ISC Adjustment & the mysterious green connector
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HT4100 4.1, 4.5, 4.9 Discussion, ISC Adjustment & the mysterious green connector in Cadillac Engine Technical Discussion; I was reading an old post of Sevillian about how he recommended using a Hayne's manual to help somebody adjust ...
  1. #1
    area310dude's Avatar
    area310dude is offline Cadillac Owners Member
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    ISC Adjustment & the mysterious green connector

    I was reading an old post of Sevillian about how he recommended using a Hayne's manual to help somebody adjust the ISC for "sail on" problems, and he mentioned stuff about grounding a green connector for the generator in order to disable it.

    PART A - Minumum Idle Speed Adjustment (page 4-15 Haynes)

    1. Warm engine to operating temp

    2. Connect tach

    3. Jumper terminals A & B of the ALDL connector

    4. Set the timing to 10 degrees BTDC (this is optional but its good to start
    from a clean slate.)

    5. Turn off engine and remove intake

    6. Ground the green test connector on the wiring harness at the rear of the alternator(gator clips and wire)

    7. Unplug the ISC motor connector

    8. Apply 12v to terminal C and ground terminal D of the ISC conn. This will fully retract the ISC plunger. NOTE: with the motor retracted all the way, the ISC plunger should not be touching the throttle lever. If it is, turn it clockwise until there is clearance between them.

    9. Start the engine and set the minimum idle speed to 450 by turning the minimum idle screw(behind ISC under throttle body)in the proper direction.

    NOTE 1: The Haynes manual says to set the idle to 525. I repeated this procedure 3 times at 525 and it still sailed until I read a post on here to set it at 450 according to the FSM. I havent confirmed what the FSM says but it worked! Try both if your curious.

    NOTE 2: The ISC plunger and minimum idle screw are 2 different things! So many seem to get this confused. The minumum idle screw is a pain in the ass to adjust but I made a nifty little tool for it pictured below.

    10. Remove the grounding wire from the green alternator test connector and turn off the engine.


    The original FSM mentions this point too . . . . . what might be the logic behind disabling the alternator? What does that have to do with ISC adjustment?

    Thanks!

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    Sevillian273's Avatar
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    Re: ISC Adjustment & the mysterious green connector

    It has nothing to do with the ISC adjustment. The above steps are for adjusting the minimum air RPM which is performed with the ISC plunger fully retracted from the throttle lever. The charging is disabled because if there is any load on the engine, this will throw off your setting for the minimum idle air RPM.

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    area310dude's Avatar
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    Re: ISC Adjustment & the mysterious green connector

    Thanks, Mr. Sevillian. I can see how that makes sense.

    Can I go further and ask your advice on my related topic:

    1) The throttle lever on my 4.9 does not rest on the min. idle screw when the ISC plunger is fully retracted.

    2) Following the procedure, I clockwise rotated the ISC plunger into the ISC motor about 1/4" until there was no thread showing above the ISC plastic sleeve. I think the plunger is now already at maximum depth, with any further screwing likely to break the motor.

    3) Replacing the ISC back on the throttle body, I see that the min. idle screw is STILL about 1/16" away from touching the lever and the throttle lever tang is STILL resting on the ISC plunger.

    4) The engine is able to re-learn its idle at 550 rpm, however.

    I have come to the conclusion that a previous mechanic must have really backed the min. idle screw off specification. I might as well remove the ISC entirely and just adjust the min. idle screw with the air cleaner off until I get about 525 rpm with no ISC at all. Does this make sense??

    I have also come to the conclusion that pretty much the ISC plunger and min. air screw adjustments are meaningless, and the real player in the car's correct idling comes from the TPS, which seems to be able to accommodate just about any improper tinkering that goes on with the ISC and the min. idle screw. (Except in the extreme situation of the min. idle screw being so extended it causes a very high idle).

    Does this statement also make sense??

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    Re: ISC Adjustment & the mysterious green connector

    Quote Originally Posted by area310dude View Post
    Thanks, Mr. Sevillian. I can see how that makes sense.

    Can I go further and ask your advice on my related topic:

    1) The throttle lever on my 4.9 does not rest on the min. idle screw when the ISC plunger is fully retracted.

    -Well It's resting on something. How do you know the plunger is fully retracted? Do you see a gap between the plunger and the throttle lever?


    2) Following the procedure, I clockwise rotated the ISC plunger into the ISC motor about 1/4" until there was no thread showing above the ISC plastic sleeve. I think the plunger is now already at maximum depth, with any further screwing likely to break the motor.
    -The ISC max extension section is the only part where you turn the plunger and this is to be done last of all. *IF* after retracting the plunger fully, it still touches the throttle lever, *THEN* you turn it clockwise until you see that critical gap between the plunger and the throttle lever.

