I've been following drm7's thread where he reads his codes and run some voltage and resistance tests on his TPS and since his symptoms are similar to mine I decided to run the same tests.

I get a code 122 which my code reader's book says "Throttle Position sensor below minimum voltage" and my Haynes book says "Sensor signal below minimum(possible short)"

My signal voltage is about .45 volts, a little below the 0.5 -1.0v it should read and my reference voltage is 4.5v again a little below the 5.0v the Haynes manual says it should be. So far so good it looks like it's the TPS.

When I test the resistance it starts at a little under 300 ohms but when I move the throttle I get no change at all. I tried connecting the meter in every possible combination and there's only 1 throttle shaft so I couldn't be doing that wrong.

What I was wondering was should my truck even be running or be drivable and shouldn't my CE light come on?

87' F-150 5.0L EFI

 

Don't understand code 122. IN 87 codes were still 2 digits. Anyway, the operating range of a TPS is .20v to 4.84 volts. So it has to go out of this range to set a code and turn on CEL. Unless the TPS is shot,in my experience the only way to see a TPS problem is with a recording DVOM or lab scope. Operate the throttle full range (KOEO) slowly and record or monitor its highest and lowest readings.

 

well actually the specs on the TPS @ idle settings is 0.66vdc to 1.22 vdc . and a max of 4.84 vdc @ WOT . optimum idle voltage is right around 1 volt .

 

Macgyver is right.

For best performance you should adjust the tps to .96 volts koeo.

Sounds like you have a bad tps.

 

When my TPS went bad it triggered no code. Voltage would climb as the throttle was open then near WOT it would fall to zero.. had a bad "spot" in the range..

 

I guess my post was misunderstood. The reference voltages are from factory charts and are operating ranges (highest and lowest acceptable value). Yes closed throttle voltages are higher than .20 (ideally around .80 ) Yes, not all glitches will set a code but the lab scope would have seen the mentioned drop in volts and confirmed the need to replace a TPS. I wanted my input to be a means of evaluating a good sensor from a bad one without having to spend money on a best guess. Sorry if I steered anyone in the wrong direction.

 

well in my case there was no guess work . my TPS was putting out 4.16 volts @idle setting . and i could get no ohms readings on it at all . so it was bad for sure ...

 

I was a little confused about wether it sould have 3 number codes at first but both the code readers book and my Haynes manual refer to them and made no axceptions for the year.

When I was getting the codes I thought at first I would get 2 numbers. It gave me 12 then I spit out another 2 the timing was slighly different so it could have been a 12. I think it still pointed to the TPS. I'll have to check when I go home for lunch.

As for the test where you can check for "flat spots" the Haynes manual has me check ohm's through the range of the throttle, idle to WOT. It was the easiest way because I could disconnect the TPS and get to the connectors. The other tests were harder because I could barely get the probes to find a connectionw/out having to expose the wire, which I didn't do. There was no change.

Anyway my theory is if the TPS is shot then the computer does't know I'm pushing the throttle and doesn't adjust fuel right away or it knows it's not working and is compensating. The parts stores in my little town were out of stock saturday and closed Easter so I have to order one today.

Hopefully it solves my mileage problem too. With the mileage I get I might as well have a 5.8L.

 

If you disconnect the TPS from the harness, and attach the DMM leads to the green wire's pin and the black wire's pin, at the TPS harness connector coming from the TPS, set the DMM to ohms, on the 10 scale, and rotate the throttle, yet the reading doesn't change, then the TPS is bad.

Take care,
-Chris

 

According to Haynes, on mine it's the 1k range otherwise it pegs the meter. Either way I'm changing it today and I'll know when I test it with the new one installed. Not to mention when I drive it.

Hopefully this will make my idle a little more normal. It idles about 800-1000 now and takes a good 4 or 5 seconds to come down to idle if I rev it to 3000 or so. It makes shifting a pain sometimes. I know it should be faster because the motor only has about 15,000 miles on it. The previous owner's mechanic probably didn't replace every little component on it when he replaced the short block. I'm sure the heads were done on it.

Thanks for the help,
I feel like I finally have at least a basic understanding of the tangle under my hood.

 

Got the new TPS in and what a difference. I barely have to touch the pedal and off I go. The only problem now is it still idles high. Higher than before. I can't seem to adjust it to come up to 1.0 volts. Actually I can't seem to adjust it at all. It reads .8v now. When I was putting it in there didn't seem to be much if any adjustment.

Think I'll hook up the code reader again.

 

well .8vdc is within specs . have you cleaned the IAC passages? also when the truck is running unplug the IAC and post what happens here (it should die).check the codes and post , we'll do our best to help ya figure it out


edit : oops . did you reset the ecm after installing the new TPS? if not the computer is still compensating for a bad TPS . disconnect the battery and turn the headlights on for ten minutes . turn lights off, reconnect the battery . idle it for about 5 minutes . then 5 mins @ 2000 rpm . turn it off ,let it sit a few mins. start it again and drive it agressivly for about 10 minutes . bring it back and see how it runs then . then read the codes .

 

Bingo.

 

Sorry for the late reply, phone problems

Anyway I disconnected the battery before I dug into the throttle body and turned on the light's. I was pretty sure before that doing that resets the computer. I'll try resetting it again and go through the routine you outlinne in your last posts.

Although the next day after my phone died (or I would have replied sooner) and fixing some water hose problems (crappy factory hose clamps) I drove it around for a couple of hours and the next morning it eventually diled at around 800 revs purr minute and the throttle responds at the merest thought while driving. I think we've got most of the problem licked.

Although my mechanic showed me today while installing some shorty headers I bought (too lazy to do that myself) that the wires to the o2 sensor on one of the original manifolds were clipped. He hadn't done the other side yet. I didn"t know there was more than one. He said they help tell the computer if there is a problem on one side of the motor or the other, like a bad injector or something and should set off the check engine light, which it hasn't. Maybe it's not the original manifold. I know the o2 sensor on the Y-pipe is good since I had my exhaust from there back replaced last year including the sensor.

The headers were just a less expensive fix for a cracked manifold. I don't expect any gain in perfomance from them. In fact I'm pretty sure I'll lose a little low end again.

Thanks again and I'll post my new mileage when I figure it out.

BTW up until now it's been about 8-10 mpg