Having a Hell of a Time Troubleshooting This Yukon...Anyone Familiar With NNBS Trucks/SUVs?

[ARAMP1]

I have approximately 200K miles on my 2011 Yukon XL 2500 (6.0L/6L90E). Its throwing a P2138 code with reduced engine power.



For troubleshooting, I followed THIS video. Resistance was over 5Ω in both low reference wires and I found one of the wires frayed so I went ahead and replaced all six wires to the throttle pedal (new wires in the harness, new plug and repined with new Delphi connector). Now both low reference wires (purple and brown) are exactly 2.2Ω (under 5Ω) and both the 5V reference wires (tan and white/black) are exactly 5.1V (over 4.8V). I replaced the throttle pedal for good measure.

One odd thing, my brown and white/black wires were swapped around from this video. I went ahead and swapped them around to match what was in the video to see if that was the fix but it didn't change a thing. Since I had replaced all the wires and plug, I still had the old one. Then, I found this diagram online which is what mine is...so I didn't accidently swap those wires.




I had a lot of the plastic wire loom under the hood brittle and missing so I decided to replace the six wires and connector from the ECM to the throttle body. The throttle body was gunked up so I replaced it to included a new TPS.

I've seen the TSB related to water intrusion and the P2138 code. I do have water intrusion on the driver side. I've cleaned out all of my sunroof drains and have drained water in them verifying they are clear. I just recently pulled the GPS antenna and put some RTV around the opening though it hasn't rained since doing this. I remember reading online where someone had a leak around a windshield/body seam. I haven't explored that yet.


So now I'm kind of at a loss. Both throttle position signals are still off from each other.



I've checked all circuit breakers and all relays appear to be fine. At this point, I guess I'm just throwing parts at it to figure out what will fix the reduced engine power and P2138 code. I'm trying to figure out what module under the dash may have gotten corroded due to water intrusion. Maybe a bad ECM (the vehicle starts and idles fine)? Appreciate any ideas or suggestions for troubleshooting.

 

[shady]

I believe the TPS system has its own little computer box also. Which on ours, a little older was on the driver's side firewall under the hood. About 4 or5" square.
Maybe something there?

I fixed a recurring issue on my kids 04 that turned out to be the new plug at the TB the PO had installed and the wires weren't crimped good enough.

 

[Blue85]

There's something weird with your measurements. You shouldn't get >5V from the PCM. Where is the negative lead to the meter? Also, both numbers for resistance are surprisingly high. 5-Ohm is probably a decent target (the sensor is probably k-Ohms, so no big deal), but it would be strange to see that in real life. Where was the negative lead for this one? Did you try touching the positive lead to the same point to see what kind of offset is in the meter?

I just wonder if it points to a bad ground to the ECM - possibly from some wet ring terminals at a body stud? (the sensor is probably directly referenced from the PCM and floating from the body ground, but your meter was probably going through the body ground.)

Can you scan the 2 pedal position signals to see if they are the same? I think in some setups one is always ~1/2 the value of the other and in some they are the same. That answers whether or not you have good signal from the pedal.

For the differing TPS signals, you could try the old TPS just for confirmation. If that doesn't match up with either sensor, that points to wiring to the throttle. Have you had the PCM connectors off to look for water/corrosion? Your description of "new wires" doesn't really say if you were de-pinning at both ends or doing some kind of splice.

 

[ARAMP1]

The system is a 5V system so anything over 5V would show that there isn't resistance in the wire. Negative lead was just attached to a ground screw under the dash.

As far as the wiring...I've had everything off looking for corrosion. Nothing looks out of the ordinary. Also, with the new pedal wires and the new TPS wires...no splicing...all wires were depinned at both ends.

 

[Blue85]

The system is a 5V system so anything over 5V would show that there isn't resistance in the wire. Negative lead was just attached to a ground screw under the dash.

No, you can't get negative voltage drop on an open circuit (or a positive one). The wire resistance of the pedal circuit is irrelevant for this voltage measurement, other than being mis wired or PCM issue. However, there is current from the PCM to ground somewhere, so resistance there would show up as a voltage drop. Here's a quick example sketch to demonstrate that 0.4 Ohms in the PCM ground, with it drawing 1/2A will make your measurement 0.2V higher. If you measured directly from 2 terminals of the pedal connector, that effect would be cancelled (the other "-" port), which is why they wire the sensors like this.



We don't really know what the resistance measurements you took mean, since there was no delta measurement to cancel the offset in the meter/leads. We only know that it went down when you replaced the wires. A better test of the pedal harness ground resistance might be from pin A to pin F (but touch the leads together first and note that reading).

All I'm getting at is that having a wonky PCM ground can cause a lot of issues. If you can confirm there are actually Ohms of resistance there, that could be a problem.

Also, with the new pedal wires and the new TPS wires...no splicing...all wires were depinned at both ends.

OK, that should be good, then.

Another crazy idea. Have you disconnected the battery since replacing parts?

 

[ARAMP1]

Yeah, I may have a corroded ground somewhere. Good suggestion with disconnecting the battery, but yes to that too. It's been disconnected. Appreciate the ideas/suggestions.

 

[Blue85]

I believe the TPS system has its own little computer box also. Which on ours, a little older was on the driver's side firewall under the hood. About 4 or5" square.
Maybe something there?

I fixed a recurring issue on my kids 04 that turned out to be the new plug at the TB the PO had installed and the wires weren't crimped good enough.

I could be wrong, but I think this function was integrated into the PCM for the GMT900 platform or when either the 6L80-E transmission or AFM was introduced.

 

[HBD]

I would think it might be a ground issue. Possibly a braided flat style ground strap from motor to the frame. Try looking on the passenger side behind the front wheel about where the passenger feet would be.

 

[bigjimmy82]

Curious if any update on this?