Have YOU checked your Throttle Position Sensor Today???

[daeron]

have you checked the TPS??

 

 

I noticed that my 3000 post mark is coming up, and since I have gabbed at you dear people SO much in the last year and change, I thought I would do something fitting for the occasion. 6-7 posts per day is alot, especially as long as my average post is...

 

So, since I figured that 3000 RPMs is a common "dead spot" for TPS troubles to start showing themselves, what better subject? Here is my little TPS write-up, complete with cut out from the 89 FSM fragment.

 

 

The throttle position sensor is an integral part of almost any Electronic Fuel Injection system; our EA82s being no exception. There are two types of TPS: potentiometer style and two contact style.

 

The older, two contact style, is essentially a simple pair of switches: one switch is closed when the throttle plate is closed, indicating "idle" position to the computer; one switch is closed when the throttle plate is all the way open, indicating WOT; when neither switch is closed, it indicates "cruising." This has no relevance to the Subaru community, AFAIK; do the EA81 Turbo cars have a pot-style TPS or the switch style?

 

The TPS in any EA82 Subaru is a potentiometer, essentially a "volume knob." Everyone has had stereos with "crackly" volume knobs, right? The most common mode of failure for this type of TPS is just like the janky volume knob. Basically, you are looking for a "crackle" in your "volume knob." This is USUALLY manifested as either a "surge" or a "fade" at a certain throttle position. Oftentimes, this fade or surge seems to be tied to an approximate engine RPM, but further inspection reveals where it truly lies.

 

 

The above diagram is for the Naturally Aspirated, SPFI TPS, but the principles are the same for ANY potentiometer style TPS, including those used on the N/A MPFI system, and also the Turbo vehicles. For that matter, the same concepts should apply directly to EJ series vehicles, and honestly, just about any FI car this side of 1980 you come across. You may have a three prong TPS without an idle sensor switch (I think the turbo cars DO have a 3-prong) and in that case, pretend that Terminal A is not there, and perform only the "Throttle Opening Signal" test. The TPS will be located on the throttle body, probably directly opposite the throttle drum (the bit that the cable attaches to) just like it is on the SPFI throttle body; it is just in a different location.

 

There is a possibility that the actual resistance values that are "correct" for the turbo are not the same as the SPFI unit, and there is a chance that "C" and "D" are swapped. However, the important part is where it mentions continuous increase and decrease as the throttle opens and closes. Measure while operating the throttle as slowly as possible, and watch your meter for a "blip." Digital multimeters will work for this, (make sure you are in the right resistance range, Kilo-ohms.) but REALLY an analog meter, with a sweeping needle, is best. The needle should sweep slowly and smoothly, and ANY jarring, jittering, or jiggery of the needle is an indication of a "crackly" spot.

 

Another potential difference is that some vehicles operate the TPS backwards, with a DECREASING resistance coinciding with opening the throttle. In fact, I KNOW that some MPFI Subarus run them backwards, but am unsure which.

 

Comments?? Corrections?? Questions???

 

This write up is going to be immediately submitted to the USRM, as a link to this thread in the archives, so discuss it up!! if anyone feels like making an appropriate "cut-out" diagram from the FSM for EA81T, EJ series, or EA82T, Post it up! I will gladly go back and edit my original post to reflect any corrections or additions.

 

EDIT

any appreciation is appreciated, but lets try to keep off-topic posts to a minimum in this thread, since I posted it specifically to link to for the USRM. If anyone wants to detail replacement procedures, great; if anyone has any corrections or expansions, great, but if you have questions about your car, please just start a new thread.

[LPGsuperchargedBrumby]

Dude.....hate to be the one to say this but you talk alot....

 

3000 posts.... thats alot, makes my 350 or so look pretty sad

 

....and some ppl on here have asked me why i want a disty&carbyed ej motor (abet on LPG/propane)?

 

ECU's are evil capsules of doom and all devices wired to them were invented by the devil for the express purpose of causing misery to mankind.

