1. Join the Community!

    DSMtuners is a massive archive of DSM information - but more importantly, it's a COMMUNITY! Join in and participate with other DSMers, and invite all of your DSM friends to make this place their home. Chat with others, create a build thread, post questions and answers. Get involved! Logging in will also remove many of the advertisements, along with this notice. ;)

1G Low idle with a/c on

Posted by benspilk09, Apr 14, 2019

Please Support Kiggly Racing
Please Support ExtremePSI
  1. benspilk09

    benspilk09 Probationary Member

    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!

    Featured Products from our Supporting Vendors

  2. BLACK'98DSM

    BLACK'98DSM Proven Member

    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
  3. ThunderChild

    ThunderChild Previously tametalon92

    Joined Jan 5, 2012
    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.

    Street Build 395  7

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

    Street Build 647  1

    1992 Eagle Talon TSi AWD
    manual · 1G DSM

    Street Build 2K  10

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

    mitsubishikid Proven Member

    Joined Jun 8, 2009
    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
  5. benspilk09

    benspilk09 Probationary Member

    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

    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
  7. steve

    steve DSM Wiseman

    Joined Feb 3, 2002
    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.

    3K  0

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

    benspilk09 Probationary Member

    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.

Share This Page

Support Vendors who Support the DSM Community
Ace Race Parts Archer Fabrication Boostin Performance ECM Tuning ExtremePSI Feal Suspension Fuel Injector Clinic Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications OHM Racing RockAuto SouthBay Fuel Injectors STM Tuned VR Speed Factory White Shed Speed