TPS issue and troubleshooting help

[Green67Stepper]

So I got in the truck the other day after not driving for a few weeks and the transmission was shifting into OD really quickly and then wouldn't downshift under throttle. (does downshift when I slow down and/or come to a stop). I noticed on my gauges that my TPS was 0% and the truck was running rough to so I assumed that was my issue. I also had P0122 and P0123 stored codes for High and Low TPS circuit voltage (CEL never came on).

I replaced the TPS and went ahead and replaced the pigtail as well. I first tested the TPS by using my multimeter to test ohms. As I moved the TPS, it varied from 0-5, like it should, with leads on the signal and reference wire of the pigtail. I hooked everything back up and then back-probed the butt splices to test more. The white reference wire is getting 5.1v with key on. With Key off, and the multimeter on continuity test, (one lead on Neg battery and one lead on TPS ground wire) the MM beeps and shows continuity.

So, the issue I can't understand is when I test the signal wire (blue), with key on, and back-probing on the new buttsplice, I get nothing. I know my new connections are good, but can't understand why I'm getting no signal, when my reference wire is good and the TPS tests good.

When I run the truck, TPS% always shows 0% now and once or twice while driving around last night it flashed to 100% for a second or two.

Where should I go from here? I should add, its a 2000 5.3l DBC with a 4l60E. bone stock with a re-worked factory harness.

[ls1nova71]

I would try ohming the wires between the TPS connector and the PCM connectors and see what you get. Try moving the wires when you do to make sure they are good and not broken inside the insulation somewhere. They just go between those two locations, so it should be pretty easy to track down a bad wire if that's what it is.

[Green67Stepper]

Originally Posted by ls1nova71

I would try ohming the wires between the TPS connector and the PCM connectors and see what you get. Try moving the wires when you do to make sure they are good and not broken inside the insulation somewhere. They just go between those two locations, so it should be pretty easy to track down a bad wire if that's what it is.

Thanks for the input. Obviously wiring isn't my strength. When doing that, Key on? Key off? Am I just checking for a break/short in the ground or signal wire? Every time I have tested the reference wire (white) I get 5.1v, so it has to be good. What confuses me is why I'm not getting no signal from the TPS blue pigtail wire when my TPS tested good, pigtail/connector is new and my reference wire tested good.

[ls1nova71]

Key off. Yes you're just checking for continuity between the plugs. Wiggle the wires around to see if you loose continuity, and also check to a ground to make sure they aren't shorted to ground. It sounds like the signal just isn't getting back to the PCM, and a broken wire could cause that.

[68c10airstream]

Maybe i can help as a past drive-ability guy. Computer supplies a regulated 5.0 voltage and goes to one terminal, and a ground is supplied to another terminal at the tps. The last remaining terminal is the return terminal going to the computer as a gas pedal position reference (computer needs to "see" the pedal position). Inside the tps this return reference is basically a "feeler wire", rubbing across a grid. The grid has ground at one end and 5.0 volts at the other end. As an example if your gas pedal (tps) was about half way open, the feeler wire would be rubbing across about a 2.50 volt position.

If the computer is not seeing any voltage, you have an intermittent connection, (open circuit) on the reference circuit.

I don't want to confuse you, but for all parties reading this i will continue into a little more in depth troubles.

If the computer is seeing full 5.0 supply voltage, the ground circuit getting to the tps has gone open circuit. Reason being is this; with the ground open circuit, inside the tps the grid that would normally have a voltage drop across it's surface is NOT a voltage drop anymore because it's an open circuit so the grid is 5.0 supply voltage all the way across the surface.

The same is true but opposite if the 5.0 supply voltage goes away because it's an intermittent open circuit. The tps grid is ground all the way across surface.

As a caution these issues can be dangerous!! Think of an intermittent 5.0 supply to the tps; you step on the gas pedal, 5.0 supply voltage goes away and the engine falls on it's face as you pull out in front of an oncoming car, only to get t-boned!!

A little more info is this; usually at idle the computer reference signal will be about 10 percent of supply voltage- .5 voltage (1/2 volt) and with the gas pedal floored (tps at max wide open) will be 90 percent of supply voltage= 4.5 volts. These values are just an approximate.

The tps will always be able to move a little farther than the gas pedal can, that way the computer should never "see 0 volts and 5.0 volts, so normally the reference circiut should always have a voltage present!

To sum up your situation, with no voltage on the reference circuit you have an open circuit at the tps area an/or the 5.0 supply voltage is an open circuit.

We have ls1nova as a great help here, eric is a real asset!!!!

