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Thread: Uber-easy DIY USB ALDL Cable

  1. #46
    Fuel Injected! phonedawgz's Avatar
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    Earlier OBD1 vehicles need a 10K resistor between pins A and B to start the data flow.

    Later OBD1 use bi-directional data flow on pin M and don't require it. Having the 10K resistor between pins A & B won't hurt however.

  2. #47
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    depends on the application. most cars don't need it, the some that do tend to need a 10K resistor connected from the B pin to the A pin(or the ground on the converter) either temporarily or permanantly.

    if you use a male OBD1 connector, then you could simply run a wire from the B pin to a rocker/toggle switch, then to a 10K resistor, then to the ground circuit. you can then hide all of it in a small project box from radioshack. i think i use the 3" X 2" X 1" versions.

    Quote Originally Posted by phonedawgz View Post
    Having the 10K resistor between pins A & B won't hurt however.
    depends. some applications go into "ALDL mode" and will change the idle speed and add up to 10* of spark advance.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  3. #48
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    Quote Originally Posted by RobertISaar View Post
    depends on the application. most cars don't need it, the some that do tend to need a 10K resistor connected from the B pin to the A pin(or the ground on the converter) either temporarily or permanantly.

    if you use a male OBD1 connector, then you could simply run a wire from the B pin to a rocker/toggle switch, then to a 10K resistor, then to the ground circuit. you can then hide all of it in a small project box from radioshack. i think i use the 3" X 2" X 1" versions.



    depends. some applications go into "ALDL mode" and will change the idle speed and add up to 10* of spark advance.
    This is for a '7427 PCM and I will probably omit the ALDL connector for a permanent install. So can I just de-pin the white/black wire that terminates at pin B from the PCM and pitch it?

  4. #49
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    IIRC, the only thing the 7427 uses the ALDL pin B is for blinking out codes if connected directly to ground.

    i'm not a 7427 expert though.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  5. #50
    EFI GearHead ! EagleMark's Avatar
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    This 10K needed for 160 Baud ECM is a horrible rumor created on internet that won't go away! Even the Moates website shows 10k resistor needed for all 160 Baud ECMs. 10K really messes up good data on 1227747 and other old 160 Baud ECMs by putting the ECM in ALDL mode, it increases idle and adds timing, not needed at all for data!

    Other then checking codes by crossing A and B I've had no use for 10K, it is needed in $6E to start data flowing. I have no idea what else it is used for except to screw up data.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  6. #51
    Fuel Injected! shimniok's Avatar
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    Glad I found this thread. I'm trying to build an ALDL USB cable using a different TTL serial to USB converter.

    WinALDL can't connect to it and neither can TunerPro RT. Only one of my terminal programs displays any data.

    Do I *have* to tie the tx/rx lines together for a 160-baud 7747?

    The usb/serial converter is on COM11. Maybe TunerPro and WinALDL can't access those ports? Or maybe some incompatibility with the driver for it.

    Since we know the FTDI thing works... I may use one of my two FT232 breakout boards from Sparkfun as soon as I can find them :)

    One is 3.3V -- what I usually do converting from 5V to 3.3V is to throw a resistor inline to limit current. For an automotive application at such slow rates I might just use a 10K instead of my regular 1k.

    More later, hopefully...

  7. #52
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    with a 7747, no, all you need is the receive part of the circuit, since it doesn't communicate in both directions.

    COM11 is fine, i'm all the way up to COM15 at times(i've connected a lot of converters) and i'm still communicating normally.

    leave it at 5V, 3.3 might work, but it might also cause more problems.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  8. #53
    Fuel Injected! phonedawgz's Avatar
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    If you are looking at a 160 baud - one directional cable, and you want to keep the 3.3v input - you should be able to do it with just two resistors to divide the voltage. A single resistor inline won't drop the voltage unless there is current flowing.

    From the pin E - connect to resistor A - then tie to both the RX pin and resistor B - and then connect to ground.

    Try using a 1K for resistor A, and a 2.2K for resistor B. Check the voltage on the RX pin since the unknown is the resistance of the USB rx input.

    You do not have to have TX and RX tied together for 160 baud pin E applications.

    TunerPro has no problem with the upper COM ports. I have been up in the 20s with no problem.

  9. #54
    Fuel Injected! shimniok's Avatar
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    Ok, I found the 5V FTDI breakout board and it worked, sort of.

    I can see data coming in over one of the serial ports. I tied tx/rx together and TunerPro RT sees it when the ECM is off or disconnected. Neither it or WinALDL can talk to the thing on COM10, however.

    I will have to go find out what to do with TunerPro RT to get it to work with this setup.

    Sparkfun did a tutorial on interfacing different voltages some time ago in case anyone finds this useful: http://www.sparkfun.com/tutorials/65

    The single resistor approach probably works best if there's a clamping diode on the 3.3V end. Hmm.
    Last edited by shimniok; 08-28-2012 at 01:57 PM.

  10. #55
    Fuel Injected! shimniok's Avatar
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    In TunerPro RT, I had to set 4800 baud and uncheck RS232 echo and it started acquiring data, and revealed an issue to chase down.

  11. #56
    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by shimniok View Post
    In TunerPro RT, I had to set 4800 baud and uncheck RS232 echo and it started acquiring data, and revealed an issue to chase down.
    My $42 ADX file is that way, not sure how yours was differant?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  12. #57
    Fuel Injected! shimniok's Avatar
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    Mine had 2400 baud (I think) and RS-232 echo checked. I forget where I downloaded it, possibly here. $42-1227747-V4.2.adx *shrug*

  13. #58
    EFI GearHead ! EagleMark's Avatar
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    Hmmm? That's one I did, but never had a reason to change that and the latest version is set to 4800 baud no echo... I've got a truck here to tune and will test it to be sure. If it works OK I'll upload the newest version ADX and XDF for you.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  14. #59
    Fuel Injected! shimniok's Avatar
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    Thanks. Not sure what happened.

    So with ftdi connected the ses light flickers really fast and sort of randomly. And data stream stops. Not sure what's up. Going to dig into that next. Hard to know why the truck won't idle if I can't log data :)

  15. #60
    Fuel Injected! phonedawgz's Avatar
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    If you shorted A- B, the ses light will blink out the SES codes
    If you had the 10K resistor between A and B - then the flashing light will tell you if the ECM is in a rich or lean condition. Note the ECM is supposed to alternate between the rich/lean states

    By shorting pin A to B the Diagnostic ALDL connector tells the computer that it should put the SES lamp into Road Test mode. You can monitor the richness of the input by looking at how fast the SES lamp flashes.

    If you've got a harness from a car and somehow neglected to get the ALDL connector (it's a GM, not Howell connector) then you can still do the test, but it will be a little more involved.

    Look at the connector on the ECM. Find pin A9. This is the pin on the connector that would go to terminal "B" on the ALDL.

    Pin "A" on the ALDL is simply engine ground and this point on the ECM is pin A12.

    So what you're doing is grounding Pin A9 to tell the computer to go into Road Test mode.

    Now when the SES lamp is flashing at the rate of about 2.5 times a second, you're in Open Loop. You'll see this happen at idle, and even sometimes when you're crusin' down the street and the engine is not working very hard.

    However, after the engin warms up, SES is Off all, or most of the time the engin is running lean.

    If the SES is on most of the time... the engine's running rich.

    See, it's pretty darned simple:

    SES on most of the time = Rich
    SES off most of the time = Lean
    Last edited by phonedawgz; 08-28-2012 at 10:59 PM.

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