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1G Low idle with a/c on

Posted by benspilk09, Apr 14, 2019

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  1. benspilk09

    benspilk09 Probationary Member

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    Joined Feb 16, 2010
    Little Rock, Arkansas
    I'm about at my wits end with this problem.

    Car is a 91 TSi AWD, all stock except for ecmlink and a wideband.

    The car makes no attempt to speed up the idle when you kick the a/c on. Normal idle is perfect at around 800rpm, it drops to about 500 with the a/c.

    I put the car in diagnostic mode and set the BISS per the VFAQ.

    The isc buzzes when you turn the key on, but doesn't visibly move. It is clean and the plunger moves freely when you take it apart. It tests perfect across all 4 coils. All the drivers in the ECM look fine. The wiring harness tests fine.

    In ECMlink, when I log ISC position and learn idle adjust they're both flatlined at 120 and 176 respectively. No changes, ever. I assume this means the problem is with the ECU? Anyone had an issue like this before?

    Any help is appreciated, Thanks!
     

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  2. BLACK'98DSM

    BLACK'98DSM Proven Member

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    Joined Feb 9, 2019
    Hoover, Alabama
    I don't know... But here's what I do know:
    - The A/C uses voltage
    - The A/C uses horsepower

    Your ecu is supposed to recognize when the A/C is on so that it can compensate for the use of power. It must not be recognizing it's on. Or somehow it doesn't have the means to compensate for it...

    When you turn the A/C on in any car it drops the rpm's, just not usually by 300.

    I know that seems really basic, but sometime people overlook the real problem because of overthinking...
     
    Last edited: Apr 14, 2019

    Street Build 514  2

    1998 Mitsubishi Eclipse N/T
    fwd · manual · 2G DSM
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  3. ThunderChild

    ThunderChild Previously tametalon92

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    Rathdrum, Idaho
    the ecu is supposed to idle up when a/c is activated.

    have you tried checking the ecu pin to ensure its being energized or grounded? i dont recall off the top of my head which it doesn. wire should be pin 7, large connector, black with green stripe.
     

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    1997 Mitsubishi Eclipse Spyder GST
    fwd · manual · 2G DSM

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    1992 Eagle Talon TSi AWD
    manual · 1G DSM

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    1992 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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  4. mitsubishikid

    mitsubishikid Proven Member

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    Turlock, California
    do you have it set in dsmlink to target idle at 800rpms... IIRC the stock or default target idle is 750rpms so thats what the ISC expects to see unless you set the it in dsmlink to target 800 rpms, I know this is obvious, but i'd start with this since you have it, and it's easy to do, also if you have a spare ISC try swapping it out.
     

    887  0

    1990 Eagle Talon TSi AWD
    manual · 1G DSM
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  5. benspilk09

    benspilk09 Probationary Member

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    Joined Feb 16, 2010
    Little Rock, Arkansas
    My target idle is 850, just so it won't stall with the a/c on.

    I did some more playing in ecmlink. If I play with the BISS I see the isc position changing and attempting to change the idle speed, but nothing happens. I'm assuming the ecu isn't actually getting a signal to the ISC and I either have a failed driver or a bad trace on the board.

    The isc does have power at the plug, it tests fine per my FSM.

    I'll have to yank the computer back out and do some testing. I can't seem to find a good walk-through on testing the drivers and traces with a multimeter though
     
  6. tk106

    tk106 Proven Member

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    Joined Sep 11, 2017
    Cincinnati, Ohio
    Have you tried adjusting idle air clamp in link? I don't have ac so I never looked into it.
     

    Street Build 1K  8

    1992 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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  7. steve

    steve DSM Wiseman

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    Boulder, Colorado
    You can also log the AC input to make sure the ECU is seeing it.
    If your ISC isn't stepping in and out when you turn the ECU on, there's a problem. If you ISC is a original metal one, change it.

    You can't always see damaged ISC drivers. The fried ones are easy but not the ones that can't sink enough current anymore to make the coil step.
     

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    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
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  8. benspilk09

    benspilk09 Probationary Member

    13
    0
    Joined Feb 16, 2010
    Little Rock, Arkansas
    Yep, I've read through those links. I have not played with anything outside of idle speed. I did set my idle switch and verified my throttle closes to 0% and calibrated the TPS. All is working fine

    It's just that the ISC position seems to be doing nothing to the idle.

    I know I don't have any significant vacuum leaks, as it will die if I screw the BISS in too far.

    I suspect an ECU problem the more I read, and the more I play with it. I see the ECU trying to move the thing, but there is no resulting change of idle. It's just locked wherever I set it using the BISS.

    Yes, it's the original ISC. I'll try and find an updated one if they're more reliable, I'd hate to repair my ecu only to have It fried when this ISC takes a dump.
     

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