1992 F-150 4.9 Liter 5 Speed With Air conditioning

Having troubles with the truck idling after it been driven 15 miles or so. Just got it out of the shop for the same reason. While in the shop they replaced a water temp sensor and an air sensor. Truck ran better but after 15 miles it still had idle problems but better than before.

At the shop they old me they found a number of codes in the computer but some codes are kicked off by other problems and they thought they had it solved with the two sensors. There was a code indicating the TPS was bad too.

Based on what the shop told me and from reading other post, I decided to check the Throttle Position Sensor (TPS). In the Haynes manual it said the voltage to the switch should be 0.5 to 1.0 volts at idle and 4.0 to 5.0 at full open. (Key on not running)

I inserted the negative probe in the top and the positive probe in the middle and read negative 3.61 volts at idle and negative 0.30 at full open. Based on this I went to the local Advance Auto and picked up a new TPS. Unfortunate after installing it and checking the volts, I get the same readings as before. So ether I have a bad TPS, I’m checking it incorrectly or I have another problem.

Any thoughts as to what may be happening?

Thanks
Bennie

 

There are three wires to the TPS,

5v reference wire
Signal wire
Ground wire

Sounds like you had the wires mixed up. Put the ground of the v-meter to the ground wire (should be black), and the positive to another of the two wires. It should have either 5v to be the vref or .8-.9ish (for it to be correct) if it's the signal wire.

When installing a new TPS it needs to be turned so the proper voltage at idle is achieved.

 

The TPS is spring loaded and I just put in in out of the box thinking this was the idle position. Should I have done something different?

 

Mine worked just fine right out of the box when I replaced it, but some here claim (Justin included) that you have to adjust them on installation. To do so, you leave the screws just shy of fully tight, then rotate the TPS against its installation screws until it gives you the proper voltages on the low and high sides. I'm not disputing that you have to adjust it, I'm just saying I didnt have to.

Jason

 

According to the Haynes manual, no adjustment is needed on the 4.9 liter.

 

Right - the Haynes manual says that *only* the 4.9L TPS doesn't need to be adjusted. This means that all others do.

 

Yes and lucky for me I have the 4.9L Straight Six

 

Really what it comes down to is the "ideal" idle voltage.

You can slap it on there and it'll probably work fine because the computer learns the voltages of idle/WOT....whether it be .6v at idle...or .9. The range is like .5v to 1v that will work...but optimal is about .9V.

My dyno guy even has a little gauge on his laptop that has green/yellow/red and adjusts the voltage to the red zone which is .96-.98 volts...which is from SCT dyno tuning software.

As long as it's in the ballpark it'll work fine.

 

Here is how i test my TPS.

Its a variable resistor, so i take it off, and put the ground on the ground wire, and the signal to the red on the volt meter, and i set it to ohms. And check for a consistiant resistance while i turn the TPS with a screw driver.
Useing an analog ohm meter will make it a lot easier to see if there are flat spots in the TPS.
When I put mine on, I try and put it back in the same spot it was before, usually there is some dirt there, and when i first start the truck, i push the gas pedal all the way down to see if it shuts off the Injectors, if it does. Then it should be right. Then i start it with no throttle, and check to see if it idles properly.

yes, as justin says, Ballpark will work fine.
The difference is so variable, that the computer will adjust it. Just as long as the idle and full position work properly.

 

Sounds like it could be bad reference voltage.

 

I don't know that it's a variable resistor...

It's called a potentiometer...which is "a device for measuring electromotive force or potential difference by comparison with a known voltage"...which in this case is a 5v reference wire (vref).


Same thing on the EGR position sensor.

 

Justin,

http://en.wikipedia.org/wiki/Potentiometer

I was an electrical engineering student for a while, but this links explains it better than I could. In all the EE labs I took, we never distinguished between "potentiometer" (or "pot" for short), and "variable resistor".

Since the TPS isn't generating a voltage to compare to the 5Vref, the computer is simply measuring the resistance, using the 5V across the TPS.

Jason

 

A key reason why I was careful in what I said and used the term "i don't know that it's a variable resistor"....incase its one in the same.

I'm no electrical genius...


well, i'm no genius when it comes to anything.


Thanks for the link

 

A pot is a resistor that is adjustable.

There is a difference between a pot and a variable resitor "circuit" wise, but they perform the same task with different methods.

A variable resistor is set and left, and pot is constantly adjusted, or changed.

Pots generally have more then just and in and an out like a variable resistor.
I can never remember which is which when wireing the damn things though, haha.
Ones ground, one is voltage in, and one is voltage out.
a variable resistor will just be in and out.

Took schooling for C++ =)

 

Talyn Is correct...... Anyways, sound like the ref voltage is bad ( bad circuitor connection). Also how is idle when cold?