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Drewgarth

Crank Angle Sensor and Idle Switch ('90 Legacy)

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Hi all,

i have a 90 legacy, where the check engine light is coming on intermitantly. Dealer tells me it is displaying codes for Idle Switch and Crank Angle sensor,....Dealer believies one is causing the other, but cant tell me which one unless the light is on when i pull up at the dealer....(easier said than done..believe me :-(

 

Dealter believes the most likely cause is the crank angle sensor, but tells me that the idle sensor isnt available anyway.

Does this sound correct? Is there any substitute part for the idle sensor if required?

 

The physical symtoms are

1)that the car occasionally has an eratic idel...with the revs suring up and down by 500 rpm or so when at a standstill.

2)When travelling along the freeway a trailing throttle and then an application of throttle results in a lurch...the only way i can describe it is that its similar to the Lockup Torque Converter kicking in and out continuously....This is painful when travelling in flowing traffic..

 

Hope someone can help me...

thanks in advance

 

cheers

Andrew

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Probably worth pointing out that if and when the check engine light does come on, it is always when i pull up to stop, at lights, in traffic etc etc

 

cheers

Andrew

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Idle sensor?? Never heard of one of those.... Could it be the throttle position sensor, which will detect a closed throttle idle situation?

 

Some cars do have a separate switch to detect closed throttle, but not Subaru EJ series engines.

 

The ECU is pretty good at identifying the crank sensor problem. The crank sensor (along with the cam sensors) controls fuel injection and spark timing, so it is rather important that it be replaced promptly.

 

Did the dealer download the codes? I would reset the ECU (with the code reader) and see if the codes return. If the crank sensor is bad the ECU should detect it in a very short period of time.

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Thanks :-)

Code number 42 is the idle switch according to http://www.legacycentral.org/library/codes.htm.

 

the dealer did download the codes and we reset the computer an within a day, both codes were back, so i dont know which one is triggering the other.

 

Im replacing the crank angle sensor on tuesday (i have to get a new steering rack done as well)...and we will see if that fixes it.

 

I will let you know..

 

cheers

Andrew

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The idle switch is contained within the TPS on the 1990-1994 legacies. You can adjust the TPS, and it needs to be done so by idle switch.

 

More instructions can be found here.

 

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/TPS_testing3.jpg

 

I would suggest you pull codes yourself. Instructions can be found on my site, and I also suggest you do the D-check mode, which may help you pin point as to the exact problem.

 

Once you see what the computer says, I'd then check the idle switch on the TPS and see if it is within spec per the link I gave above. If not, adjust it. Also, you may want to check the TPS, to make sure it is functioning properly. More testing instructions:

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/TPS_testing1.jpg

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/TPS_testing2.jpg

 

If TPS is fine, look at the crank sensor, or whatever other codes you find.

 

Please check out this post. It will give you links to where you can d/l the engine section of the factory manuals and tell how to test some of the various engine sensors.

http://usmb.ultimatesubaru.org/forum/showthread.php?s=&threadid=8399

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Wow, thanks :-)

thats a more detailed response than i was ever hoping for :-)

 

Cheers...I will have a look over the weekend and let you know what i find

 

Just out of interest, do you know what the physical symptoms of the TPS playing up are? or are there too many symptoms making it difficult to describe?

 

 

thanks again

Andrew

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Originally posted by Drewgarth

Wow, thanks :-)

thats a more detailed response than i was ever hoping for :-)

 

Cheers...I will have a look over the weekend and let you know what i find

 

Just out of interest, do you know what the physical symptoms of the TPS playing up are? or are there too many symptoms making it difficult to describe?

 

 

thanks again

Andrew

 

TPS problems include:

 

* idle speed way off or fluctuating

* engine not accelerating properly (sluggish or not smooth)

* engine knocking (due to too lean air/fuel ratio)

* black exhaust (due to too rich air/fuel ratio)

* other miscellaneous issues

 

The TPS measures how open the throttle is, which is in effect measuring what your foot does on the gas pedal.

