Is my TPS bad?

[Fast64ranchero]

I did search, not saying I didn't miss the thread with the answer but here's my question for those of you that know, my car has not liked to idle for some time now, I've been thinking it was the tune, new tunes have been writen and have not cured the problem, the car just flat out hunts and dies, I found this thread that talks about TPS voltage

http://www.s197forum.com/forum/showthread.php?t=40020&highlight=idle+adjustment

So I checked mine, closed throttle with key on showed 0.7 volts, I'm thinking yes found the problem, compaired the GT500 TB to my stock TB, you can see light through the butterflies on the stock one, the GT500 you can not,
stock TB reads 1.27 volts. next I open the butterflies by hand and watch the volts, at 50% throttle the TPS read full voltage (4.51 volts) I put it on my stock TB and it reads full voltage at 50% throttle, WTF??? this can't be right can it??? everything I've ever worked with reads full voltage at 100% throttle not 50%, so I'm thinking two things,
1) TPS is bad and
2) throttle stop was not set correctly on the GT500 TB

here's where things go wrong, I tried to adjust the stop, and of course Ford locktights that so I break it off, fucking plastic crap,
any of you Guru's checked voltage like this? and yes I have read the threads that state don't fuck with it, so I don't need those comments, if any one can help me out, I'd sure like to hear what you have to say

 

[rojizostang]

subscribing. sorry i'm no help to you.

 

[MesaRec]

Did you break the adjustment screw??? I don’t think the TPS is bad, but it will need adjustment. If you replace the TPS, you will have to adjust it anyway.

Since I started that thread, I've learned a bit more about the TPS and how it works with the PCM. If I was to guess, I would have to say that my voltage with a voltmeter probably would have been around where yours is on the GT500 TB before I started tweaking the screw, but I didn't measure it before I started to adjust it initially. Point is that my voltage was too low. Well, since you read the thread I won't go into all of my measurements and adjustments as it’s posted there.

Since then though, I've readjusted the screw, and I will explain how. But first a little background info. The voltage you measure with a voltmeter is not the voltage that the PCM sees (more on this in a minute). Also, the TPS is actually 2 units in one. One with a positive slope (voltage rises when the throttle opens) and one with a negative slope (voltage lowers as throttle opens). So TPS1 has a negative slope, and TPS2 has the positive slope. When you measure the voltage with a voltmeter, you are actually measuring TPS2. The spec that each should have are TPS1 should have 4.1V KOEO and TPS2 should have 1.2V KOEO (throttle closed on both accounts). The screw actually adjusts both at the same time. Tightening it will lower the TPS1 voltage and raise the TPS2 voltage, and vice versa for loosening the screw. Also, the voltage slopes for each TPS unit is different. The rise over run is greater with the TPS2 compared to the TPS1. This is why you get max voltage on TPS2 a little over 60% throttle. Now with all of this in mind, you need a tuner to see what the voltages for both TPS units are that are reported to the PCM.

Like I said, the voltage on TPS2 measured with a voltmeter is not exactly what the PCM sees. Measuring that way, I actually got different readings depending on where I grounded the voltmeter (battery, engine, body). So what I did was set up my tuner and laptop, and I data logged the KOEO voltages. This is going to give you the true voltage readings that the PCM reads. I was able to set TPS1 to 4.116V while TPS2 was at 1.216V. Remember that as you adjust the screw, the voltages change in opposite directions.

Another bit of info on how the PCM uses both units has to do with the slopes. TPS1 has full range, meaning that its voltage is minimum at full throttle (negative slope). TPS2 voltage maxes out at about 60% throttle. Now to explain why. The PCM uses TPS1 to read the actual throttle angle. TPS2, since its voltage range is crammed into a smaller sweep, has a greater resolution at throttle under about 60% and is used to sense small changes in throttle. The PCM uses both to determine how fast the throttle opens and by how much. Obviously TPS2 is designed primarily with normal part throttle driving in mind to provide good throttle response.

Since I've adjusted the screw while data logging, a voltmeter shows TPS2 at 1.235 Volts, while the PCM actually sees 1.216V. You might think that the small difference doesn't affect the PCM. However, the PCM response to a resolution of 0.004 Volts. You want to make sure that the PCM sees a voltage lower than 1.250V, because it starts counting the voltage at this starting voltage and expects it to be lower. Whatever the PCM sees at KOEO it sets to closed throttle, and a measured 0.004 voltage change on either TPS unit is considered part throttle.

Bottom line is that if you drive an automatic, you can really tell a difference in how these adjustments affect the car. I believe the PCM uses the TPS1 and TPS2 data to determine when to shift at part throttle. Since adjusting mine, I can tell that the car is much more responsive and shifts at higher rpm at part throttle. Also it's quicker to downshift with less throttle. Definitely makes the car more fun to drive.

Hope this all helps shed some light on how to adjust the TPS.

 

[dysan]

Dude, that's a great write up. Thanks for taking the time for that!

 

[Fast64ranchero]

Thanks for the write up, yes I broke the adjustment screw, that explains it, I just ordered another gt500 throttle body, I'll put it on and talk with the tuner again!