[GeneralDisorder]

It is a lot - considering he just joined a little over a year ago. Jeez.... have I really been here for 5 years? Good god.

 

GD

[GeneralDisorder]

ECU's are evil capsules of doom and all devices wired to them were invented by the devil for the express purpose of causing misery to mankind.

 

I rather like their tendancy to be boolean with respect to failure. If you have the right tools, and the knowledge needed to perform the proper tests then it's a simple process of elimination to get at the cause of the problem. Solid state components are much less likely to exhibit partial, or intermittant failure modes like many mechanical systems tend towards.

 

But as with most things in life - often what works for one person/situation is entirely unappropriate for some other. There are places on this earth where I would rather have to fix a mechanical system than try to source a solid state replacement part.

 

My policy personally is to use the right tools for the right job, and to know as much as possible about as wide a range of systems as possible so as to be confident in whatever equipment I happen to be using. If EFI is appropriate (as it often is in most developed nations), then I learn about EFI. If I were in lower-armpit Africa then I'm confident my knowledge of carbs and their fuel systems would serve me just as well. Sometimes all it takes is an Ethiopian in the fuel line to ruin your day.

 

GD

[calebz]

 

ECU's are evil capsules of doom and all devices wired to them were invented by the devil for the express purpose of causing misery to mankind.

 

 

Thats exactly how I feel about carbs. ECUs are easy to diagnose/fix usually.. No moving parts!

[crazyhorse001]

Excellent writeup! This can be used on a variety of cars, not just Subies. The vast majority of TPS are 4-wire potentiometers.

I've never seen the 2-contact TPS, but it sounds like a PITA to deal with.

[Cougar]

Welcome to the 3,000 club Daeron. Good writeup on the TPS sensor. "Obi-Wan has taught you well", young Jedi of the Soob.

[daeron]

Dude.....hate to be the one to say this but you talk alot....

 

YAR, I know. But I don't watch TV; I would far rather Stimulate synapse growth through communication and research than stifle it through mindless input without processing.

 

ECU's are evil capsules of doom and all devices wired to them were invented by the devil for the express purpose of causing misery to mankind.

 

No they aren't; they control things to a much more precise degree than a Toilet tank :-p (no offense intended, you know my feelings about YOUR vehicles.. LPG FTW) Besides, if you have electronic ignition then you have a solid state computer in your car anyhow; may as well have a computer that controls fuel and spark together. Some systems are far better than others; give me the soob hitachi SPFI or good ol Bosch L-Jetronic style any day.

 

Excellent writeup! This can be used on a variety of cars, not just Subies. The vast majority of TPS are 4-wire potentiometers.

I've never seen the 2-contact TPS, but it sounds like a PITA to deal with.

 

Actually, the "two contact" TPS is a much SIMPLER critter to deal with.. but it is a MUCH More primitive beast, and allows much less computer control of fuel mixture. As far as coping with failures, though, it can only be in one of three states: Idling, cruising, or WOT..

 

My policy personally is to use the right tools for the right job, and to know as much as possible about as wide a range of systems as possible so as to be confident in whatever equipment I happen to be using.

 

GD

:headbang: :headbang:

 

Daniel-san paint fence. Paint fence good, Mister Miyagi?

[misledxcracker]

I think my TPS talks to me. Even when I'm sleeping and the car's parked half a mile away :-\

[Gloyale]

The older, two contact style, is essentially a simple pair of switches: one switch is closed when the throttle plate is closed, indicating "idle" position to the computer; one switch is closed when the throttle plate is all the way open, indicating WOT; when neither switch is closed, it indicates "cruising." This has no relevance to the Subaru community, AFAIK; do the EA81 Turbo cars have a pot-style TPS or the switch style?

 

The TPS in any EA82 Subaru is a potentiometer, essentially a "volume knob."

 

 

Note about comments in red:

 

EA81T and Early EA82T(flapper Maf) use the 2 switch type, not a potentiometer. They are a much simpler design, easy to fix(you can open them up and clean the contacts). But they do not provide the precise capability of the newer Potentiometer style.