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isc circuit problem 

 

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Posted: Fri Oct 06, 2006 11:55 pm 
Oompa Loompa
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dont know if it will be an easy fix for me, i dont know much about cars, ive only just started trying to fix things myself, sounds like you just about checked everything. will try some of the sugestions here, but if there was a problem with any other sensor it should come up on the ecu diagnostics id think.
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Posted: Sat Oct 07, 2006 12:18 am 
Getting Side Ways
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The ecu diagnostics will only report a bad sensor if it is short circuit or open circuit therefore in reality becomes practically useless unless its a 'hard fault'
If a sensor say engine coolant temp sensor is meant to read a certain voltage at 50 degrees, lets just say 2.5 volts for example, but in reality your sensor is giving the ecu a reading of 2.5 volts at 35 degrees, the ecu thinks the temp is 50 degrees, therefore will fuel the engine accordingly, but in reality it will be running a bit leaner than it should.
Because the voltage is still within a 'normal' range, the ecu wont log a code, the only way to find it is backprobe the sensor, or ecu pin and take the measurement and compare it with what it should be.
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Posted: Sat Oct 07, 2006 2:23 pm 
Oompa Loompa
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i see what youre saying, sensor can become inacurate.
The throttle body is clean, backprobed the TPS with ignition on, it checked out ok, backprobed the MAP sensor for voltage thats ok, only got a cheap multimeter which hasnt got frequency but can get one so will check it better later.
hard to tell if the valve is moving in the ISC, i pulled the valve open and turned ignition on but couldnt tell if it moved or not, dont know if that means it didnt move or that it moves so little its hard to notice, then did the opposite, pushed the valve close turned ignition on but still could tell if it moved.
then i took the ISC off, pulled the valve open and bolted it back on, disconnected the wiring for it and started it up, it still idled low but it started.
then i connected the wiring back on the ISC and it wouldnt start, disconnected it again and it started but not as well, rpm was going up and down for a few seconds, which is what happens anyway, until it stablises, maybe the valve moved slightly, i dont know.
while the engine was still running cold i disconnected the ISC and it didnt seem to make any difference, ive got the idle bumped up otherwise the engine would stall.
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Posted: Sat Oct 07, 2006 2:27 pm 
Getting Side Ways
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ef_ford wrote:
while the engine was still running cold i disconnected the ISC and it didnt seem to make any difference, ive got the idle bumped up otherwise the engine would stall.


Theres your answer.
Try borrow another ISC and do the same test again, see if it makes a difference. If it does, it is the ISC, if it doesnt, id be looking back toward the ECU.
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Posted: Sat Oct 07, 2006 4:44 pm 
Getting Side Ways
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Ride: 98 EL Falcon - Last of the EL's

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it might be as simple as the Battery or s**t house battery terminals, i know fron exp the my isc hates it when the battery gets a bit low or the terminals are crap or if i dissconnect the battery for longer then a minute.

i replaced the terminals a couple of days ago and it made a noticable differance even though the car was running good in the first place.

just food for thought.

 

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Posted: Sat Oct 07, 2006 9:50 pm 
Oompa Loompa
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yeah.. i might try replacing it but the voltage in the circuit isnt right so im not sure if it will work properly.
The battery is relatively new, think it was changed either early this year or late last year.
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Posted: Thu Oct 12, 2006 6:10 pm 
Oompa Loompa
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back probed MAP sensor, checks out ok.
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Posted: Sat Dec 16, 2006 10:09 am 
Oompa Loompa
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just to update i replaced the isc with one from a wrecker and reset base idle but still the same and i backprobed the isc again but this time the other wire not the red one and when i put the aircon. on the voltage dropped from 13.6v to 12.6v so the ecu seems to be signaling.
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