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Thread: Affect of idle switch on stock idle ECU

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    Affect of idle switch on stock idle ECU

    I tested the effect of the idle switch on the stock idle ECU. What it does is set the minimum amount to where the idle motor will close. When idle switch is grounded the idle motor will close a little more than if the switch is open if your RPM is still to high. The unit still runs closed loop with the idle switch open or closed and of course with the switch open you will get vacuum advance with a warm engine at idle. I don’t know why they decided to have the ECU do that function other than open the idle motor a little more when your foot is on the gas.

    The stock ECU will never fully close the idle motor (my ECU does fully close it if your RPM is still too high).

    By the way, new 6 pin connectors for the idle ECU or RPM relay are available from mouser.
    PN 571-88029-1
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #2
    Senior Member Christian Dietrich's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    I tested the effect of the idle switch on the stock idle ECU. What it does is set the minimum amount to where the idle motor will close. When idle switch is grounded the idle motor will close a little more than if the switch is open if your RPM is still to high. The unit still runs closed loop with the idle switch open or closed and of course with the switch open you will get vacuum advance with a warm engine at idle. I donít know why they decided to have the ECU do that function other than open the idle motor a little more when your foot is on the gas.

    The stock ECU will never fully close the idle motor (my ECU does fully close it if your RPM is still too high).

    By the way, new 6 pin connectors for the idle ECU or RPM relay are available from mouser.
    PN 571-88029-1
    I wonder if thats been my issue? Been wanting to get this upgrade next! Seems like a great unit!

    Sent from my SM-N920P using Tapatalk
    Vin 11035 wide stripe, flat hood, 5 speed, Spec 1 exhaust, custom grey/black interior, custom lighting, custom stereo and custom alot of stuff!

  3. #3
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    Dave - what it does it increases the idle speed when not pressed.

    The reference voltage is higher than with switch pressed.


    for most people only one thing is important: the idle switch does NOT turn the
    idle motor/system on or off !

  4. #4
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Elvis View Post
    Dave - what it does it increases the idle speed when not pressed.

    The reference voltage is higher than with switch pressed.


    for most people only one thing is important: the idle switch does NOT turn the
    idle motor/system on or off !
    I found with my bench test it still holds the 775 RPM setting with or without the switch input grounded. For my ignition simulator I have a switch to set 675 RPM or 875 RPM so when I select 675 RPM the idle motor will swing open until it's at max. When I select 875 RPM it will swing the idle motor closed until at max. It does that in both modes of the idle switch grounded or open but with it grounded it will close the idle motor a little further than if the switch was open.

    Now if you were testing on the car, it probably would increase the idle RPM but you would not be running closed loop because the idle motor is open to far.
    Last edited by Bitsyncmaster; 08-20-2018 at 01:40 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  5. #5
    Delorean Guru
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    Wither it actually turns the idle system on or off really isn't important. It "deactivates" the function by moving the idle motor to a closed or almost closed position so it doesn't affect the motor and isn't trying to change the RPM's. The power of the idle system is very dependent on a closed "tight" system with no air leaks (false air) bypassing the idle motor. The more vacuum leaks the less control the idle system can have. BTW, closed loop and open loop refers to another system, the feedback loop consisting of the O2 sensor, the Lambda ECU and the frequency valve. Whole different system.
    David Teitelbaum

  6. #6
    Administrator Ron's Avatar
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    I'm confused -- I understood that it was the Thermistor that signaled the ECU to step the idle up (separate internal circuit) and the Idle Switch (closed) enabled the ECU to monitor the RPM and adjust it by rotating the ISM as needed.

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Ron View Post
    I'm confused -- I understood that it was the Thermistor that signaled the ECU to step the idle up (separate internal circuit) and the Idle Switch (closed) enabled the ECU to monitor the RPM and adjust it by rotating the ISM as needed.
    I think the thermistor has to be very cold to trigger the high idle mode. I will try to test that value when I get a chance.

    From my testing the ECU always tries to hold 775 RPM with the idle switch on or off.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
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    It might be helpful to re-read D:05:01-D:05:04 in the Workshop Manual. It explains in detail how the system operates. The thermistor activates at 59 degrees. So not only must the motor be cold, the ambient temperature must be low enough too. You are not going to see any fast idle in the summertime. The manual says when the idle micro is activated the idle ECU "begins operation" doesn't say turns it on or off, just that it starts working.
    David Teitelbaum

  9. #9
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    It's a PID controller.


    The thermistor als influences the P-part of the controller coupled into the circuit with a diode.
    Once the temperature is lower -> voltage higher -> the speed setting increases

    Thermistor and idle switch circuit influence the I-part of the controller. Looking at the schematic
    makes me think it just kicks the idle motor open when pressing the idle switch - momentarily,
    not permanently like the thermistor when cold.

    The D-part also adds to this point -> D-part is meant to "catch" the engine when speed
    suddenly drops - it prevents the engine from dying - for example at an intersection when pushing the clutch.
    If you don't trust me - just unsolder R107 or T8 and drive the car.


    The manual is not describing it in details, it is written to satisfy a car mechanic
    and give him a basic understanding for trouble shooting.
    Last edited by Elvis; 08-21-2018 at 03:28 AM.

  10. #10
    Senior Member NckT's Avatar
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    The ecu uses pulse width modulation on both the open and close outer pins to effectively operate it to fine position control. This is done by transistors the rapidly switch the negative side(s) of the idle motor. The centre pin of the idle motor has the 12v supply. Usually when the idle motor windings go short, it blows one of the transistors with the result of idle at 2000+ rpm due to switching fully open, or instantly stalls due to switching fully closed.

    As Elvis says, it uses a PID control for this (as near as, as quaff op anps are used for comparitors). The Setpoint is the fixed 775rpm value using the switched ignition coil side as the Process Variable ie speed reference.

    The thermister is a variable resistor dependant on temperature, and if the resistance is above a certain value then this increases the Setpoint ie increases the Idle speed in relation to the temperature. You can substitute a variable resistor (0-100k) in place of the thermistor and make the engine speed go up and down.

    When I bench tested the ones I fix (20 plus repairs), on key in ignition 2 pre engine start the idle valve fully positions open to aid starting.
    With engine idling with the throttle closed microswitch engaged, the idle motor modulates as my first paragraph in this post.

    With the variable resistor test, I've found repeatedly on numerous cars that adding 10k ohm in series with the thermister makes the idle increase in colder weather better. Mind you this is UK weather and works ok on mine in winter.

    I hope the above helps.
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