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'ALDL information?'
2005\12\12@135406 by Danny Sauer

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This weekend, I bought a new car.  Well, not really a "new" car, since
it's a '95 Caprice Wagon (I have to be the only person who buys a 10
year old full-size station wagon with a V8 to get *better* mileage),
but it's new to me.  The check engine light comes on occasionally.
I'm pretty sure I know what the problem is, but I'll have to ask the
computer to back my guess up.

This car would use the 8192 baud ALDL connection to communicate.
I think I have a couple of 16F688s as well, I'm sure there's a 16F684
in here.  Either way, it shouldn't be terribly difficult to establish
communication with the car, since it uses 5V TTL for asynchronous
serial comms from the documentation I've read.  I've also got a cute
little serial VFD begging to be used for somethign entertaining.
Being a big geek, I feel like I oughtta use the stuff I have to build
something that'll show trouble codes and maybe some semi-realtime data
or similar.  Spending the money on a cheapie scan tool just feels
wrong when I should be able to do it myself.  :)

So, I'm doing some research on the ALDL data stream my '95 LT1 should
be generating, but I haven't found any good, comprehensive
documentation - just the bits and pieces I've mentioned so far.  I'll
keep reading and playing with Google, but I figured I'd ask if anyone
up here had experience with this stuff (I don't need to program, just
monitor, though eventually I may well want to compensate for gear
ratio / tire diameter changes).  Any good descriptions of what I can
ask the computer for, how to ask it, etc?  The info seems to be spread
out all over the place, and I've found some decent stuff, but I
figured it was possible that someone up here had already done this
searching so I could piggy back on their work.

A source for the square 12-pin GM ALDL connector would be handy, too.
:)

Thanks.
--Danny

2005\12\12@170909 by Lee Jones

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> This weekend, I bought a new car.  Well, not really a "new" car, since
> it's a '95 Caprice Wagon (I have to be the only person who buys a 10
> year old full-size station wagon with a V8 to get *better* mileage),
> but it's new to me.  The check engine light comes on occasionally.
> I'm pretty sure I know what the problem is, but I'll have to ask the
> computer to back my guess up.

> This car would use the 8192 baud ALDL connection to communicate.
> it shouldn't be terribly difficult to establish communication
> with the car, since it uses 5V TTL for asynchronous serial comms

No, it shouldn't.  But since the protocol is considered proprietary...

> Spending the money on a cheapie scan tool just feels wrong when
> I should be able to do it myself.  :)

Would a paperclip be acceptable as a scan tool?  Chevrolet had a
technique where you ground one of the pins on the OBD connector,
turn on the ignition, and the service code(s) blink out through
the service engine light.  I use a paperclip to short the special
pin to a ground pin (both on the OBD connector) on my 87 Suburban.

I strongly advise getting the factory service manual for the car.
It will describe this procedure and show you which pins to use.
The descriptions of the various causes of the service codes are
quite usefull too.

                                               Lee Jones

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