isc motor question
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HT4100 4.1, 4.5, 4.9 Discussion, isc motor question in Cadillac Engine Technical Discussion; my 83 eldo w/4.1, when the ignition key is turned on ,the isc cycles 3 times, enough to move the ...
  1. #1
    springmaker is offline Cadillac Owners Member
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    isc motor question

    my 83 eldo w/4.1, when the ignition key is turned on ,the isc cycles 3 times, enough to move the gas pedal/ is this normal?also in the colder months the isc would cycle when the motor was running during the first 10 seconds of running? could this be related to my earlier post of my car cutting out fora second when coasting downhill? I put another isc from the motor that went kaboom last year just to see if anything would change.. nope same story...thought the tcc could be messing up when coasting so I unplugged the harness,took a ride and same cutting out....can't notice it as much, probably because the converter isn't locked up. any Ideas?

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  3. #2
    DrDave is offline Cadillac Owners Fanatic
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    Re: isc motor question

    It's probably the throttle position sensor. Check the connections to the ISC and TPS, but that can be what happens if the TPS goes out or gets disconnected.

    Good luck!!

  4. #3
    Anthony Cipriano is offline Cadillac Owners Fanatic
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    Re: isc motor question

    Actually, if the idle speed control motor is moving like you say it's probably okay. If the TPS were very far off it would set a code in all likely hood. It sounds like the idle speed control motor may simply need to be adjusted correctly.

    With the engine idling, hold the throttle lever open slightly to get the RPM to about 2500-3000rpm. Simultaneously, depress the plunger of the idle speed control motor with your finger. The plunger should start retracting and continue retracting until it's fully retracted and stops. Disconnect the idle speed control motor wire connector to freeze it in the fully retracted position. Return the throttle lever to the closed bore position. The engine should almost stall or idle at 450-500rpm. There should be about .030 clearance between the throttle lever and the ISC motor plunger. Turn the plunger to adjust the gap until it is .030. Reconnect the wire connector to the idle speed control motor and cycle the key several times. Drive and report back.

  5. #4
    springmaker is offline Cadillac Owners Member
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    Re: isc motor question

    did as you instructed, car idled down to 500rpm, the isc plunger was screwed so far in it had about 3/8 inch gap. i adjusted it out to the .030 clearance. after, i cycled the key and it seemed much better, start the car it idled good in park{about 1100 no tach connected at this point}but when i pressed down on the gas pedal the idle rose up. way up, i stepped on the pedal again,pushing further and the idle raised up even further....any ideas?

  6. #5
    Anthony Cipriano is offline Cadillac Owners Fanatic
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    Re: isc motor question

    Quote Originally Posted by springmaker
    did as you instructed, car idled down to 500rpm, the isc plunger was screwed so far in it had about 3/8 inch gap. i adjusted it out to the .030 clearance. after, i cycled the key and it seemed much better, start the car it idled good in park{about 1100 no tach connected at this point}but when i pressed down on the gas pedal the idle rose up. way up, i stepped on the pedal again,pushing further and the idle raised up even further....any ideas?

    It sounds like the closed throttle switch inside the plunger of the ISC motor is not "making" when the throttle is closed or something is restraining or dragging on the throttle cable/accelerator pedal keeping some degree of pressure on it.

    You can use the onboard diagnostics to tell what's going on, now. With the engine idling, enter into the onboard diagnostics by simultaneouslly pressing and holding the off and warmer buttons until the diagnostics activates. In this mode, the little lites above the various climate control icons take on new meanings.They're indicators of certain ECM functions. Look at the light above the lo button on the climate control. It's on when the ECM is seeing the closed throttle switch closed ie. your foot is off the accelerator. The light goes out when the closed throttle switch is open ie. when you're pressing the accelerator. Watch the light and work the throttle. See of the light doesn't come on when you touch the throttle and release it as you described. If not, then the closed throttle switch is either not "making" when the throttle is released due to something holding the throttle or the closed throttle switch is failed, sticky or dirty. Replacing the ISC motor with a new one is adviseable at that point. If you open the throttle by hand and depress the plunger you can feel the closed throttle switch detent as it clicks - that's the switch that you're checking with the lo indicator light when in diagnostics and that's the switch that tells the ECM to move the actuator to control the idle speed. Without that switch operating the system doesn't know that the throttle is closed and that it has control.

  7. #6
    springmaker is offline Cadillac Owners Member
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    Re: isc motor question

    I ran system check on the climate control and the light did not come on,tried stepping on accel several times and still nothing,, back under the hood I pulled the harnes off the isc and checked it with a ohm meter and the switch operates as it should { opening and closing continuity wise}and is closing with the return spring, i.e. nothing hampering its operation, then i traced the wires under the hood for that circuit,{using a rather short on info haynes manual} black wire shows continuity to ground and the other{pink} wire in the circuit shows continuity to the firewall connector. did not check for volts at pink wire as not sure what would be normal..where to next? thanks in advance for all the help, I really should grab a factory manual, boy I miss working in a dealership{parts}access to data was sooo easy back then....

  8. #7
    DrDave is offline Cadillac Owners Fanatic
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    Re: isc motor question

    The reason the light isn't coming on is because the ECM sees that the idle is way too high (1100rmp?), so the ISC is detracting itself, trying to lower the idle. That's why the LO light doesn't come on, because the throttle lever isn't making contact with the ISC plunger. Check to see if the throttle lever is making contact with the ISC motor plunger--it probably is not (keep in mind, it has to be pressing the plunger enough to press the switch).

