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Thread: IAC testing

  1. #1
    Electronic Ignition!
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    IAC testing

    My 57 Chevy with a 89' 350 TBI is having an idle issue. I believe I tracted it down to the IAC but need to confirm as it is a new IAC valve. The car doesn't increase idle at startup or the normal idle increases. It actually will not start unless I crack the throttle. I'm getting 52 ohms through A to B and C to D on the IAC. The wiring diagram for the specific motor showed the wiring different that the connector. I switched wires with no luck. I get 10.5 volts on both sides of the 4 pin connector. How do I tell which side is high side and which is low side? The IAC does go back and forth when I switch wires around. I've been goofig with this for a year and I need to fix this. I can push the pintle in and the car fires right up reaching through the window. After a few starts the pintle close and it's back to hard starts and low idle. I need to esting information and how to determine which wire goes to which pin on the IAC. I have the wiring diagram if needed.

  2. #2
    RIP EagleMark's Avatar
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    Re: IAC testing

    EDIT: If someone is looking for procedure to test IAC, wiring and ECM here is the write up and a diagnostic PDF from GM.
    viewtopic.php?f=23&t=327
    ----------------------------------------------------------------------------

    What did your wiring hrness come off of? What IAC do you have flat or round?

    Did you upload you wiring diagram here?

    EDIT: I have neve seen one push in so it may be a bad IAC. They spin in on a gear drive.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  3. #3
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    Re: IAC testing

    being able to push the pintle in isn't a bad sign, it's actually a recommendation when installing a new one, along with reseting the ECM.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  4. #4
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    Re: IAC testing

    Here is the wiring diagram:

    The IAC connector is square 4 pin. I remember pulling the block stamping numbers at it is a 350 TBI for a 1989 G-10 van. The wiring harness is also from a G-10 based on the wiring diagram. The IAC screws in but I've seen were some wiggle in. The IAC seems to function in and out.

  5. #5
    RIP EagleMark's Avatar
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    Re: IAC testing

    Quote Originally Posted by RobertISaar
    being able to push the pintle in isn't a bad sign, it's actually a recommendation when installing a new one, along with reseting the ECM.
    Weird I can't move a couple I have here?

    Blue wire white stripe to A on IAC connector.
    Blue wire black stripe to B on IAC connector.
    Green wire white stripe to C on IAC connector.
    Green wire black stripe to D on IAC connector.

    HTH

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

  6. #6
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    Re: IAC testing

    Thanks, I'll give it a shot tonight.

  7. #7
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    Re: IAC testing

    Tried the above wiring suggestion but it didn't work. Well, I should say it worked backwards. The pintle opened in diagnostic mode (A & B jumpered). I'm going to reverse the colors and see if that works.

  8. #8
    RIP EagleMark's Avatar
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    Re: IAC testing

    What ECM do you have? What BCC do you have?

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

  9. #9
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    Re: IAC testing

    Wow....haven't had time to touch my car in forever.....the ECM is a 1227747. I don't remember the BCC. I need to finish my car and get it running correctly. Any help I can get will be appreciated. Finally getting back to fixing the problem

  10. #10
    Super Moderator Six_Shooter's Avatar
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    Re: IAC testing

    I've played with the ECM to IAC wiring a few times, and it doesn't seem to make a difference, which I know is odd, because you would think it would work backwards, but it doesn't for some reason.

    I plan to revisit playing with the IAC in the not so distant future.
    The man who says something is impossible, is usually interrupted by the man doing it.

  11. #11
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    Re: IAC testing

    Well for some reason the BB chevy has 2 wirescrossed compared to SB chevy. But I never tried to figure out why...

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

  12. #12
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    Re: IAC testing

    Six, are you talking about reversing wires across the coils?
    That makes sense and it seems that all wiring diagrams for my year and the years around mine are all different. Some have the wires reversed across the coil on one side and the same on the other side.
    I have wired it like the stock wiring diagram has shown and the results have been no idle adjustment from the IAC. I reversed the blue and green wires across the coils and get a high idle which would indicate that the stepper motor is pulling in and working the opposite of what it needs to. When it was hooked up to a computer the stepper motor was within spec steps but the car just wasn't running like it should so either the scanner was lieing or my ecm is but I need to make sure my wiring is correct to verify.

  13. #13
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    Re: IAC testing

    Maybe this will help diagnose the IAC correctly.
    Attached Files Attached Files

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

  14. #14
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    Re: IAC testing

    Thanks Eagle. Some differences in my wiring diagram and the one you posted. The wiring diag. for my motor says that A&D are one coil and B&C are the other coil. So, Lt BLu-Blk goes to D and Lt Blu-Wht goes to A. Lt Grn-Wht goes to B and Lt Grn-Blk goes to C. Buttttttt....when looking at other connectors as well as the way my original connector was wired, Blu-Wht goes to A, Blu-Blk goes to B, Grn-wht goes to C and Grn-Blk goes to D........
    I'm not sure if GM didn't screw something up or something else is wrong but here is what my plan is:

    I'm going to check resistance between the terminals. Across the coil pins I should read 20+ ohms, across the coils I should get infinity ohms.

    All I do know is that something is not telling the IAC to open during cranking. Any ideas as to what that would be?

  15. #15
    Super Moderator Six_Shooter's Avatar
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    Re: IAC testing

    The IAC doesn't usually move during cranking. It does however, get parked at key off, of the previous engine run cycle. This park setting is to help the engine during start on the next run cycle. After the ECM sees the right number of DRP, it then moves to close the IAC to a point where it will idle, in correlation to engine temp.
    The man who says something is impossible, is usually interrupted by the man doing it.

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