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Thread: idle air control test procedure

  1. #11
    Senior Member Rich's Avatar
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    Thanks for the DMC-specific ISM resistance info, Dave. My reference page was generic.

    Apparently there's nothing electrically wrong with this motor. No opens, no shorts.

    Clean the valve internals twice more, using carb cleaner. Then check its operation again. Something's probably causing the valve to bind when the motor tries to move it.
    March '81, 5-speed, black interior

  2. #12
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Rich View Post
    Thanks for the DMC-specific ISM resistance info, Dave. My reference page was generic.

    Apparently there's nothing electrically wrong with this motor. No opens, no shorts.

    Clean the valve internals twice more, using carb cleaner. Then check its operation again. Something's probably causing the valve to bind when the motor tries to move it.
    If the thermistor was not connected it would only close the the idle motor (just key on no start) about to 3/4 open. If the thermistor is below 5 Kohms then the idle motor will close almost all the way (about .1" left open).

    I just bench tested this with a OEM idle ECU. I was not sure if it functioned the same without engine running but it does.

    As I have always been saying the thermistor just limits how closed the idle motor would go. It has no other function. That is how the design provides a higher cold start RPM.
    Dave M vin 03572
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  3. #13
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    If the thermistor was not connected it would only close the the idle motor (just key on no start) about to 3/4 open. If the thermistor is below 5 Kohms then the idle motor will close almost all the way (about .1" left open).

    I just bench tested this with a OEM idle ECU. I was not sure if it functioned the same without engine running but it does.

    As I have always been saying the thermistor just limits how closed the idle motor would go. It has no other function. That is how the design provides a higher cold start RPM.
    Hi Dave,

    I was out of town this weekend but last night experimented a bit with trying to "jump out" my thermistor. If you recall, cold I am getting about 8.0 on the 20K scale and hot (fans on) about 0.85 when measuring between black/yellow and black/slate on the idle ECU inner plug.

    I then pulled those two wires out of the plug and replaced them with a jumper so that the two pins on the ECU were joined. The results were inconclusive and I'm going to try again tonight.

    What would joining those two pins do? Would it allow the idle motor to close completely, or only close to the "fast idle" condition you mention? I want to be 100% sure I am effectively ruling out the thermistor switch and the thermistor plug and all thermistor wiring up to the idle ECU so I don't keep chasing that ghost or needlessly spend all day replacing the thermistor, draining coolant, refilling, bleeding, etc.

    Thanks a million!!!
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
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  4. #14
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    Hi Dave,

    I was out of town this weekend but last night experimented a bit with trying to "jump out" my thermistor. If you recall, cold I am getting about 8.0 on the 20K scale and hot (fans on) about 0.85 when measuring between black/yellow and black/slate on the idle ECU inner plug.

    I then pulled those two wires out of the plug and replaced them with a jumper so that the two pins on the ECU were joined. The results were inconclusive and I'm going to try again tonight.

    What would joining those two pins do? Would it allow the idle motor to close completely, or only close to the "fast idle" condition you mention? I want to be 100% sure I am effectively ruling out the thermistor switch and the thermistor plug and all thermistor wiring up to the idle ECU so I don't keep chasing that ghost or needlessly spend all day replacing the thermistor, draining coolant, refilling, bleeding, etc.

    Thanks a million!!!
    The jumper lets the idle motor close to almost fully closed (about 0.1" still open). I don't know why the stock idle ECUs don't close it fully. Just watch is for about 30 seconds with the key turned to on and engine not running.
    Dave M vin 03572
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  5. #15
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    The jumper lets the idle motor close to almost fully closed (about 0.1" still open). I don't know why the stock idle ECUs don't close it fully. Just watch is for about 30 seconds with the key turned to on and engine not running.
    OK great, so it sounds like the test I am attempting to do will simulate a thermistor in its "hot" state and will force the idle motor to the most closed state in regards to normal engine operation.

    If my idle is still elevated with this jumper in place, then I have effectively ruled out the entire thermistor circuit, do you agree?

    By the way, thanks for all this detailed information on the idle circuit. This information would be hard to find if it weren't you staying active on DMCTalk!
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  6. #16
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    OK great, so it sounds like the test I am attempting to do will simulate a thermistor in its "hot" state and will force the idle motor to the most closed state in regards to normal engine operation.

    If my idle is still elevated with this jumper in place, then I have effectively ruled out the entire thermistor circuit, do you agree?

    By the way, thanks for all this detailed information on the idle circuit. This information would be hard to find if it weren't you staying active on DMCTalk!
    Yes the ECU will think the engine is hot with that jumper. If it does not close than I would suspect the idle motor is damaged on the valve.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #17
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Yes the ECU will think the engine is hot with that jumper. If it does not close than I would suspect the idle motor is damaged on the valve.
    Hmm I am going to have to test this! I'll try it cold without the jumper but everything plugged in, then I'll try it with the jumper to see if it closes any differently.
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  8. #18
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    Hmm I am going to have to test this! I'll try it cold without the jumper but everything plugged in, then I'll try it with the jumper to see if it closes any differently.
    Here is what I found when following this test procedure.

    First I checked the 20K ohm value between Black-yellow and Black-Slate on the "inner" plug of the idle ECU (the one with only four wires on it).

    Stone cold engine but hot summer day in my garage - 4.xx on 20K scale (indicating the thermistor is warm enough due to ambient that no "fast idle" is required)

    Test 1 - I pulled out the idle motor but left it plugged into the black plug. I turned the key to ON (Position II) and quickly went back to look at the motor with a flashlight. I saw the "gate" inside had opened completely and after a few seconds it began to retract towards the left (as installed). It closed until there was but a small space the thickness of a slotted screwdriver blade remaining for air to go though.

    Test 2 - I then turned off the key and removed the black-yellow wire from the white plug (insert a small jeweler screwdriver under the female spade and push up while pulling the wire out to release the lock clip). This resulted in open-circuit and lack of continuity to the thermistor. I repeated the test and found the motor will only close about half way.

    Test 3 - I then removed all four wires from the white plug and made a new jumper that connected the terminals that the thermistor goes to on the idle ECU itself, then plugged in the other two wires (black-green and white-slate) being very careful that they were secure and cannot touch. I turned the key to ON and checked the motor - exact same behavior as the first test, closing nearly completely.

    I reinstalled the idle motor and then ran the car up to normal operating temperature with the jumper installed. After about 20 minutes of idling my idle started to climb as I have been trying to solve for weeks now (thermistor totally removed from the circuit so now I know the thermistor is not my issue!). I pulled out the motor once again (quite hot by now!) and repeated Test 3 and found the motor closed to the same nearly closed position even though the idle had been about 1,000 RPM and erratic. So I believe I have effectively ruled out the idle motor itself as defective when both cold and hot.

    Thoughts on my logic?
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  9. #19
    Senior Member Bitsyncmaster's Avatar
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    Try hot gluing the deceleration valve closed.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #20
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Try hot gluing the deceleration valve closed.
    That's my next plan - actually first I might run a temporary ground from the Idle ECU directly to the battery, a temp ground from the RPM relay to the battery, and a temp ground from the idle speed microswitch directly to the battery. Maybe also from the ignition ECU to the battery. That should totally rule out any kind of weird grounding problem.
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

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