1994 Sidekick RPM Question

[fixkick]

on a perfect running engine and idle controls work right, and idle control ISC is working
and you set the duty to 50% (using the bleed screw)
if the TPS idle switch is not closed at 0vdc there will be no idle controls.
The ECU must not be stuck in limp-home mode. ( no CEL codes stored or worse live errors)
The ISC must not be bad , stuck or jammed, or , leaking air closed or  dead coil inside.
No huge illegal air leaks.



the bleed as set to 50 % dutycycle on the coil of the ISC,  using scope or duty meter,  180f coolant is hot not 140f ever. the IAC closes at 150f so, that must happen first.
i made this drawing that shows all parts related to idle







the scope here is wrong, cold engine but the hot engine we turn the bleed until the ISC signal is 50/50  , high only 1/2 the time, 200 times  second it pulses



vertical scale above is 5vdc per division, the alternator is active ad 14.7vdc so looks like 3 division at 15v.   the IAS is Pulse width modulated, PWM.

[fixkick]

The ECU forms a servo action, a closed loop, control that has control of hot idle at all times foot off the throttle and idle switch in TB closed at 0vdc at the TPS pin called idle pin.
The ECU then takes command of idle and can get lost , (means loss of servo control)
if there is a huge air leak (aka, vacuum leaks) the air leak tiny is hidden by the ISC (and o2 sensor) and the pulses to the ISC go more closed to hide said leak.
if the combined (crude estimate) is a leak that adds up to more than say 400 rpm? the ISC (actually the ECU) looses control and goes dead. (8v car like to hunt idle. speeds illegally)
the HOT RPMs are then near 1000 or more.
THEN the next horror happens (hacking)
the owner of the car (or others) would then unknowingly turn the bleed screw in, and set the idle back to 800. ending all ISC controls forever.

the cure is find the leaks and end them then re-calibrate duty cycle. of the ISC.


in the last drawing, the MAP is not on your 16valve engine , the drawing does both engines 9/16v, and I have bypass lines drawn to show my tricky universal drawing.
The TV must be 99.9% closed, it is so closed in fact that water will not go past it , TB in hand, you pouring water to the brass TV valve. plate. (feeler gauge 0.0005" clearance)
All air leaks must be found and corrected.
once all air leaks are corrected the DUTY must be set, to 50% hot idle, never cold, never below 180F. coolant, (ECT reads 300 ohms hot)

all modern cars work the same way until car has throttle by wire,(no TV cables !)
many do not use solenoid ISC but use a stepper motor there, seen on vast jeeps. but work the same way exactly.

The test for the ISC is easy, (basic) is just get engine to 180f, idle switch closed, 0v, and overload the Alternator.
on M/T car, Headlamps on and blower heater on at same time or even rear defrosts glass heater on if present.
or A/C car , shift to drive. engine stalls or goes way to low not 800 RPM (750 is spec min.) ISC is dead. (means the servo is dead, why only remains)
on M/t same rules 750 is spec, min. allowed the ISC is touch slow and can take 1 second for it fully gain control of you actions.

that is all there is on ISC.
as any engine gets older vacuum goes lower, and the ISC corrects this, with more open , PWM, the Servo has limits on how many leaks it can hide.

[tmanstein]

Worked again last night.
The ISC is reading 19.6ohms.
I think that the TB Bleed screw is my problem. There is no screw, looks like maybe JB Weld in there or something. Not sure really what to do.
Currently, 1200-1300 cold and 800-900 hot. Thinking I may have to replace the entire TB, any ideas?

[fixkick]

Worked again last night.
The ISC is reading 19.6ohms.
I think that the TB Bleed screw is my problem.  There is no screw, looks like maybe JB Weld in there or something.  Not sure really what to do.
Currently, 1200-1300 cold and 800-900 hot.  Thinking I may have to replace the entire TB, any ideas?

the bleed missing is the proof of tampering and more.
if the ISC passes the hot wire test then , only the bleed needs to be addressed,  but only if 900rpm is a problem for you,.

when hot wiring any ISC do not get the polarity wrong. the coil inside has snubber DIODES  that blow up if hot wired backwards.
one pin is ground, on the connectors


unplug iSC
then ground the correct pin
https://fixkick.com/ECU/92-95MPI-eng-sch...e-huge.png

the black green wire is ground side B12
the blue black is  12vdc.A12

just ground the wire with green first.
then with 12vdc battery of some kind connect plus 12v to the blue pin , for one second, and see of ISC is alive. some are jammed.
some the rubber ring seat inside is a total wreck.