    If the throttle lever doesn't appear to be touching the ISC plunger, it's possible that you may need to adjust the mininum air screw. There is a little screw near the same area as the ISC, but hidden inside a hole on the passenger-side of it. Use a flat screwdriver to feel around (this may take some doing), and turn it counter-clockwise to lower the minumum throttle opening (and thus lower the base idle). You might want to check first that the screw is indeed holding the throttle blades open during idle--try to look for it, it's a small nub holding open the plate that's connected to the throttle lever (near/under the throttle body). You'll probably need a flashlight to see it. There's plenty of other instructions for this if you do a search for "minimum air" in the forums, but the setting should ideally be adjusted when the ISC is fully retracted and unplugged, and adjusted until the idle is 450-500 when warm/luke-warm.

    Side note--when you said the ISC was 'moving the gas pedal', did you mean simply moving it, or was it cycling so fast that you could hear the gas pedal clicking. If the gas pedal was making a loud clicking sound at startup, the problem probably lies with the TPS circuit. If not, check the minimum air screw, and also check that the throttle linkage cable isn't binding and holding the lever back, and that the Cruise Control mechanism isn't holding it back also.

    WHEEW!!

    I think if you follow that, you'll HAVE to find the problem somewhere.

    If anyone else has more to add, please do! I thought I'd save Anthony from having to spit out the general instructions again.

    Good luck!!!

  9. #8
    Anthony Cipriano is offline Cadillac Owners Fanatic
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    Re: isc motor question

    Wait. Time out. I don't think there's any need to tamper with the min air setting just yet. Per the previous post, "springmaker, did as you instructed, car idled down to 500rpm, the isc plunger was screwed so far in it had about 3/8 inch gap. i adjusted it out to the .030 clearance..."

    That sounds like the min air is fine if the engine idled down to 500 when at the closed bore position. I agree with DrDave in that the closed throttle switch sounds like it isn't closing which is why the idle is so high. When you open the throttle the idle speed control motor extends slightly so as to be ready to "catch" the throttle when you lift off the accelerator. As soon as it sees the throttle close via the closed throttle switch the idle speed control motor then retracts per ECM commands to control the idle. If the ECM is not seeing the closed throttle switch for some reason then it wont issue the retract commands and the system will do what you're experiencing. The idle speed motor plunger will be extended from you opening the throttle and then it'll hold the idle high at 110 because it doesn't retract because it never sees the closed throttle switch.

    Go back into the diagnostic mode with the onboard diagnostic system and see if you can manaully operate the ISC plunger to make the indicator light come on for the closed throttle switch. Turn on the key, enter into diagnostics (engine off) and see if the closed throttle indicator is lit. If not, open the hood and just depress the ISC motor plunger with more force and see if the light comes on. If the indicator is lit, open and close the throttle several times to see if it always comes back on. If not, then go press it manually at the ISC motor tose if you can override it and cause it to come back on. If not, it may very well be a wiring or circuit problem based on the fact that you have confirmed that the switch is operating per the VOM. The question is does the ECM "see" that contact. If you trace the wires to the PCM directly and check for continuity across the closed throttle circuit when the switch is closed then it may just be the ECM itself. If the electronic driver for the closed throttle circuit is failed the ECM willl be incapable of seeing the closed throttle switch.

    Are you positive that nothing can hold or catch the throttle when it returns to idle position against the ISC plunger? Retract the ISC motor plunger as before and let the throttle return to the closed bore position. It should idle at 450-500rpm as mentioned. Make sure that the cruise link is not too tight so as to restrict the movement of the throttle lever to the closed bore position.

    Even if everything checks out the closed throttle switch may be sticky and require more force than normal to close it when the throttle is released. Some people have reported using a secondary return spring to add force to the throttle when closing. If this works for a diagnosis tool then I would say that the idle speed control motor needs replacing as the closed throttle switch is starting to fail and is sticky.

    It seems to work when you deliberately manipulate the idle speed control motor plunger as evidenced by the fact that it retracts when you hold the throttle open and depress the plunger. When you depress the plunger it closes the closed throttle switch and signals the ECM to retract the plunger. So, the closed throttle switch seems to work then - and when you manually work it with the VOM attached. I think it's just starting to stick and that the throttle return spring does not have sufficient force to activate the switch on it's own.

  10. #9
    DrDave is offline Cadillac Owners Fanatic
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    Re: isc motor question

    Ah the spring... could be, I had to replace that on mine a couple years ago--it was having trouble closing the throttle switch.

  11. #10
    springmaker is offline Cadillac Owners Member
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    Re: isc motor question

    well , this problem seems to be solved, someone previous had replaced the tps and broke one of the screws off, seeing it was going to rain and the car sitting outside i figured i would pull the tbi unit off and fix the broken screw, and although the tps moved it was binding, due to the "hacked" install, put it back on and all is well "lo" light works,isc functions well and idles as it should, I was hopingthat would cure my other problem of the car cutting out momentarily when coasting downhill but that problem still exists,,, any clues where to start on that one?????

  12. #11
    Anthony Cipriano is offline Cadillac Owners Fanatic
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    Re: isc motor question

    I'm glad you found the problem. It's hard to predict what might have happened to a car/engine over the years but it really seemed like a throttle binding issue like you found. Persistence pays off, eh?

  13. #12
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    SoundAdvantage is offline Cadillac Owners Enthusiast
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    Re: isc motor question

    Thanks Anthony for your info on how to adjust the ISC motor, this has helped me adjust mine properly as it was way out of adjustment.

    Anthony your wealth of Cadillac knowledge is priceless!

    You should write a Trouble Shooting Service/Repair manual to market for all of us Cadillac Owners.

  14. #13
    peteski is offline Cadillac Owners Enthusiast
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    Re: isc motor question

    Yeah, Anthony Cipriano is 'da man!

    But this type of procedure is already described in the Factory Service Manual (HELM).
    So, there is no need for Anthony to reinvent the process.

    Peteski

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