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Thread: Bench interface for LT1 PCM

  1. #1
    Electronic Ignition!
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    Bench interface for OBD1 LT1 PCM

    I was running into errors trying to read my PCMs using the cable and a laptop, so I got the idea of using a 12V power supply to power the PCM and then making a short interface harness using GM type connectors so it would plug in. This made my 21 year old unreadable in the car PCM readable. I used Winflash and with a 0 setting on the communication delay it gave me fast results. The attached photos compliment the text, one is my setup before I got the remaining connector parts. I set everything on a small sheet of plywood for portability.

    Disclaimer: My first go at this omitted 2/3 of the connector parts as I went by the part number in the shop manual. This was a workable solution, but I had to manually plug/unplug each wire and then seat the connector. I have added the other connector parts based upon research and datasheets. They are currently on order and if they are incompatible I will update this post.

    Here's what I did (part numbers are linked where possible):
    1. For the 12V power supply, I used an old computer power supply. This page tells you how to do it. I did not bother with the 5V and 3.3V wires. It was a tight squeeze as there were a lot of black and yellow wires.

    2. I bought the following parts from Mouser.com. You can buy any of these w/o an account and you do not need to buy minimum quantities. Shipping is very reasonable as well. Note that a connector is the plastic piece(s) while a terminal is the metal part that fits inside a connector:
    a) 1x binding post terminals #565-6883. I drilled two pairs of 29/64 holes for these for a second set in case I wanted to have 5 and 3.3V posts later. They mount in pairs. It may be easier to find some single posts, but these were cheap. You can paint the knobs so you know what does what.
    b) 4x Delphi 12129025 connectors (part 1 of 3). This is the part that holds the wires. I was initially unaware of the other parts and as able to use this by itself, but it is hard to connect/disconnect this way.
    c) Connector retainer locks: 1x 12129021-B (black), 2x 12129022-B (clear), 1x 12129023-B (blue). The red connector is apparently 12129024-B and is a special order. Use a clear one and mark it with a red sharpie marker. These are part 2 of 3 parts for the 32-way connector.
    d) 2x Delphi 12129028 strain relief connectors (left exit) and 12129030 (right exit). These are connector part 3 of 3, the gray part that fits over the above parts.
    e) 16x Delphi 12084913-L female "Micropack" terminals. These fit into the connectors and the wire fits into them. Always order more than you think you'll need.
    f) 2x 1/4" ring terminals for 10-12 gauge wire #19044-0167. These can also be found at a hardware store.
    g) 1x OBD connector (16 pin) #12110250. This connector will work for 1995+ cars that use a 16 pin OBD connector. The tuner cable will plug right into it.
    h) 8x 12129484-L female terminals for OBD connector.
    i) 2x 12160437 connector locks- these hold the terminals in place and are essential.
    j) Inline fuse holder #576-913-007 or similar- must be cabaple of 15 amps.

    3. I had 18 gauge TXL wire on hand, but it can be found in shorter lengths for reasonable prices at Autosports wiring. Be sure it is AutosportS wiring, as there is a similar site that does not sell wire. The terminals are really small, so you could get by with 20 gauge if need be. You'll need the following:
    a) Pink for +12V power. You could use red or orange, anything to show the power feed wiring.
    b) Black for ground. You can use a sharpie marker to make a stripe if you want- I used silver where a black/white wire was shown.
    c) Tan for data and one ground. I again used a silver sharpie to stripe the tan ground wire.
    d) White wire for data ground, you can use a black sharpie to stripe it. I did not have any, so I used a black wire and striped it.

    4. Crimping tool- I have a Milbar 25E, these are around $50, or you can look for a Metripack or Weatherpack crimping tool on eBay, I found one for around $32 shipped. If you are careful you can use some small needle nose pliers. This would be the most expensive part of the project. A quality tool will pay for itself in time & trouble saved.