[Green67Stepper]

Thanks, that’s a lot of great info. So here is what I did yesterday. I unplugged the computer and did a continuity test on all three wires, from the TPS pigtail to each of the corresponding PCM connector pins. All three wires showed good continuity so I would assume I have not breaks/shorts in any of the three wires. I called the guy that re-worked my harness and tuned the PCM originally and he seems to think that I might have damaged something inside the PCM which is causing a short. I did pull my PCM and give it to a friend to test on his new LS swap before his computer got tuned. When I got it back and went for the first drive is when I started having this issue.

[68c10airstream]

After doing driveability work for years my rule of thumb is this; i like to check circuits live with a voltmeter. Checking circuits for continuity is a great follow up after you have done the live voltage checks. Doing live checks is great because it's under load.

I'll use this as an example that usually helps; take a garden hose, full of water, nozzle on the end so water is not leaking out. Imagine taking a needle and poking into the hose in various places. Water and pressure are present. The nozzle when opened just didn't blast out of it like it used to. You did the static (at rest) (continuity) tests and checked fine. Pressure and water were present. What you didn't know was the hose was pinched almost closed. Now you test it in use and the upstream pressure is good, but the downstream pressure dropped off a lot. Problem is in between these two points. You never will find it at rest, but in use it's obvious. You can now relate this to checking a circuit in use. You poke an upstream circuit, it's good. downstream voltage check is low voltage and the trouble is between these two points.

I suspect what you have is poor connections at the tps. I'm not a programmer so your story above is worth considering looking further into. Work with what you have in front of you. If you still have your old tps, carefully cut it apart and save the metal terminal(s) and do what i call a connector drag test. Take the tps connector and slide it over back and forth on the connector in the harness. You should be feeling a slight drag. If not, you will need to replace/repair as needed.

With the vehicle running, pull the backside of the tps plug in harness back by sliding the weather pack rubber back to expose the metal connectors. 5.0 volts has to be there before you can go any further. If not, other items driven off of the 5 volt supply are dragging it down, been there, done that before. You would also need to check at the pcm for the 5 volt supply to be coming from the pcm.

During my jeep days, from memory i had a bad map sensor drag the 5 volt supply down, but the tps was the circuit with the fault with no voltage. Disconnected the map, 5 volts was now avaliable at the tps. Seemed weird because map should be more important than tps in a speed density fuel injected system.

[Green67Stepper]

Originally Posted by 68c10airstream

After doing driveability work for years my rule of thumb is this; i like to check circuits live with a voltmeter. Checking circuits for continuity is a great follow up after you have done the live voltage checks. Doing live checks is great because it's under load.

I'll use this as an example that usually helps; take a garden hose, full of water, nozzle on the end so water is not leaking out. Imagine taking a needle and poking into the hose in various places. Water and pressure are present. The nozzle when opened just didn't blast out of it like it used to. You did the static (at rest) (continuity) tests and checked fine. Pressure and water were present. What you didn't know was the hose was pinched almost closed. Now you test it in use and the upstream pressure is good, but the downstream pressure dropped off a lot. Problem is in between these two points. You never will find it at rest, but in use it's obvious. You can now relate this to checking a circuit in use. You poke an upstream circuit, it's good. downstream voltage check is low voltage and the trouble is between these two points.

I suspect what you have is poor connections at the tps. I'm not a programmer so your story above is worth considering looking further into. Work with what you have in front of you. If you still have your old tps, carefully cut it apart and save the metal terminal(s) and do what i call a connector drag test. Take the tps connector and slide it over back and forth on the connector in the harness. You should be feeling a slight drag. If not, you will need to replace/repair as needed.

With the vehicle running, pull the backside of the tps plug in harness back by sliding the weather pack rubber back to expose the metal connectors. 5.0 volts has to be there before you can go any further. If not, other items driven off of the 5 volt supply are dragging it down, been there, done that before. You would also need to check at the pcm for the 5 volt supply to be coming from the pcm.

During my jeep days, from memory i had a bad map sensor drag the 5 volt supply down, but the tps was the circuit with the fault with no voltage. Disconnected the map, 5 volts was now avaliable at the tps. Seemed weird because map should be more important than tps in a speed density fuel injected system.

What your saying makes complete sense.

I actually think I just figured it out. After reading all of that, you made me want to go out and test things one last time. I really looked over the PCM and noticed a few pins that were slightly bent, probably from connecting it over and over when I let my friend borrow it. After straightening them and testing everything again, i'm getting 5v reference from the white and a signal back from the TPS on the blue. I checked my OBD2 reader as well and i'm getting TPS response on it as well with throttle. Hopefully that's all it was. Once it stops raining i'll take it for a road test.

thanks again for all the feedback.