 

If defective or incorrectly adjusted it may not know when the engine should idle (throttle closed), and not read your accelerator pedal inputs to know when to increase/decrease fuel flow.

 

With a defective TPS the ECU must use the Mass Air Flow (MAF)/ Manifold Air Pressure (MAP) and oxygen sensor to set the proper air/fuel ratio, but since these sensors do not provide the immediate readings that a TPS would, the engine response will seem delayed. The engine will not increase power output immediately when you press the accelerator, and it may also not reduce power immediately when you back off the throttle a bit.

 

If the sensor is improperly adjusted, your engine may not idle properly, or may be in idle mode for some of the early open throttle position, and you may not get the full open throttle (FOT) condition for maximum power output.

 

The best way to test the TPS is by plugging in an advanced diagnostic device which can display the TPS reading as you press and release the accelerator. The diagnostic device will also indicate when the TPS indicates idle mode. You want to see the TPS read idle at the proper time, and not early or late in the pedal travel, and also the TPS should read a smooth increase throughout the pedal travel range, and also read a FOT condition when the accerator is just about fully depressed (FOT occurs a bit before the pedal hits the floor). You should not have the engine running when testing and adjusting the TPS (its not required).

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OK checked out the TPS last night....

 

Its upposed to lose continuity with a 0.7mm feeler between the stop screw and the stopper... Mine was losing continuity a little under 0.6mm... So i have adjusted it....Now we will wait and see it might solve some of my problems :-)...such as fluctuating idle etc

 

 

Now the puzzling bit...The manual says to back probe the TPS for voltages...Only trouble is i get exactly the opposite voltages than whats in the manual..

 

According to the manual i should be getting 0.7 to 1.x volts with throttle fully closed and about 4.8 volts fully open...

 

Im getting about 4.8 closed and 0.9 fully open.

 

Is the manual wrong??? Because i cant imagine the TPS working at all if it was truly inverted.

 

cheers

Andrew

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Where in, what manual did you read the info?

 

In the links I provided on page 1 of the testing procedure it gives

 

4.7 closed

1.6 open

 

You're in the ball park, so it should be fine. Other thing you want to check is that the resistance changes smoothly as you open & close the throttle.

 

If everything checks out.....I'd say your tps is fine.

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Thanks Josh,

i was going off the link on your page

 

http://www.surrealmirage.com/subaru/engine.html#tps

 

which is the same as my manual...

 

After you pointed out that the Idle Switch was part of the TPS, i was using my manual (Haynes)

 

Unforunately, i cant check your links here at work...our firewall wont let me get out on port 8080...i will have a look at home.

 

But if your link says the opposite to my manual and the other link on you page. i would believe it...given thats what i measured :-)

 

cheers

Andrew

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Originally posted by Drewgarth

Thanks Josh,

i was going off the link on your page

 

http://www.surrealmirage.com/subaru/engine.html#tps

 

which is the same as my manual...

 

After you pointed out that the Idle Switch was part of the TPS, i was using my manual (Haynes)

 

Unforunately, i cant check your links here at work...our firewall wont let me get out on port 8080...i will have a look at home.

 

But if your link says the opposite to my manual and the other link on you page. i would believe it...given thats what i measured :-)

 

cheers

Andrew

 

Your south of the equator, things are reverse down there!

 

Your readings seem to be fine, my Haynes manual is reversed as well.

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Funny, you should mention the south of the equator bit....we were having that laugh at work :-)

 

Well i have been driving it for a few days now and no more check engine light...

 

Driving in flowing traffic is back to being smooth on a trailing throttle and the idle is better :-)

 

So looks like the problem is fixed...although because the fault was intermitant, i wont break out the champagne until i have driven it a bit longer :-)

 

thanks for everyones help :-)

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