly stressed machines with turbo boost, variable valve timing, direct injection, and so on, all to squeeze out a bit more power while sipping less gas. I watch YouTube videos from mechanics like Scotty Kilmer, who has over 50 years experience. Valuable info. But those stressed little engines, coupled with their fragile CVT's, are not likely to last. For that matter, I'm worried that Jeep will ultimately fail with FIAT in the manager's chair. Just look at this year's color offerings at a Jeep dealer... the Wranglers in various bright "Tonka Toy" colors. Yikes...

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post #14 of 14 Old 09-25-2019, 11:07 AM
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Quote:
Originally Posted by Mattyjm View Post
We used to use terms such as a "warm restart", which was just restarting the operating system to give it a new look at recent changes. We also used to use a term "cold restart", or "hard restart", meaning you let the PC power down and reboot to adjust to major changes or upgrades.

Our WJ PCMs aren't $3500 units from the factory at time of manufacture. They're vehicle PCMs, at best....We can get into an open and closed loop discussion real quick on here, and maybe we will. Resetting your vehicle's PCM before AND after ANY major changes could bring you a variance of results, and a variance of different codes.

I used to HATE defragging my desktop computers. Even more, I hated installing a new version of Windows on them. BUT, I used to love the results of an old machine with a brain and facelift. It was like Christmas morning all over again....Same old machine with a new way of thinking and operating.

Your PCM works, thinks, and keeps the WJ running to what is supposed to be the manufacturer's expectations, whether they knew the WJ would last this long, or not. I'm telling you, and all the doubters, that SOMEONE designed and built these vehicles to last longer than the "throw-away" mentality that we witness today.

Yes, the "cold boot" was the last resort when some odd problem seemed to be memory related. It worked so often that it became the solution of first resort; we'd shut down (if possible, sometimes the system would lock or have a BSOD), remove all power then reboot.


I've cured a lot of odd problems with cars acting funny by simply disconnecting the ground for a few seconds for a hard reset of that little brain.


On the computer way-back machine, I defragged many a hard disk back in the day; it was amazing how much that speeded up an early Windows machine. Back in the days of the 640k RAM barrier, I had to become an expert on expanded/extended memory and writing effective config.sys and autoexec.bat startup files. Really dating myself, there - I started in the days of DOS 2.0 and remember when Windows 286 came out (I bought a copy because Win286 was the only OS with drivers for the 256-color NEC Multisync monitor).


Ditto on the joys of system upgrades. I still can't believe how much my CJ runs with the MPFI conversion. I inherited my Dad's 1997 Silverado, which has the "Vortex" engine. Turns out that setup was kind of a throttle-body injection system disguised as multiport. Now there's a replacement part that turns it into a true MPFI system and I'm really looking forward to doing the upgrade on the Silverado, too.

"Let all the laws be clear, uniform and precise:
To interpret laws is almost always to corrupt them."

~ Voltaire
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