Oh, one more thing I forgot to mention, for anyone planning on building the two transistor circuit I mentioned earlier -- make sure you check the datasheets on the transistors you use for the specific pinouts, especially if you buy an assortment from Radio Shack like I did. The back of the package lists all the transistors as having a pinout of EBC (emitter - base - collector) looking at them from the front (pins down, flat side toward you), but depending on what transistors were cheap when Radio Shack packaged them, that might not be the case. 2N2222 transistors are EBC, but ECG289As (or NTE289As, they're the same thing), for example, are ECB -- and most certainly will not work if connected as if they were EBC, and may end up damaging your computer, your ECM, or both if energized as such. So be wary. I very nearly hooked up one transistor incorrectly until I noticed what transistor it was, and thankfully I'd used ECG289As quite a bit in the past and caught myself before all was lost.

If in doubt, you can always check them using a multimeter with a diode check function. Place the red lead on what you think is the base, and the black lead on what you think is the emitter. You should see a forward diode voltage drop of approximately 0.7V. Reverse them, and you should see an open circuit. Between emitter and collector, no matter what the polarity, you should see an open circuit.

-b

Thanks for all the info yes, I was using the MAX232 chip.

Unfortunately I don't think I'll ever get the chance to try this again on an OBD-I car, mainly because I'll probably be trying to sell my '92 Bonnie soon (that would be a sad day...).

I'm thinking of getting myself a 2000/2001 Impala w/ the 3800, that'* the closest thing to a Bonneville that'* currently being imported here (except the LeSabre, but I don't really see myself driving one.....). There are relatively many 2000-2004 Chevy Impala'* that were sold here, a few hundreds of them I guess.

The Bonneville was regularly imported here only until 1993, so newer ones are very very rare and almost impossible to find, and when I do see one, of course it'* never for sale. also, if I buy a 1996-1999 Bonnie, replacement parts could be a real nightmare. Also there are absolutely no parts for the 3800 */C here, no matter what year.

Anyway, I'll probably try building an OBD-II to PC interface one day for a newer GM car I'll be driving

DAMM YOU OBD 1 USERS !!!

Not so lucky for us OBD II freaks.

Yes, you must watch those for the difference between the PNP and the NPN transistors

I went to a computer store to get the adapter from RS232 to USB, they even had one in stock but they want 59.95 for it. I says pardon? I saw one on the net (CAD) for 18.99 Needless to say, I am going to be ordering the one from the net...the best he could do was 44.00...still not even close!

That'* what ticked me off! They were ALL NPN! But Radio Shack was too cheap or indifferent or both to indicate on the package that the actual components contained therein may have different pinouts.

Yikes -- that'* pretty terrible. But I guess they can charge whatever they want to if people have old legacy serial devices and no serial ports to connect them to, eh?

-b

Very cool and geeky! I, too have one of the freak 94-95 transition years and it is very expensive to buy a scan tool for. This electronic stuff seems pretty straightforward and I think I will give it a try. If it works, maybe I can assemble a few of these for those interested (I've had experience making my own circuit boards).
I'll keep you posted.

Steve, I just noticed you're in Blue Springs -- we're practically neighbors! I'm right down 70 in Columbia.

-b

Hey, we're all neighbors, globally speaking. I've been through Columbia many times...
Question on the coding on your circuit:
For the PC serial connection, the pins used on a DB9 would be 2, 3 & 5 while a DB25 would use 3,2 &7? Am I reading that right?
Also- any programs out there for us counter cultural Mac users?

are there any cables/programs that work with the OBDII systems? Are there computer-based scanners that we can use like what you are? I'd be interested in this if I can buy a cable for relatively cheap, and use that free program.

Correct.

I did read a few things about the possibilities of such programs being used on Macs, but a lot of them seemed to say that the Mac'* UART is somewhat finicky and probably wouldn't be able to lock on to a non-standard 8192 baud rate. But I know little to nothing about Macs, so...

-b