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

Tags:
Please Support STM Tuned
Please Support STM Tuned
  1. benspilk09

    benspilk09 Probationary Member

    26
    3
    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!
     
  2. BLACK'98DSM

    BLACK'98DSM Proven Member

    581
    169
    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

    Road Race Build 2K  15

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

    286  3

    2012 Murray Explorer Dune
    rwd · automatic · Misc Vehicles

    199  6

    2009 Troy Bilt Bronco
    rwd · automatic · Misc Vehicles
    Loading...
  3. ThunderChild

    ThunderChild Supporting VIP

    2,541
    710
    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 189  3

    1990 Plymouth Laser N/T
    fwd · manual · 1G DSM

    283  8

    1987 Toyota Pickup/Hilux
    awd · manual · Misc Vehicles

    Street Build 835  7

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

    Street Build 1K  1

    1992 Eagle Talon TSi AWD
    manual · 1G DSM

    Street Build 2K  10

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

    mitsubishikid Proven Member

    810
    171
    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.
     

    961  0

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

    benspilk09 Probationary Member

    26
    3
    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 Supporting Member

    1,409
    294
    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.
     

    115  3

    2000 Jeep Cherokee Sport - Classic
    awd · automatic · Misc Vehicles

    119  4

    93 Civic 4 Door -sold-
    fwd · manual · Misc Vehicles

    Street Build 2K  6

    1992 Eagle Talon TSi AWD
    12.625 @ 111.42 · 1G DSM
    Loading...
  7. steve

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    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 your ISC is a original metal one, change it regardless.

    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.
     
    Last edited: Apr 26, 2019

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
    Bud92gsx likes this.
  8. benspilk09

    benspilk09 Probationary Member

    26
    3
    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.
     
  9. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    Allright, I've had some time to mess with the car some more.

    I bought a brand new black ISC and it does the same thing. It should be good, ~40 ohms across all coils.

    I verified again that the wiring harness is good, there is no resistance between each pin in the ISC plug and the corresponding pin in the ECU plug. I'm almost sure it's an ECU problem at this point.

    I pulled the ECU again and verified that all 4 coil pins have connectivity to the ISC drivers themselves. I'm not sure where to test after that. I can't find anything on testing the drivers or motherboard traces themselves. Anyone have experience with this? Or do I just need to call up ECMTuning?
     
  10. lasthope05

    lasthope05 Proven Member

    916
    206
    Joined Mar 31, 2006
    Philadelphia, Pennsylvania
    Did you bother checking the AC wire/pin like some members said. The ECU needs to see the AC turning on for it to even bump idle. Check to make sure the wire is good from the compressor back to the ECU.
     
    ThunderChild likes this.
  11. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    It's not just an a/c issue, the computer has zero control over idle. The ISC is completely inoperable. Whatever I set the BISS at, that's the idle speed

    I have not checked the a/c pin, the ISC needs to function before I dive into anything else
     
  12. ThunderChild

    ThunderChild Supporting VIP

    2,541
    710
    Joined Jan 5, 2012
    Rathdrum, Idaho
    You don't happen to have the box checked in ecmlink disabling the IAC do you? Past a bad ECU at this point I can't think of anything else really.
     

    Street Build 189  3

    1990 Plymouth Laser N/T
    fwd · manual · 1G DSM

    283  8

    1987 Toyota Pickup/Hilux
    awd · manual · Misc Vehicles

    Street Build 835  7

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

    Street Build 1K  1

    1992 Eagle Talon TSi AWD
    manual · 1G DSM

    Street Build 2K  10

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

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    So with the new ISC does it rehome when you power the ECU up?
    Do you have a working IPS?
     

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
  14. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    Nope, there is no movement when you turn the key on. Just a slight buzz then nothing.

    The IPS is working correctly, I can see it switch on and off in ECMlink

    As for disabling the ISC, just the diagnostic pin? No, that's not checked. I can see ISC position change like I'd expect when I play with the BISS, but the plunger isn't moving because the idle doesn't speed up/slow down when the computer tells it to.
     
  15. steve

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    Sounds like wiring or a bad driver(s).
     

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
  16. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    I'm looking for the method to test the drivers.

    I've already ruled out the wiring harness
     
  17. steve

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    I built special software and hardware to do that so I didn't have to drag one of my oscilloscopes around.
     

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
  18. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    Ahh, if it takes an o-scope I'm probably better off sending it somewhere. I was hoping I could rule them out with a simple multimeter.

    I contacted ecm tuning a few days ago about diagnosing and repairing the ECU, but they haven't responded yet. Are you affiliated with them? Or do you offer a repair service? I'm ready to get this thing idling right
     
  19. steve

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    Once upon a time both were true. I have heard that it's been difficult reaching them lately.
     

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
  20. benspilk09

    benspilk09 Probationary Member

    26
    3
    Joined Feb 16, 2010
    Little Rock, Arkansas
    Problem is resolved, I got ahold of ECM tuning and sent my computer in for diagnosis and sure enough one of the drivers had failed. It showed no external signs of failure so they aren't always obvious when they go.

    Car idles great now and speeds up like it should with the a/c on
     
    AllanL and BLACK'98DSM like this.
  21. steve

    steve DSM Wiseman

    12,731
    518
    Joined Feb 3, 2002
    St. Charles, Illinois
    That was my experience as well. About a quarter of the ECUs I saw had driver issues that weren't obvious. That's why I build tools to test them.
     

    3K  0

    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
    Loading...
    BLACK'98DSM likes this.

Share This Page

Support Vendors who Support the DSM Community
Archer Fabrication ECM Tuning ExtremePSI Feal Suspension Fuel Injector Clinic Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications OHM Racing RockAuto SouthBay Fuel Injectors STM Tuned Track Decals Track Sculptures VR Speed Factory WheelWell.com