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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: SERVICE ENGINE SOON light
Date: Sat, 24 Jun 2006 20:29:53 -0400
Message-ID: <>

On 24 Jun 2006 10:45:32 -0700, "Rick Onanian"
<> wrote:

>Stan Birch wrote:
>> On Thu, 22 Jun 2006 06:28:08 -0400, "Eisboch" <> wrote:
>> >b). to remember to tighten the gas cap after fueling.
>> OBD I used on '99 rigs doesn't monitor that kind of stuff.
>I thought OBD II had entirely replaced OBD I by something like 1996. I
>had a 1997 Pontiac with OBD II.

95 for most models, 94 for a few.

>To address another point in this thread: Disconnecting the battery for
>long enough to change it will NOT clear most ECUs and cause the vehicle
>to run bad.
>Also, even if you DO clear all the training in the ECU, the default
>programming will result in a smoothly running vehicle; the training is
>just a matter of the vehicle adapting to the driver (if it has that
>feature) and it's own aging parts.

This is completely wrong.  There are a couple of levels of learning
for mixture control.  The Long Term Trim is the most important.  This
table trims the mixture back to stoich based on oxygen sensor data
averaged over a long period of time.  This is the trim that adjusts
for the changes in the engine as it wears.

Even the long term trim is fairly short term so there is another
table, the name of which is not standardized and is not accessible
over the OBD-2 interface, that stores the LTT table, averaged over a
much longer time period.  BTW, time for these purposes is measured
internally in terms of engine starts and not clock time.

All of these tables are volatile and are erased when the PCM is
powered down.  I know authoritatively how this works because I have
the source code for the PCM firmware for several popular GM PCMs and
because I've done a whole bunch of work on a few GM PCMs.  I can't
comment on Ford or Chrysler systems because I've not seen any sources.

>I've run the battery dead (won't
>even light the dome light a little) and left it that way for days
>before realizing it on my 2002 GMC, and it runs the same immediately
>after I jump start it. Same on the 1997 Pontiac.

All your anecdotal experience means is that your cars' engines are
still in good enough condition that the original tables still work
well enough.  And the fact that GM PCMs from as far back as 94 look at
the instantaneous engine speed and make immediate corrections to
correct for gross misfire.  This is done very quickly because gross
misfire can rapidly burn out the catalyst.  I hesitate to say that
this is universal but it worked that way in every engine I've had the
occasion to look at in detail.

>I've never heard of mechanics keeping the system live with an external
>power source while changing a battery, and although it sounds great to
>preserve the radio presets,

I can't recall talking to a mechanic, as opposed to a parts changer,
who DIDN'T know to keep the system live.  Certainly any mechanic with
factory training does.  I would expect any ASE certified mechanic to
know the same.

Late model cars have a LOT more stuff in memory than just PCM tables
and radio presets.  My mom's Lincoln LS, for example, has those plus
the seat position presets, the built-in garage door opener
programming, the multi-CD changer playlists and the learned
transmission shift programs (this is the part that adapts to the
driver's habits.)  It sure is a lot simpler to send keep-alive power
through the cig lighter than have to go back and reprogram all those

>I'd guess it's an OSHA violation.

WTF does that mean?


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: SERVICE ENGINE SOON light
Date: Thu, 22 Jun 2006 20:09:52 -0400
Message-ID: <>

On Thu, 22 Jun 2006 15:58:06 GMT, "Nate"
<> wrote:

>>> Disconnecting the battery cable also resets the auto set stations on the
>>> radio, the clock, the temp controls.  Easier and better to just pull the
>>> ECM fuse from the fuse panel.  Or go to Autozone, have the code read, and
>>> then have them reset the computer properly with their scan tool.
>>> Nate
>> Who sets the computer properly when you replace the battery?
>> LZ
>I don't often replace the battery as a result of the MIL indicating a
>malfunction code.  What the hell are you talking about?

Mechanics possessing a clue (which excludes LZ) have a double-ended
male jumper cord to power up the computerized items during a battery
change..  One end goes into the cig lighter plug on the car and the
other end goes into the similar plug on the portable booster pack.
Said cord is attached while changing out the battery to preserve all
the programmed settings as well as the learned tables in the PCM.

Especially if the vehicle is high mileage so that the PCM has had to
adapt to wear, resetting the learned tables can result in quite poor
running for some time.  Not a great disaster but if one is working on
other peoples' cars, one doesn't want to return the car to the
customer idling rough or surging, the only alternative being to drive
the car around until it tunes itself which could take hours.
Especially since the alternative - the booster pack and cord - is so


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