Re: CarChat and cable ...

["Programmer" <nwester@eidnet.org>]

Hey Andrew,
Should my cable be updated ? It's SN #17--maybe to become a
collectors item ?
Lyndon.
-----Original Message-----
From: Andrew K. Mattei <amattei@mindspring.com>
To: gmecm@diy-efi.org <gmecm@diy-efi.org>
Date: August 27, 2000 8:22 PM
Subject: Re: CarChat and cable ...


>> On a '95 Z28 PCM.....
>> I made the 2 trans converter from Sanja's picture, using port power, and
>it
>> almost worked.  I forget now exactly what the symptoms were, but it
didn't
>> work 100%.  The two diodes were added upon Andrew Mattei's suggestion,
and
>> it has worked since.  It has successfully reflashed the pcm a good half
>> dozen times.  Seems too simple to be true, but it works, and it is pretty
>> small cable wart too.  Don't you love cable warts.......
>
>I love cable warts!
>
>This is because a logical zero is 2 volts and below in GM speak, and the
>no-diode single transistor input will only flop to a logical zero at .8V
and
>below. So with the addition of the two diodes, you are essentially moving
>the logical low detect up to about 2 volts. We have seen on 1995+ cars that
>the GM ALDL line is only pulled down to about 1 volt during certain reads,
>and then pops back up to 5V again. This was causing checksum errors with
>flash reading and scan tool programs. We even got a flash corrupted PCM out
>of it! ;^P
>
>Ever since I added those two diodes to my schematic and cable, I've never
>had a problem since. So I highly recommend adding them to ALDL circuitry.
>Earlier cars are less prone to this problem than the later (more "chatty"
>with ABS, DERM, TCS, dash, yada yada yada) vehicles.
>
>Andrew
>http://www.mindspring.com/~amattei/akmelect.htm
>
>
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