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Thread: Scan Tool says not connected but it is

  1. #1
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    Exclamation Scan Tool says not connected but it is

    I just sold my 00 Chevy S10 to my son who is stationed in Alaska. They have an OBD scan emissions test and it failed because the scan tool shows that it is not connected, but it actually is.

    He has very little money to put into this truck and what he does have, he should be putting into things like snow-chains and other safety items. I feel terrible that I sold it to him with a problem like this.

    Does anybody have any ideas what to check, besides the obvious like fuse and connections?

    Thanx for any help you can provide.

    Beetle

  2. #2
    VENDOR - OBDPros
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    What scantool is in the vehicle?

  3. #3
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    None, the Alaska emissions tester hooked one up to it for the emissions test and it wouldn't show anything because it said that it wasn't hooked up, but it was. It's like it can't detect the computer. Truck runs fine though.

  4. #4
    Constant Bitrate joeyoravec's Avatar
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    On that vehicle you should only need 3 wires - Power (16), Ground (5), and Class2/VPW (2) so there's not much to diagnose.

    Sometimes the power line shares a fuse with the cig lighter or some other accessory. Check that you're actually getting voltage. Places like autozone usually let you borrow a scantool free to read codes. You could borrow that to verify communication to the PCM. At that point you should be fine.

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    Quote Originally Posted by joeyoravec View Post
    On that vehicle you should only need 3 wires - Power (16), Ground (5), and Class2/VPW (2) so there's not much to diagnose.

    Sometimes the power line shares a fuse with the cig lighter or some other accessory. Check that you're actually getting voltage. Places like autozone usually let you borrow a scantool free to read codes. You could borrow that to verify communication to the PCM. At that point you should be fine.
    Thank you so much sir. He just told me that the cigarette lighter hasn't worked in ages. That is likely the problem then.

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    @joeyoravec: pin 2 is an "SAE +" communication line, which requires pin 10 ( SAE - ) as this is a twisted pair network. Pin 7 is the ISO K line which allows Global OBD-II communication across a multiplexing single wire network. Emissions tests require access to the OBD-II readiness monitors, this is done over the SAE lines.

    I have never seen a PCM or any other module share the same circuit as a cigarette lighter, ever. No manufacturer would open themselves to the liability of having a major system fail because a customer plugged in some garbage 12v accessory from jc whitney & blew a fuse!

    "...so there's not much to diagnose."

    HA!

    If a communication line has excessive resistance this can cause attenuation of the signal. In other words, it takes what is normally a digital signal & makes it look like an analog one. This can cause the PCM & other modules to do bogus things or not work at all. The only way to see this is with an oscilloscope.

    @beetlebailey: your son's cigarette lighter not working is a separate issue. Myself speaking as an automotive technician, I recommend your son take the car to a GM dealership. They are the ones who will have the Tech-II scanner, the GM information system (diagrams), O-scope, & the expertise.


    ...and to all the kids out there! Remember! The internet can give you just enough information to get yourself into a horrible jam!

  7. #7
    Constant Bitrate joeyoravec's Avatar
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    Quote Originally Posted by dieselray View Post
    @joeyoravec: pin 2 is an "SAE +" communication line, which requires pin 10 ( SAE - ) as this is a twisted pair network. Pin 7 is the ISO K line which allows Global OBD-II communication across a multiplexing single wire network. Emissions tests require access to the OBD-II readiness monitors, this is done over the SAE lines.
    Although you dug-up an almost two-year-old thread it's worth correcting your correction. You may be confused between the two variations of J1850. Ford implements J1850PWM which uses differential signaling so it needs both pin2 (J1850+) and pin 10 (J1850-). General Motors, Chrysler and anybody else implements J1850VPW which is single-ended and needs only pin2 (J1850+). The original poster asked about a 2000 Chevy S10 which uses J1850VPW so the information in my original post is correct.

    Quote Originally Posted by dieselray View Post
    I have never seen a PCM or any other module share the same circuit as a cigarette lighter, ever. No manufacturer would open themselves to the liability of having a major system fail because a customer plugged in some garbage 12v accessory from jc whitney & blew a fuse!
    Perhaps I was not clear. The PCM is obviously powered if the vehicle runs; what I meant to question was the diagnostic connector and wiring. It's common for the OBD-II connector to share a fuse with something else and it's very easy to overlook a defective power, ground, or communication line if your scantool gets power from batteries or the PC's USB port.

    Whenever you think to yourself "this should work" then it's important to drop back to the basics. Does the same scantool work on a different car? Does a different scantool work on the same car? Is the scantool powered and grounded? Is the scantool configured correctly for this car? Is the OBD-II connector damaged? Is the wiring between the diagnostic connector and the PCM intact? Work through some easy-to-verify tests and eventually you should isolate the problem.

    The only additional suggestion I'd make today is to check the grounding scheme. The vehicle is supposed to have pin4 and pin5 for ground. Many vehicles ground both, but what if the vehicle grounds one pin while the scantool relies on the other. Checking that your scantool is actually powered and grounded is important when you have a mystery like that.

    Quote Originally Posted by dieselray View Post
    Myself speaking as an automotive technician, I recommend your son take the car to a GM dealership. They are the ones who will have the Tech-II scanner, the GM information system (diagrams), O-scope, & the expertise.
    Speaking as an engineer that designs service tools I have to agree. Lots of things require specialized tools and aren't worth the time, money, or effort to do yourself. It's important to have a trustworthy shop that can take care of the vehicle.

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