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Thread: Unable to read vfm on lq4 swap

  1. #1
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    Unable to read vfm on lq4 swap

    HPT can not find the vcm. I checked the for 12v and ground at the obd2 connector and both are present. Any ideas what else could be the problem?
    Last edited by LVJimmy; 12-11-2010 at 03:16 PM.

  2. #2
    Advanced Tuner jakebdb56's Avatar
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    I'm assuming you mean VCM...

    What kind of swap is it?

    Have you checked for grounds on the engine itself? The PCM and fuse center grounds through the engine block.
    '12 Duramax - Tow Rig
    '90 S10 - Alleged "Race Truck"

  3. #3
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    It's a 2004 6ltr out of a savannah van that has factory drive by cable. The engine is is a 71 GMC Jimmy (blazer). I belive all the grounds are good but I will recheck them.

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    I have 5 good grounds. Battery to body, battery to engine, engine to frame. Two more coming out of the rear of the harness which I assume are the PCM and coil grounds. I did find a connector that was not connected to anything, and still isn't. I'll see if I can post a picture of it when I get to a computer. Any other ideas on not reading the vcm?

  5. #5
    Senior Tuner Google's Avatar
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    make sure you have your 4 grounds at the pcm.. 1 batt supply power and 2 key fwd power supplys. make sure you have your aldl power and ground.. then your serial communication wire from the pcm to the aldl.. thats all thats needed to flash a pcm in a car or on a bench..
    We Can Fix Your Bricked PCM Or Your YYYYYY OS ID

  6. #6
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    Quote Originally Posted by Google View Post
    make sure you have your 4 grounds at the pcm.. 1 batt supply power and 2 key fwd power supplys. make sure you have your aldl power and ground.. then your serial communication wire from the pcm to the aldl.. thats all thats needed to flash a pcm in a car or on a bench..
    What is aldl?

  7. #7
    Advanced Tuner jakebdb56's Avatar
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    ALDL is your OBDII port. Most common issue people has is no power to OBDII port and/or bad serial wire from the PCM to the port. The pin is #58 on the blue connector (dark green serial data) for GenIII's IIRC.

    '12 Duramax - Tow Rig
    '90 S10 - Alleged "Race Truck"

  8. #8
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    Quote Originally Posted by jakebdb56 View Post
    ALDL is your OBDII port. Most common issue people has is no power to OBDII port and/or bad serial wire from the PCM to the port. The pin is #58 on the blue connector (dark green serial data) for GenIII's IIRC.

    I have 12v and ground at the obdc. I also have a predator that works when connected to the obdc. As soon as the rain stops I'll check the grounds on the pcm

  9. #9
    Advanced Tuner jakebdb56's Avatar
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    Are you trying to use the stock body harness too? Reason I ask is we had the same problem the other day when a friend of mine was trying to use the BCM and complete body harness. We tested continuity from the OBDII plug to the plug on the BCM and there was resistance. Scantools were able to poll DTC's from the PCM and BCM, but HPTuners wouldn't connect. I jumped the Dark Green Serial Data Wire from the PCM harness to the plug and it worked just fine.

    If you have power, ground, and switched power to the PCM and you have the serial data wire connected to pin 2 on the OBDII you should be able to read/write the PCM. Otherwise, you may have a fried PCM.
    '12 Duramax - Tow Rig
    '90 S10 - Alleged "Race Truck"

  10. #10
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    Turned out to be a bad interface which HPTuners repaired.