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Throttle positon sensor


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8 replies to this topic

#1 mikec03

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Posted 25 September 2009 - 09:34 PM

I adjusted the TPS in order to try and correct hesitation at low rpm's on a '95 2.2 automatic. It is easy to do since the idle air valve is energize to close, and I can hear it pick up as I move the TPS clockwise from the extreme counter clockwise position. It was out of adjustment by about 1/8"!

It seems to have reduced the hesitation! The question I have is why would it help? I see from one of Skip's post that the TPS also sends a positon signal to the computor. But what could the computer do with this information?

#2 Fairtax4me

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Posted 25 September 2009 - 09:46 PM

It reads the angle of the throttle plate. It's used to adjust fuel mixture, ignition timing, transmission shift points (on automatics), just to name a few.

The proper way to adjust the throttle position sensor is with a multimeter. I would check the adjustment the correct way before calling the issue "fixed".

#3 mikec03

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Posted 29 September 2009 - 02:46 PM

After doing some searching this site, I found the the procedure for setting the throttle position sensor. It basically requires that the TBS be set to actuate [the top two contacts] at 0.028 to 0.035 in. with a feeler gage inserted into stop for the throttle.

This made a world of difference in the hesitation and acceleration of the car. I wonder why this isn't the go-to solution when someone posts a hesitation problem [and yes I have read the sticky].

PS. I used the sound of the idle solenoid actuating, instead of a multimeter, to set the TBS. It's very difficult to get the multimeter on the TBS plug outputs. If someone wants to tell me why this is not correct, I'm open to understanding.

#4 Log1call

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Posted 29 September 2009 - 09:23 PM

The idle valve works on a modulated pulse width, that is to say, it is getting turned on and off hundreds of times a second and the ratio of off time to on time is being altered to make the valve wider open or more closed. It doesn't actualy get turned on, or off in the traditional understanding of the word as it would normaly be applied to a solenoid. Whatever sound you heard wouldn't have been the solenoid opening or closing. That's the first problem with trying to use the valve "opening" as your clue to the throttle position contacts opening.
The pulse width is happening all the time, even when the revs are up and you are driving.

You wanted to know what the TPI does, what it tells the ecu to do. Ok, It has a set of contacts in it that tell the ecu when the throttle is in the fully closed position. When the idle contacts are closed the ecu thinks it should be able to control the idle speed, if the contacts are open it disregards that idea. That means that if the contacts are closed, and the ecu can't regulate the speed(air leaks for instance) it sets an ISC trouble code. If the contacts are open though, it doesn't care because you are giving it throttle and it knows that's normal and not to worry, it doesn't set a trouble code.

When the contacts are closed, the ecu also sets the ignition timing to a fixed setting. Just what the setting is, depends on other factors like if the engine is at normal operating temperature, whether the engine temperature is too high, if the transmission temperature is too high, if the idle speed is getting too high or too low and the ISC can't control it adaquatly. It is set to a fixed setting though, unlike when you have your foot on the throttle in which case the ignition timing changes as load, temp, throttle position, knock sensor, air flow all change. It sets/locks the timing at the idle because otherwise the revs would hunt around as the airflow changed making the timing change making the revs change making the air... you get the idea I hope.

There is also another system in place in the throttle position sensor which is a variable resistor linked to the butterfly shaft, this sends varying voltage to the ecu as you move the throttle. That voltage is used to calculate the appropriate fuel mixture and ignition timing for the conditions and load at the time. The varying voltage also has a second function, it is monitored not only to see what voltage it is at, but also to see how fast it is changing. The rate and direction of change is used in the ecu to calculate whether you are putting your foot on or off the throttle and how fsat you are putting it down or off. This calculated figure is factored in to the fuel mixture to overcome the lag in airflow caused by inertia of the air in the manifold.

So...
When you adjust your TPi, you should firstly adjust it to set the contacts to close at the right position of the throttle. You should do that with a multimeter. Then you should check the swing of the variable resistor is still within the recommended voltage range. If both figures/settings aren't correct you can get problems.

Hope all that makes sense and helps. There is always a reason for there being a recommended way of doing things, even if it's not always obvious to us.

