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  #1  
Old 12-06-2006
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Throttle Position Sensor Install

I know this is a silly question but well, it's, well, just me...

Anyway, I was poking around under the hood just to see where everything is, oh, it's a '97 XLT Super Cab w/4L OHV and about 98k miles on it...so I was poking around and decided to take a part off and look at it just for giggles. It didn't look like it did anything special so I just re-attached it then plugged it back in...

Turns out is was the TPS and I now have a CEL. So I went and bought a new code read cuz I should have one anyway. I am getting code P0122. OK, so now the question is, I find it hard to believe that the sensor went bad just by taking it off to look at it...I know stranger things have happened to me, as I carry around my own pocket Murphy Field most days. Sooo, is there a special way to install the TPS or did I do it right by just putting it right straight on and screwing it in. And by special I was wondering if it needs to be twisted a couple times to open the butterfly or whatever it's attached to?

If it really needs relpaced was it simply that it was going bad already and somehow disconnecting it screwed it up? Or are they that fragile?

Like I said this is a really basic question so pity me because I am stoopid!! :)

Thanks!!
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Old 12-06-2006
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and this is why i dont touch stuff that i dont know what it is. next time, keep your paws to urself and leave the poking to the professionals
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Old 12-06-2006
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FMD:

hehehehe...oh SURE you say that NOW, where were you to tell me to keep my paws off it the other day...and thanks...but REALLY I just disconnected the power connector then undid the screws and took it off...

Besides, toys are for tinkering with!!
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Old 12-06-2006
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With everything plugged in and key on, the normal closed throttle voltage on the TPS signal wire (gray/white) should be 0.50~1.25v. The P0122 code says that the voltage is less than 0.17v which is way below normal.

Obviously, the first things to check are that the connector shell is properly seated, the pins are not pushed back out of place within the connector and the wires are undamaged.

I don't have a shop manual for 97, so I can't give you anything definite. However, I do know that some Ford TPS installations are straightforward boltons and others require an extra step or two to ensure that the TPS engages the throttle shaft correctly.

You'll need to get access to a 97 service manual to know which you have.


Ford service info:
Quote:
P0122 - Throttle Position (TP) Circuit Low Input

The TP sensor circuit is monitored by the PCM for a low TP rotation angle (or voltage) input through the comprehensive component monitor (CCM). If during key ON engine OFF or key ON engine running the TP rotation angle (or voltage) changes below a minimum calibrated limit, the test fails.

A TP PID (TP V PID) reading less than 3.42% (0.17 volt) in key ON engine OFF, continuous memory or key ON engine running indicates a hard fault.

TP sensor not seated properly
TP circuit open to PCM
VREF open to TP sensor
TP circuit short to GND
Damaged TP sensor
Damaged PCM
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Old 12-06-2006
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Thanks Bob,

I'll check the voltages tomorrow and grab a Haynes or whatever shop manual tomorrow to check on the install procedure.
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Old 12-06-2006
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Did you unplug it while the engine was running?
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Old 12-07-2006
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deapee:

Nope, I know enough to know bad things can happen to electrical parts and you disconect them when the engine is on...maybe I should have disconnected the neg battery terminal first? That I did not think of doing.

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Old 12-07-2006
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Nah as long as it was off, you should be ok...

Even if it was running, you'd be ok, but that would explain the light...they take a while to turn back off...3 trip cycles actually.

So I dunno, double check your connection and go from there. Otherwise, I think they sell them at auto parts stores pretty cheap...maybe 30 bucks or so.
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Old 12-07-2006
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OK, here is the info I got this morning. I decided to go get the manual, they had a Chilton's (eh, good enough). Found out the TPS is a plug-n-play install with no special install procedure or fiddling. So, I went ahead and grabbed a new TPS since I was a at the store anyway.