    At this point, you have built the power supply and are now ready to build the harness. The following diagram was gleaned from a 1995 GM F-Body (Camaro/Firebird) shop manual. It is essential that you make sure your PCM is properly wired for power, ground, and data. This diagram is known to work for 1995 PCM #16188051.

    There are 4 connectors for this PCM, the shop manual calls them A, B, C, and D. They are color-coded, with A=Red, B=Black, C= Clear (sometimes called gray), and D= Blue. There are 32 pins in each connector, and the terminals they use are called Micropack. The pin numbers are visible using a magnifying glass to look at the corners of the connector and run from 1-16 then 17-32 for the second row (see pictures). Be advised the connectors are numbered on the PCM, and one set will be upside down when the ones you are reading are right side up:
    Example:
    Black
    1-----16
    17---32

    Blue
    32---17
    16----1

    Wiring to A or red connector:
    Pin 2: Black/White wire to ground
    Pin 18: Tan/white wire to ground (stripe the wire or just use a black wire)

    Wiring to B or black connector:
    Pin 15: Pink wire to +12V
    Pin 30: Pink wire to +12V
    Pin 31: Pink wire to +12V

    Wiring to C or clear (sometimes called gray) connector:
    Pin 32: Black wire to ground

    Wiring to D or blue connector:
    Pin 1: Black wire to ground
    Pin 3: Pink wire to +12V
    Pin 20: White wire to data ground
    Pin 30: Tan wire to data feed

    Wiring the OBD connector:
    Pins 4 and 5: Black wire to ground
    Pin 6: White data ground wire to blue connector above
    Pin 9: Tan data wire to blue connector above
    Pin 16: Pink wire to +12V
    Last edited by 95_C4; 01-12-2016 at 09:32 PM.

  2. #2
    Fuel Injected! jthompson122183's Avatar
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    97z28 A4 obd1 swap(16188051)
    Tunerpro Newbie

  3. #3
    Electronic Ignition!
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    Good info, especially for the OBD2 PCMs. My el-cheapo power supply was putting out 12.2V and it worked OK for the two PCMs I read and the one I flashed. Night & day from them being in the car.

  4. #4
    LT1 specialist steveo's Avatar
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    i just used a 1.5 amp 12v wall wart from an old external hard drive, and some leftover pins i saved from a previous harness job.

    but seriously, fix your car. there's only one data wire and one ground wire to the aldl port, one of them must be faulty.

  5. #5
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    The error messages I got had to do with the cable as far as I could tell. I had interface problems with both of my 1995 cars, and then my laptop crapped out (unrelated- power jack). The PCM is not too hard to access on either the TA or the Corvette, and it's not like I'll be doing this on a regular basis. I really just needed to remove a heavily modified bin and replace it with a more or less stock one, so this bench system works OK for me.

  6. #6
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    Tough crowd here. I thought at least one person would appreciate the connector info if nothing else.

  7. #7
    LT1 specialist steveo's Avatar
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    Quote Originally Posted by 95_C4 View Post
    Tough crowd here. I thought at least one person would appreciate the connector info if nothing else.
    it's a nice write-up. i especially appreciate that you hunted down fresh part numbers on mouser. most people would simply chop a wrecker harness.

    it might take more than two hours before the very slim population of ameture lt1 tuners stumble across it, so you might want to increase your patience level a bit.

  8. #8
    Electronic Ignition!
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    LOL in most threads where I deal with wiring most people say I have too much patience, and they can't put up with electrical problems. My point was of the two that replied, the first cited another method that has problems:

    • One part number listed: The power supply (which no longer is sold by Radio Shack), where exactly do I find the other parts I need?
    • A dead link for Andrew Mattei's website.
    • Where do I find Andrew's cable since his site went away some time ago?
    • What if there are no PCM connectors available at a junkyard?


    Despite these shortcomings, I used the adage of "If you don't have something nice to say, don't say anything" and commented it was good info for the OBD2.

    Then you suggested another method and said to "fix your car". I won't go into detail about using "pins" and a "wall wart" other than to say it is not a method I'd suggest for anyone to use since uninsulated/non-isolated terminals could contact other pins if a wire moved, causing unintended problems or possibly damage.