#5 edrach

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Posted 01 October 2009 - 08:50 AM

Nice write up Log1call! You should submit it to the USRM so it doesn't get lost.

#6 Log1call

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Posted 01 October 2009 - 03:31 PM

Thanks, it's not a complete picture but it hopefully answered his questions.

I am a mechanic... I don't know how to operate a computer so well as to submit stuff! I did keep a copy though in case anyone asks again.

#7 mikec03

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Posted 01 October 2009 - 09:34 PM

Wow! Who would think that the effect of the TPI was so complicated? Thanks Log1call for the explanation.

What I hear when I push the TPI open [and the power is on] is a buzzing sound coming from the area of idle air valve. I have often heard this sound from industrial solenoids when they are being energized.

#8 Fairtax4me

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Posted 02 October 2009 - 11:32 AM

On a side note... That's why it's bad to try to adjust idle speed by turning the throttle stop screw.

I made a post in the USRM for you Log1call. Just linked it to this thread.

Edited by Fairtax4me, 02 October 2009 - 11:40 AM.


#9 Log1call

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Posted 02 October 2009 - 03:52 PM

Whow, I'm flattered! Perhaps this would also be of interest too then, it's something I wrote for a local subaru forum and it's related to that butterfly screw we must not touch....

Getting an idle valve code can just mean that the idle valve hasn't been able to keep the idle speed within the range the ecu thinks it should be in. That though could be caused by air leaks, a faulty temp sensor, a faulty neutral switch or adjustment, a faulty throttle position sensor or adjustment, the throttle butterfly setting has been changed or the air bypass valve on the throttle body has been changed, there is a low compression on one cylinder etc... Anything that changes the idle speed can cause that code. Defective wiring or electrical faults can also cause them.
If you are sure the valve is clean and all the wiring looks good, have a look at the butterfly adjusting screw and see if the paint on it looks untampered with. Have a look at the air bypass screw on top of the throttle body and see if that looks like it has been tampered with. Check the throttle cable is returning properly and check the throttle position sensor is adjusted properly. The TPS does wear and need adjusting every so often. Use an electrical gauge and check the temperature sensor readings. Then, and only then......

For the ISC valve to operate properly you need to have it near the centre of it's travel at the ecu's programmed idle speed. If anyone has played with the butterfly adjusting screw, the idle bypass screw on the throttle body(if it has a screw), or the idle control valves solenoid on top of the valve, then the valve can be at the end of it's travel at idle speed and that makes them unresponsive, which causes surge and erratic idle. Only try adjusting it if you have checked everything else first.
The first thing to adjust is the butterfly. The butterfly adjusting screw is only there for one reason... to stop the butterfly digging into the throttle housing when it's shut and possibly wedging shut. The best way to adjust them is with the throttle body off so you can see what is happening but if you just back the screw out till the butterfly touches the wall of the housing(you may need to back off the cable while you do this), then turn the screw back in again till it just starts to open the butterfly. There is no specifications given for the setting but about an eighth of a turn of the screw open should be plenty. Less woulld be ok, as long as the butterfly isn't touching and isn't going to touch when the lock-nut gets tightened or the engine warms up.
Next thing if it has it is the solenoid on top of the valve. Set it to the centre of it's travel if the screws have slotted holes.
We will perhaps be adjusting this again later.
Now you need to connect the car to a laptop or scan tool. The ISC valve should have a duty cycle of 40 at idle if everything is correct. By correct I mean the motor is in good mechanical condition, there are no air leaks, the temperature sensor, throttle position sensor and neutral switch are all working properly.
If the duty cycle isn't at 40 or close to it at idle then you need to adjust either the solenoid or the bypass screw on the throttle body. If you have it, it's best to use the bypass screw. If you don't have the bypass screw then you need to use the solenoid. Adjust till the D.C. is close to 40 with the motor warm and idling. Once it seems right, rev the motor to approx three thousand revs for about one minute then let it back to idle and recheck the D.C. Repeat the adjustment till you find the best compromise between steady prolonged idle and the idle immediatly after the reving up. Ideally the setting should be the same for both situations.

If you think it's any use to you guys over there feel free to cut and paste.




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