2005 Silverado - 5V TPS circuit help

Hi all,

I've been working on a 2005 Silverado for a bit now, but I'm not getting anywhere and am hoping someone might help figure out what the heck is going on here.
Intermittently, the truck will go into reduced power mode and set code P0220 - TPS2 circuit fault.

I've attached the wiring diagram I'm working from and a lab scope screenshot.

The scope screenshot I attached is taken from the TPS 2 signal wire, at the throttle body. Other than the obvious oscillations to ground, minimum and maximum voltages are expected values. TPS 1 signal line looks normal. No pulsing to ground, just a strong steady voltage. If I scope the 5V line for TPS 2, I get a solid 4.9V signal with the same chatter to ground. I'm working from the assumption that my signal is oscillating because my 5V reference is oscillating.

Looking at the wiring diagram, this circuit is isolated to the TAC module, the throttle body, and the accelerator pedal. Diagnostic information from Identifix and testing confirm that the TPS 2 5V line is internally connected to the APP 2 5V line inside the TAC module. All wiring related to this 5V circuit (5V ref, signal, and lo ref) has been load tested with a light bulb. Minimal voltage drop was measured across the powered circuit for each line. I did this for the wiring between the TAC module and the throttle body, as well as between the TAC module and the APPS. No wiring issues were found. I also checked for shorts to ground on each line, and found nothing. At this point, I suspected the TAC module. The last step I did was to load test the power and ground wires for the module. Again, no issues found.

A new throttle body had already been installed before we got the truck, so I was not too suspicious of that. I also noted that if I unplugged the connector to the APP, the chatter in the 5V line was still present. I did not suspect the APP. I called the TAC module and ordered an OE replacement.

As you might have guessed, this did not fix the issue. The new TAC module produces the same chatter in the same 5V line. I double checked all the wiring again but still did not find any issues. I also de-pinned the connectors at the throttle body and the TAC module and verified good pin contacts and continuity through the connectors.

After some more testing, I found that if I unplugged the throttle body, the 5V line cleared up completely, no more chatter. I'm assuming this has something to do with no longer having a load on the circuit...? Anyway, after talking with the customer, they brought me a warranty replacement for the throttle body that they installed and we tried installing that. I got the same results again.

After reading about some folks having issues with aftermarket throttle bodies, I suspected that using a non OE throttle body might be the issue. I went ahead and ordered an OE throttle body and installed that, but still no change to the circuit.

At this point, I'm at a loss. I feel like I've run out of relevant things to test. Now I'm just throwing parts at it and hoping, which is a sure sign of defeat, I admit. My biggest issue is I've never seen a waveform look like this one. I don't know what kind of fault or problem could even cause it. I'd assume any kind of short would just pull the voltage up or down, not induce oscillations like that.

Any help or pointers in a direction I have missed would be greatly appreciated. Hopefully I've covered everything and explained it well. Thanks and Merry Christmas.

. I went ahead and ordered an OE throttle body and installed that, but still no change to the circuit.

​​​​​​Just to be clear, there was no change to the circuit, but does the truck still set codes and go into reduced power mode?
I have a 2500 with a 6.0 I always work on, and it had very similar symptoms (more than once actually) that got multiple new throttle bodies in the course of a couple years. Only a used old junkyard GM throttle body has kept the light off in this one.

To be fair, I have not driven it. The customer has been having it happen every couple days for a while now. I've not been able to get it to go into reduced power except one time, and that was before any of the testing I've done. Once I found the waveform for the signal circuit looking it does, I knew that was my problem. I've been focusing on trying to cure that.

I'm leaning toward the throttle body myself, but I expected the new OE unit to fix it if that was the case. I'm thinking I'll try to round up a good salvage one and see what happens there.

Any other ideas still welcome and appreciated.

I wasn't suggesting that you need to try a used one.
The point I was trying to make is that it's an intermittent symptom, but that toggling to ground is always there.
If that was the problem, then the symptoms would always be present. I was thinking you should drive it and see if the problem persists. I don't recall from the last time I was working on the truck I mentioned, but it's not uncommon for manufacturers to toggle one leg of a redundant app or top signal to ground as part of an internal circuit integrity test.

You hit the nail on the head with the toggling to ground being normal....I just so happened to have another truck come in, same YMM. So I scoped that same 5V circuit on a different truck and got the exact same results.

This was my big problem. I had never seen that before, and I assumed it was not normal. Everything I've done has been in the name of getting the toggling ground signal to clear up.

When I realized this, I put the truck back together with the replacement TB and TAC module installed and drove it. So far so good. What bothers me about that is that leaves me not knowing what the actual problem is/was. Through all the testing, I never was able to confirm any other issues.

Thanks again for the replies and help. I definitely learned something new. If the truck comes back or I figure something else out, I'll update y'all here. Otherwise, it looks like we're done with this one.

Excellent, from my experience with these trucks, I think you nailed it.
All the after market throttle bodies I have installed on GM trucks come back in about year with the same or similar codes/symptoms.
Really makes you second guess your work...