When I got home i disconnected the old TPS and removed it from the car. The manual used the resistence measures on the TPS itself as a way to test the sensor. Seemed reasonable to me. Checked the old sensor and on the low side there was nothing, which would mean that side was dead (?) and the high side game a number in line with the manual. Check the new sensor, both sides showed resistence in line with the manual (sorry I forget the numbers but they likely don't matter with this method???)

Short story long, after installing the new TPS, CEL went off and truck was fine. So thanks a bunch to for the help.

Not so bad an adventure I got a new code reader (Innova 3100c), the shop manual I should have bought the day I got the truck and relpaced an worn out part all for about $150. Considering it would have cost me that much if I took it into the shop I think I came out ahead, though I'll be eating PB&J for a week to make up for spending the extra cash...

I was really suprised how cheap the scanner was at ICK, WallyWorld...$100. I know I coould have found the fancier 3110 on eBay for around $150 but i needed something now...

So, really, thanks for the suggestions and help!! It was just weird to me that the sensor died just from removing it to look at it. It must have been teetering on the brink of failing, yeah, that's what I say...it was dying already.
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Old 12-07-2006
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Thought I would drop back in and mention something I left out of my original post because I was unsure if it was related to the TPS problem or not.

The 2nd (maybe 3rd) trip adter the CEL came on my tranny started shifting VERY HARD. I thought "...WTF, now the tranny is going, what kinda POS truck did I just empty the bank account to buy..." There was also a double, sometimes triple, shift early from a stop. I was worried on one hand because I gave up my last Ranger because of tranny and drive train probs. But on the other hand I did buy an extened warranty for just such a problem.

After reading info here and around the web I got the idea of what the TPS really does and that the two problems might be somehow related. I figured if the computer is having a problem reading the throttle position it might have a problem shift correctly.

Since the problem went away immediately with the new TPS I am assuming the two issues were connected. Am I correct in my thinking?
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Old 12-07-2006
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Another self-inflicted CEL ends happily.
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  #12  
Old 12-07-2006
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...yeah, well, well...fine!!

I knoooooow I took a perfectly functional engine and had to "fix" something. But then again how ya gonna know how it works if ya don't look at all the shiny parts inside? Like they say if you wanna make an omlette you gotta brak some eggs...

Had a buddy in college took his '79 Formula Firebird (it was new then) and took a perfectly good car and took the motor apart to have it blue printed. Then what is had was a bunch of butcher paper on the floor of his garage with engine parts scattered over it.

So, it could have been worse!

BTW, I promise not to do anything this stooooid again...well, for at least a few months anyway... :D
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Old 12-08-2006
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Quote:
Originally Posted by grumpy3b
After reading info here and around the web I got the idea of what the TPS really does and that the two problems might be somehow related. I figured if the computer is having a problem reading the throttle position it might have a problem shift correctly.

Since the problem went away immediately with the new TPS I am assuming the two issues were connected. Am I correct in my thinking?
You are correct. The transmission shift point and pressure tables are heavily dependent on the TPS input. When the TPS value is questionable, the PCM ignores it and relies on a backup calculation that guesses the throttle position based on some other data. It is good enough to get you home but not as accurate as the actual measurement.
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Old 12-08-2006
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Thanks Bob....

It's interesting the way the cars today are so dependent on the computer control. A simple part, like the TPS, can induce what cuold appear to the the avg. guy, like me, as a problem with the tranny. It sure must keep some of the less then ethical shops out there in some cash...in the short term anyway.

They should really make these code readers required equipment with new vehicles. Had I not been reading here and around the web I might not have known to get a reader and see what the CEL light was from. For all I knew it was the tranny. And here it turns out the tranny will not trigger a CEL (or that is what I read somewhere)...who knew??!!
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Old 12-08-2006
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Dedicated transmission codes will not light the CEL but they will cause the O/D OFF light to flash in the cluster.
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Old 12-08-2006
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ahhhh...thanks for the tip...filed away for furture reference!!
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