    What if the car was not the problem? There was a problem that others have experienced (I know because I read a lot of threads about OBD1 cable communication issues), and I offered a proper and repeatable solution to it.

    When I write a how-to post such as this one, I write it from the perspective that the reader knows very little about the subject and try to make it as easy as possible for them to make use of the material, which includes finding the parts and showing pictures. It's a lot easier to be like the 99% that fix something and never share how they did it, or ignore the right way to do it. If people that try to help are dumped upon, it's human nature to not help anymore. I've worked on & modified vehicles for over 30 years and have seen helpful people dumped on at car shows years before the Internet, so it's nothing new. If I truly lacked patience, I'd zap the thread and let the next guy figure out how to do it.

    Contrast this thread with my other contribution here: EE Speedometer correction spreadsheet with tire size calc

    Notice how no one jumped in there and said they have another way to do it? It appears a couple of people actually benefited from its use and were appreciative, especially when I took the time to follow up on the problems they had.

  9. #9
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    The effort compiling all this information is admirable.
    Very usefull with all the part numbers needed.
    It will be a great reference not only for the lt1 pcm but all pcms that share same connectors.
    It is trully a professional approach. Something that I want to have, because I really hate my setup now.
    It consists of couple wires, some home made terminals, some tape for insulation and fully charged battery.
    I might upload a picture of my diy setup for reference how not to do it.
    It will be on the extreme low budget end.

  10. #10
    LT1 specialist steveo's Avatar
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    Quote Originally Posted by 95_C4 View Post
    If people that try to help are dumped upon, it's human nature to not help anymore.
    nobody is "dumping on you" by simply making comments about their own experiences and alternative methods of building a bench harness, no more than you are now "dumping on me" by telling me my simpler method is no good, even though it's been in continuous operation for long enough to develop and test tens of thousands of lines of code with.

    sorry nobody has run up and pleasured you for your overly detailed write up, which, like i said, was nice.

    get ahold of yourself, man

    What if the car was not the problem?
    car, cable, power, or computer. what else could it be? flashing on your bench as a solution to unstable aldl comms is like moving to a different house with no grass because your lawnmower is broken.

  11. #11
    Electronic Ignition!
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    Quote Originally Posted by kur4o View Post
    The effort compiling all this information is admirable.
    Very usefull with all the part numbers needed.
    It will be a great reference not only for the lt1 pcm but all pcms that share same connectors.
    It is trully a professional approach. Something that I want to have, because I really hate my setup now.
    It consists of couple wires, some home made terminals, some tape for insulation and fully charged battery.
    I might upload a picture of my diy setup for reference how not to do it.
    It will be on the extreme low budget end.
    You're welcome. My goal was to provide the info for others to use.

  12. #12
    Electronic Ignition!
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    Quote Originally Posted by steveo View Post
    nobody is "dumping on you" by simply making comments about their own experiences and alternative methods of building a bench harness, no more than you are now "dumping on me" by telling me my simpler method is no good, even though it's been in continuous operation for long enough to develop and test tens of thousands of lines of code with.

    sorry nobody has run up and pleasured you for your overly detailed write up, which, like i said, was nice.

    get ahold of yourself, man
    I'm holding on just fine, thanks for the concern. The plus side to this thread was that it has allowed me to learn there is an ignore user feature here.

  13. #13
    Super Moderator Six_Shooter's Avatar
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    I locked this, simply because there is some unnecessary bickering going on.

    95_C4: I suggest you don't take other's experiences or suggestions as an attack of your own contributions, if you continue to do so, you will not last long here. We simply try to give as much relavent information on a subject here as possible. Sometimes that means that people offer their own alternative, or improved/improving solutions and ideas, related to the original topic.

    steveo: Lets keep the comments a tad more tactful in the future.
    The man who says something is impossible, is usually interrupted by the man doing it.

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