    3) Replacing the ISC back on the throttle body, I see that the min. idle screw is STILL about 1/16" away from touching the lever and the throttle lever tang is STILL resting on the ISC plunger.
    -Well, assuming that the ISC is installed correctly, I'd suspect that the motor is not yet fully retracted. Minimum air cannont be set with the plunger touching the lever.

    4) The engine is able to re-learn its idle at 550 rpm, however.

    I have come to the conclusion that a previous mechanic must have really backed the min. idle screw off specification. I might as well remove the ISC entirely and just adjust the min. idle screw with the air cleaner off until I get about 525 rpm with no ISC at all. Does this make sense??
    -Makes sense but something still isnt right. As I said you should be able to gain that clearance without removing the motor. Either way, minimum air must be set correctly since this is what is used to center the TPS, and the ISC behaves as a direct result of what the TPS reads to the computer.

    I have also come to the conclusion that pretty much the ISC plunger and min. air screw adjustments are meaningless,
    -Not true. The TPS needs these adjustments to be in the proper range of operation. Once the the TPS is in its range, then at least in real-time they are sort of meaningless...

    and the real player in the car's correct idling comes from the TPS,
    -Yes but as I said, it must be in range. This range I'm speaking of is the relation ship between the TPS's physical installation angle and the throttle blade's angle (from closed to wide open).

    which seems to be able to accommodate just about any improper tinkering that goes on with the ISC and the min. idle screw. (Except in the extreme situation of the min. idle screw being so extended it causes a very high idle).
    -Again, as long as the amount of correction needed falls within the magical range.

    Does this statement also make sense??
    So, do what you have to do to get the minimum air speed correct, then you can move forward. Everything in this 3 part system hinges upon that.

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    Re: ISC Adjustment & the mysterious green connector

    OK those comments seem fair, but as far as I've seen, there has never been any clearance between the throttle lever tang and the ISC plunger.

    I believe this is true even when the plunger is fully retracted because I've used three methods to check for maximum retraction of the plunger:

    1) Forcing the plunger back manually with one hand, while pulling on the throttle pulley with the other hand to "trick" the ECM.

    2) Using the PCM buttons (A/C buttons) in the Driver's Information Center to retract the ISC.

    3) Simply turning off the car engine, waiting until everything cools off, and then observing for any gap between the ISC plunger and the throttle lever.

    Never seen any gap yet! So I guess I will use a copper wire to make a jumper for the green connector and then use a #25 Torx bit with a 1/4" box wrench to adjust the min. idle screw until I see 525 rpm. I will completely remove the ISC motor when I do this since it is interfering with everything anyway.

    Does this sound like a good plan ??

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    Re: ISC Adjustment & the mysterious green connector

    Yeah, before removing the motor, get that plunger in there as far as possible. When you are 100% sure that it's as far as it will go, screw in the min air screw until you do see a gap. Then, see what rpm you have. (Do all the RPM adjusting/checking with the motor unplugged.) After this, if rpm is at or below spec, then you would be right in suspecting that the previous owner unscrewed the min air screw.

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    Re: ISC Adjustment & the mysterious green connector

    I recently became the owner of a 1993 Sixty Special. It drives like a dream..until two days ago. The idle increases after a few minutes and the service engine light illuminates. The idle is definitely high. I have two codes 30 and 98 which both have to do with fast idling. These are the only two.

    I tapped on the isc and the idle dropped to normal. I drove it and the idle problem returned. When the engine is shut off, I notice that there are 3 short burst of noise which I believe I have isolated down the isc motor. I suspect it is trying to drive to full retract. Not sure, though.

    1. Can these things be cleaned/overhauled internally?
    2. Prices for them vary from less than $50 to over $150. That is a huge spread. What brands are preferrable?
    3. What controls the unit? Could it be getting false input? What part does the throttle position sensor play in this? Any?

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    drewsdeville is offline Cadillac Owners Master
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    Re: ISC Adjustment & the mysterious green connector

    Quote Originally Posted by keumnam View Post

    1. Can these things be cleaned/overhauled internally?
    Yes. Most common problem is a dirty closed throttle switch, located in the ISC. It sounds like that's your problem as well. When you let off the gas, the throttle hammer rests on the ISC plunger, closing the switch. The car then knows to idle down. If the switch isn't closing, the car will not idle down...

  10. #9
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    Re: ISC Adjustment & the mysterious green connector

    You can get an ISC motor for about $70. I went through three of them in 100.000 miles and ten years.

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    Re: ISC Adjustment & the mysterious green connector

    ISC motor adjustment/testing should be a sticky in this forum.

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