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How does an ISC work? 

 

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 Post subject: How does an ISC work?
Posted: Sat Mar 29, 2008 10:31 pm 
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Wanting to fix the chronic "rev hang" inbetween gear changes ive tried swapping the isc.... no fix

Ive disconnected the isc and turned up the throttle body screw until it held idle, and runs perfect when warm, BUT first few minutes of a cold start the revs drop too fast between gear changes and thumps into gear

So im thinking about leaving the isc connected, but blocking off the gasket area whilst turning up the screw to hold idle, although i would like to know exactly how it works as i can see its wired up to the injectors and a better understanding i might come up with a better solution.

Yes im running a manual ecu, and have tried other ecu's to no avail.
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Posted: Sat Mar 29, 2008 10:40 pm 
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yeah my revs drop quickly when freshly cold. once it warms up its normal

 

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Posted: Sat Mar 29, 2008 10:43 pm 
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here is something to try.
disconect your isc with the car running turn the idle screw down untill it just stalls (about 450 rpm and plug the isc back in).
i know the manual states the idle speed without the isc should be something like 600 rpm but try it it worked for me.
also you can make a gasket for the isc with the air by pass holes smaller than the original holes this also seems to work..
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Posted: Sun Mar 30, 2008 8:13 am 
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TROYMAN wrote:
here is something to try.
disconect your isc with the car running turn the idle screw down untill it just stalls (about 450 rpm and plug the isc back in).


yeah tried that, then when i plug the isc in and rev it, it stays at 3000rpm. As it is, the idle screw is only just turned enough to take the weight of the metal 'tab'

The smaller gasket idea might work, will try
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Posted: Sun Mar 30, 2008 8:59 am 
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s**t 3000 rpm thats crazy.

have you tried making the isc gasket holes smaller?
this will restrict the amount of maximum air flow past the isc .
so when the isc is fully open there is less air flowing which = less rpm at idle
even though you have a manual ecu you should check the voltage on the gear position switch aswell.(Neutral 0V-0.1V, In gear 5Volts) as with manual gear box there is a gear position switch to tell the ecu if your car is in gear or neutral.
depending on what model your car is there might also be a switch on the clutch pedal this is also conected to the neutral/gear curcuit..
and again if the ecu gets the wrong voltage from the switch it will think the car is idling in gear. and it will bump the idle up..

otherwise it sounds like you have a vacuum leak.
also check your pcv hose and valve for external leaks.
the 2 things i mentioned have worked for me and a few others..
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Posted: Sun Mar 30, 2008 9:09 am 
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yeah all this mixes up with another issue.

the ecu always sees 5 volts, so it thinks its in gear. I have tried wiring up the neutral switch to the box as per Voxace's guide, and i have also tried bypassing the neutral switch and joined the two wires together with the same result

The car starts and runs per normal, but still sees 5 volts. These two wires must be the correct ones, as if theyre not connected to the neutral switch or if their not joined together, the car wont crank over.

Im thinking the smaller gasket holes wont work, as from what i gather, the car always needs more air on a cold start, but less air when its warmed up to idle properly without rev hang. Making smaller holes will make it run fine when warmed up as its decreasing the air flow, BUT will affect idle when cold.

In other words, it will be too hard to replicate the change of airflow needed when the engine is hot or cold
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Posted: Sun Mar 30, 2008 9:23 am 
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FordFairmont wrote:
yeah all this mixes up with another issue.

the ecu always sees 5 volts, so it thinks its in gear. I have tried wiring up the neutral switch to the box as per Voxace's guide, and i have also tried bypassing the neutral switch and joined the two wires together with the same result

The car starts and runs per normal, but still sees 5 volts. These two wires must be the correct ones, as if theyre not connected to the neutral switch or if their not joined together, the car wont crank over.

Im thinking the smaller gasket holes wont work, as from what i gather, the car always needs more air on a cold start, but less air when its warmed up to idle properly without rev hang. Making smaller holes will make it run fine when warmed up as its decreasing the air flow, BUT will affect idle when cold.

In other words, it will be too hard to replicate the change of airflow needed when the engine is hot or cold


the car does need more air to idle when cold. but what does your car idle at cold now with the isc??
and with the smaller hole in the gasket the isc still will work the same it just restricts the max airflow.
i think its worth a try..
its not hard to remove the isc and make a gasket from thin card board.
if it dont work fair enough and it wont cost anything to try..
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Posted: Sun Mar 30, 2008 9:35 am 
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TROYMAN wrote:


the car does need more air to idle when cold. but what does your car idle at cold now with the isc??
and with the smaller hole in the gasket the isc still will work the same it just restricts the max airflow.
..


