Another TPS Voltage question. 88

[88xltranger]

So my truck has always revved pretty hight on a cold start and after making various repairs to many things, it still does. However after all my repairs once its warmed up fully the idle is lower than it used to be. So low that it will stall when put into gear. I decided to check tps voltage. It has 5v where its supposed to but I'm reading almost 5v at closed throttle and .6v at WOT. Is that right? Shouldn't it be the other way around?

 

[Shran]

That does seem backwards. Should be between 0.6 and 1.0 volts closed and up to 4.5v at full throttle. I'm assuming you don't have your probes backwards?

See this link for adjustment procedure

 

[RonD]

Yes, reverse your probes, TPS uses a Pull Down circuit

Should be under 1v closed and 4.5v or higher Wide open

The TPS is a "learned" sensor, so computer can adjust to variations, takes a few drive cycles
There is an anti-diesel screw on the throttle linkage, if the TPS doesn't have slotted holes for adjustment this can be use to fine tune the voltage
Key on engine off
You want to raise closed voltage to .9 and WOT voltage to 4.5, so +.3 from current readings
See if you can rotate TPS first to gain the +.3v
If not adjust the anti-diesel screw, looks like an "idle screw" its not

Computer sets RPMs using IAC Valve, RPM is based on engine/coolant temp and input from Transmission sensor on automatic, and AC circuit if you have AC

TPS is not used for idle, TPS gives the computer a "heads up" to add extra fuel for acceleration faster than MAP sensor can, serves same purpose as accelerator pump on carb
TPS is also used to shut off fuel injectors while coasting, when RPMs are above 1,500, this saves on fuel
TPS voltage level is also used to compare to MAP voltage/frequency, as a comparison to see if BOTH are in sync

When you shift into R or D(actually out of P or N) the computer should "see" that and bump up RPMs by 75-100 depending on software, same if AC is on

But computer should open IAC Valve more if RPMs drop below warm idle regardless and its pretty fast so odd that engine stalls when put into gear, almost stalls and then recovers means computer and IAC are working, but maybe trans sensor is not

Its called the NSS, its on drivers side of trans just above shift rod, its a plunger type sensor, 4 or 5 wires, its also the Reverse Light switch

 

[88xltranger]

Thank you for all that information. Looks like I have some things to check. I'm not familiar with a Pull down circuit. Typically on my meter reversing the probes will read the voltage the same, it will just put a - in front of the reading.

 

[Shran]

My meters will read negative voltage the same way yours does then... I was just not sure if your meter was compensating for reversed probes or something.

There are sensors from other trucks that will fit but they rotate the opposite direction. 4 cylinder trucks are notorious for that, I have a pile of TPSs in my drawer and they're all slightly different. Usually if you put the wrong one on, the throttle blade simply won't turn it though, resulting in no change in voltage, or it'll turn it and break it.

Why yours is reading backwards is odd, assuming you're probing the wires correctly.

 

[88xltranger]

There are sensors from other trucks that will fit but they rotate the opposite direction.

That was my initial thought. This truck has been in my family for at least 10 years before I inherited it. My dad and brother have been known to rig things to work if needed. I dont know if they ever changed the TPS though. It seems to run more or less fine, it just seemed strange to read like that. Idle seems to be the correct rpm, maybe just slightly low. I was attributing the stalling to the "drop" as I describe it when I shift from park to reverse. Its very distinct. The truck has ALWAYS done that.

 

[Shran]

Hey, I'd take low RPMs on a 2.9 truck any day... all of mine have seemed to run on the high side for one reason or another. The automatics were especially bad, going from park to drive almost felt like neutral bombing it.

 

[88xltranger]

Im not a fan of automatics in general and this one does seem pretty bad. It just feels wrong. That is a pretty good description of what it does though.

 

[RonD]

For signal OUT wire test it using engine metal as ground, not TPS ground

Variation of the Pull Up and Pull Down circuits are use for a more accurate reading in circuits that can have varying voltages like in a Vehicle
So computer is not using an actual voltage number, but a difference in numbers, so 5v doesn't need to be 5v, could be 4.8v or 5.2v, and reference ground doesn't need to be 0v, or 0 ohms to vehicle ground, can be -.2v or +.2v

 

[88xltranger]

Thats the info I needed. I was using the TPS ground. Even still, that wouldn't make the reading completely opposite of what you would expect to see right? I can't make it out to check it correctly right now.

 

[RonD]

The signal return wire, grey/red stripe or black/white stripe(pin 46), is technically a 5v wire and a reference ground but different that the 5v pin 26

Most sensors only use the signal return wire, and then have a sensor return wire

The TPS and MAP sensors used the pin 26 5v wire

ECT, IAT(ACT), knock sensors just use the pin 46 "voltage", 5v

I know confusing, lol

Very good reading here: http://www.fuelinjectedford.com/

These pages have been moved and saved over the year
Check out the Sensors "main page" then each sensors page
There are some mistakes but very good read generally on how things work in the older Fords

 

[88xltranger]

Just reading a few sentences in and it already makes much more sense and why my readings were opposite of expected. Definitely different than what I'm used to working on.