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will car run bad after battery disconnect or replaced

 
Old 12-31-2001, 06:50 PM
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onthedge928s4
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Post will car run bad after battery disconnect or replaced

I was told buy a tech that in my 88 928 if the battery goes low or has to be charged
the engine will run rough or idle bad till it relearns itself.I beg to differ if the car has to re learn it should run good rather
than bad unless it has To compensate for a engine condition .to clearify I don`t have a draw a battery concern it`s more about should the car run bad because it loses stored values,without changing a part that would change inputs to the brain.
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Old 12-31-2001, 08:46 PM
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John Struthers
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Eh, Piasan,
This is strictly, anecdotal.
1. I am lazy.
2. Once I do get to tinkering on the sHARk one thing leads to another and I get side-tracked to associated problems.
3. Electrical problems including battery drain issues are time consuming and consequently, expensive to fix - those freakin' manhours!!! - because a 'wrench' instead of 'spark chaser' is working on the problem it is seldom flat rated. $$$$$$$$$
4. Though there have been numerous posts with great curative info. from a variety of sources which include simple tests and tools, I have yet to solve MY battery drain problem. Reasons: See #1, #2, and part of #3.
I have experienced similar problems; rough idle, sometimes stalling, sluggish off the line... And for what its' worth when my battery - only 6 months old - has been charged to capacity I have ZERO problems under the hood and my '82 is a performer to be contended with. However, the remapping in my case - if that is what it is - seldom takes more than 15 minutes and, honestly, seldom more tham a few minutes rompin' thru the neighborhood and on to the freeway.
I'm sure one of our Guru's and keepers of 928 wisdom will post a better explanation... failing that start glomming through the archives.
Bayonne? Jesus!!! My wife is still haunted by our trip to the OUTPORT on our way to Germany. Even the cops and mailmen couldn't help. We got split up at a light by a small underpass and she got stuck in a Twighlight Zone Loop to Manhatten and back - 8 times - I finally got the toll booth lady to hail a cab and he, eventually, led her back to Bayonne. The tollbooth lady and my wife were best friends by the end of that day.
HTH/LOL
John S. 82' Weissach, Auto. 'Pattycakes'
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Old 12-31-2001, 09:37 PM
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Jerry 87 928S4
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My battery was going bad for about 4 weeks and the car right absolutely perfect. Then one morning it didnt start so I guess I have to disagree with your mechanic.

However, when I was adjusting the mass flow to reduce the emissions I disconnected the battery for about 15 minutes so it would have to learn the new settings.

Hope this helps.
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Old 01-01-2002, 09:52 PM
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Steve Cattaneo
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As we all know, our cars have PCM’s {power control modules .LH EZK) which are computers. Any computer needs memory to function properly. There are several kinds of memory. PROM (program read only memory) and ROM (read only memory). They hold (library) the basic set of instructions (map) on how the input from sensors (O2, RPM, MAF) will be handled, and also vehicle specified data (weight, axle ratio, engine type, etc.

Maps are programs stored in the PROM and ROM. Maps are a set of instructions for different driving conditions. An example: if you push the gas pedal to the floor the PCM knows the car is under a load by inputs from the RPM, MAF, and TPS sensors, for these set of conditions the PCM will advance the cars timing.

RAM (random access memory) and KAM (keep alive memory) is where temporary information from sensors is stored, updated and rearranged. It’s a journal of driving and operating conditions that have been recorded. Fault codes are also stored in RAM and KAM. Even though the PCM has a map, it also has the ability to relearn new strategies. It learns from systems or sensor failures. An example would be the 02 sensor, having a lazy reading due to age. The PCM “looks up tables” (map) to find the adaptive setting for fuel injector pulse and timing. The PCM can’t compensate for a total electronic or mechanical failure. This information is stored in the KAM as fault codes. If the battery were disconnected in this example the adaptive strategies would be lost on startup, which would cause a rich or lean drivability problem until the PCM reads the O2 sensor is out of range and adapts to its new setting.

928 S4’s have adaptive memory strategy for idle speed and CO level. Porsches exact words are “Due to the storage of carried-out curve displacement in the memory, the engine will have to be run about 10 minutes at first after an interruption of the permanent pulse to the control unit, e.g. caused by disconnecting the battery or pulling off the 35-pin plug on the injection control unit, which cancels the displacement, in order to permit correct adaptation to the nominal values.”

If any electronic sensors on the motor, which would set a code, were changed the keep alive adaptive memory would have to be erased for the PCM to return to its factory map setting. If you don’t have a scan tool that can clear codes, you should disconnect the battery and touch the disconnected negative cable end to the batteries positive disconnected cable end. This will discharge any capacitors in the system and remove the adaptive information stored in the KAM.

In conclusion, disconnecting the battery on any computer control vehicle will have an adverse effect on the vehicles performance until it has relearned its adaptive strategy or map values.

Steve C
The great white
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Old 01-01-2002, 10:07 PM
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Dave H.
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it's also helpful to remember that this "adaptive" brain isn't exclusive to porsches or 928's, any car that used a Bosch brain will do the same thing.

every time i do a tune up on my jetta, the engine runs rough until it adapts to the new cap, rotor, plugs etc. since it adapts to these parts as they wear out...
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Old 01-01-2002, 10:53 PM
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Jay Wellwood
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Bob-

My experience has been that if I disconnect the battery - the next start if the car WILL produce some weird effects as the 'LH' brain figures things out. In fact, it will be sluggish/almost want to die until at least 1o minutes have passed (btw - this is an excellent time to check for free engine movement = motor mounts).

As I understand it - this is normal 'behavoir' for the Sharks.

hth-
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Old 01-01-2002, 11:20 PM
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John Struthers
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Bob,
What they said!
Don't know if it helped or nullified your arguement. But the archives follow the thread that Steve, Dave, Jay AND Jerry set forth.
On the up side, in all likelyhood, you have more knowledge about this problem than your wrench does.
Aren't these cars a trip?
There were some smart puppies in the Zuffenhausen engineering group once upon a time.
Later, keep learnin' -I am- and keep em' rolling.
John S. & Pattycakes
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Old 01-01-2002, 11:28 PM
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Chris Lockhart
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Steve, that reply is a keeper!!!!!!!! Probably the best explanation of the system that I've heard from anyone in the 2 years I've owned my GT. Thanks for taking the time to post that. Cheers,
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Old 01-03-2002, 06:34 PM
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onthedge928s4
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Cool

to all that replied!

NICELY DONE, YOU ARE TRULY A VERY GOOD BUNCH OF PEOPLE.NOT TO MENTION A FEW THAT ARE TECHNICALLY WERE A PLEASURE TO HEAR FROM.
THANKS

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Old 01-03-2002, 11:01 PM
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Steve Cattaneo
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Chris and Bob,

Thank you, I'm just sharing a little knowledge.

Steve C,
The Great White

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