Just took it for a drive, very cold start, and it ran perfect..... no rev hang and idles at 800rpm. But i know once it warms up it will rev hang, and sometimes hover at 1000rpm for a couple seconds when slowing down at a set of lights.

I dont want pay 150 dollars for a new isc and find it does the same thing. But yes i understand what your saying and will give it a go today, but will have to wait till tommorow morning to test it when cold.

Maybe a resistor inline with the power wire, might make it only open 85% for example instead of 100% all the time might also be a solution?
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Posted: Sun Mar 30, 2008 12:58 pm 
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sounds like your just fixing the simptoms, not the problem.

go to a wreakers and get a second hand one, cost you $30. and if its no good, take it back, then you know to look at something else thats not working

 

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Posted: Sun Mar 30, 2008 1:30 pm 
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i pulled off the isc and it looks to be dead, valve doesnt move at all, its not seized or dirty, its just buggered....... it appears that the airflow had opened the valve and just stayed there all the time.

So until i can afford a new isc, ive made a new gasket to block off the isc whilst leaving it connected.

However, I assume the position of the valve in the isc gives off certain voltages for the ecu, i wonder how the ecu uses this info?

While ive got the isc blocked off, should i leave the isc valve shut, or in the open position, again assuming the ecu uses this info...... i guess trial and error will sort it out

Tell me im wasting my time, but so far no-one has really answered the thread title yet? Yes the isc valve bypasses air around the throttle body valve, but what does the ecu do with the voltage readings from it? The ecu defintely has other ways of controlling idle
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Posted: Sun Mar 30, 2008 2:06 pm 
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For anyone who's interested how these things work, heres a detailed writeup...... your looking at the stepper motor type.... i think :)

http://www.autoshop101.com/forms/h26.pdf

After reading this, im worried that a new isc still wont fix the problem, as the ecu has special parameters for the isc when car is in neutral, and like i mentioned, since the manual conversion i cant get the ecu to see the correct neutral voltage regardless if im using manual ecu or bridging wires

Which leads me to, what voltage does the ecu need to see for neutral? And would putting a resistor inline with the ecu wire reduce the 5 volts its seeing, down to the correct neutral voltage..... but then i guess the driveability would be affect if the ecu sees neutral all the time?
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Posted: Sun Mar 30, 2008 2:09 pm 
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have you done a diagnostic on the ecu to see if there was any fault codes?
as faults from other sensors (power steer pressure sensor,airconditioning,gear position,tps and map sensor) can all have an efect on how the isc controles the idle. eg; if the power steering pressure switch had a fault it may give a signal that your steering is turned and will push your idle up to compinsate for the load of the power steer pump..
also the ecu supplies the different voltage to the isc to tell it how much to open..
and afaik the return signal is to determin if the isc is within its operating range. if its not it should give a fault code..
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Posted: Sun Mar 30, 2008 2:19 pm 
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FordFairmont wrote:
For anyone who's interested how these things work, heres a detailed writeup...... your looking at the stepper motor type.... i think :)

http://www.autoshop101.com/forms/h26.pdf

After reading this, im worried that a new isc still wont fix the problem, as the ecu has special parameters for the isc when car is in neutral, and like i mentioned, since the manual conversion i cant get the ecu to see the correct neutral voltage regardless if im using manual ecu or bridging wires

Which leads me to, what voltage does the ecu need to see for neutral? And would putting a resistor inline with the ecu wire reduce the 5 volts its seeing, down to the correct neutral voltage..... but then i guess the driveability would be affect if the ecu sees neutral all the time?


the voltage for neutral is 0V-0.1V,
i noticed that you say if you disconected the gear position switch the car wont start??
are you sure your playing with the right wire??
have you checked the voltage at the ecu? or at the point where the switch is?
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Posted: Sun Mar 30, 2008 2:21 pm 
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car wont go into diagnostic mode

keeps getting 500 range codes, relating to not being in neutral etc

ChipTorque said theres a way to force it into diag mode but wouldnt tell me how. He said they came up with a coolant temp sensor error, so ive replaced it, plus ive rewired the f**k, and still wont go into diag mode.

If a new isc doesnt fix it then i give up, theres no more wires to test for the neutral switch. Ive even ran the ecu wire straight to the gearbox AND tried bypassing the switch, and the f**k still sees 5 volts......
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Posted: Sun Mar 30, 2008 5:56 pm 
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gee you have had some issues with it..
it might be a long shot but you might need another ecu??

im out of ideas??
maybe someone else can shed more light on the subject..
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