Okay, so today I got the chance to mess with the TPS. [91 F-150 5.8 for those who need reminding] I just pulled it off and reinstalled it, just to make sure I did it correctly this time. I made sure that there was spring tension on the TPS as I installed it.

But I'm still not getting the voltage readings I should be. Which brings me to stupid question #1- when checking the voltage for the TPS, should the ignition be on or off? My service manual says "With the ignition F..." I just assumed that was a misprint and was supposed to say "ignition OFF." Of course, silly me, I didn't think to check to the voltage with the ignition on, either.

But that was backprobing the harness. After making sure the TPS was installed correctly and still not getting a reading, I decided to check the voltage straight from the harness. I unhooked the connector [it's the half round 3 pin type] and grounded out the VREF pin, and still didn't get any readings.

By the time I got home it was after dark, so in the morning I'm going to check codes. If 121 comes up again, I think I'll need to start tracing the TPS wires. But I suppose I should know if I'm even checking it right.

Thanks guys.

 

The key needs to be on or the 5 VDC will not be there.
Do not ground the VREF (5VDC) pin.

 

Okay thanks.

Just to be clear tho, I meant I checked the VREF pin against ground- meaning I was touching a lead to the pin and grounding the other lead. It wasn't direct contact to ground.

 

Alright, so I'm getting 0.6v at closed throttle, which according to fordfuelinjection.com is within limits. It's the lowest value acceptable, but it's within limits. Full open throttle reads at 4.4v- which is slightly lower than the table on FFI.com

The codes I'm getting still point to the TPS as well. KOEO produced 121 again [closed throttle voltage higher or lower than expected]. A new code out of CM is 452, which is insufficient input from vehicle speed sensor. And KOER generated codes 121 and 412 [cannot control rpm during KOER high rpm check] again.

So where should I go from here? Trace the wires from the TPS harness to make sure there's no bad ground or connections?

Thanks again.

 

Do you have 5.0 VDC VREF voltage?

 

Yeah I forgot to mention that- exactly 5 volts from the VREF pin when checked against ground.

 

The 0.6v closed throttle reading may be the problem, if the TPS won't swing any on it's attaching screws adjust the thrrottle stop screw to raise this to 0.8-0.9v and clear the codes again.

 

Well it would appear you had the right idea, Conanski. My TPS is non-adjustable so I ended up adjusting the stop screw. I adjusted it to where the volts just barely changed from 0.8 to 0.9v. Right now it's idling smooth as I'm warming it up and the idle is up to 650 RPM like it's supposed to be.

So we'll wait and see what happens, but I think I'm okay now. I will give the codes a check after I do some running around with it and see if anything comes up.

Thanks guys- your advice is much appreciated.

 

Well I had to do some more adjusting with the stop screw. Once the engine got warmed up, I took it out for a drive. Didn't even make it to the first stop sign and the engine went nuts. I had to shift into neutral so I could get it stopped safely [we just had our first snow last night and so there's accumulation on the roads], and the idle jumped to 950 RPM! KOEO and CM were system pass, but KOER threw out 412, 213, and 411.

So I readjusted the screw so I was reading 0.8v, reset the codes, took it out for a drive and nearly the same thing. It's just way too fast.

So after getting it back home again, I readjusted so it was reading 0.7v, and that appears to be my magic number. But I don't want to jump to conclusions again so we'll see what happens. I reset the codes again and after I get the chance to drive it around some more I'll pull the codes again. But so far, so good- and it's idling around 450 RPM again, which is where it was at before I installed the new TPS.

 

Personally, I wouldnt have messed with the Idle stop screw, I would have elongated the other screw hole on the TPS too allow for the necessary adjustment.

 

Yes you should never touch the throttle stop screw. Very hard to put the throttle stop screw back where it needs to be. His voltages were OK. When you have 0.6VDC closed throttle WOT should be 3.36 VDC or above and his reading were OK.
You just remove the neg battery cable for 15 minutes and the the computer will relearn the closed and WOT voltages.

 

I tried clearing the computer so it could relearn the voltages and it didn't do any good, which led to messing with the stop screw. I'm not too concerned with it tho- the truck is running just fine now. No sputters or bucking while driving or at idle.

Thanks for the advice tho.

 

No it won't, these numbers are programmed in and cannot be changed without a tuner. The purpose of the stop screw is to set the closed throttle position so the IAC valve operates at around 30% duty cycle when the TPS is in the closed position and produces a voltage between about 0.6v and 1v.

 

Paul O.
OK I had to stop and look this one up.
A tuner can not change this one either.
I do not know where you got your information but it seems to be wrong.

What happens is the programming compensates for any differences in sensors, readjusting to a base voltage when the throttle is closed.

When the computer sees that the voltage is 2.76 volts above the base voltage it takes this as WOT.

 

I got tired of jacking with my TPS (weird readings) and decided to bite the bullet and buy a new one. Autozone has them fo $26 w/2 yr. warranty. Half as much as Napa and O'rielly. If I would have known this earlier, I wouldn't have wasted so much time. But hey, I learned a lot from you guys from the experience.