CarChat and cable ...

[Andrew K. Mattei]

> 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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