P2138 Issues

wardmichael

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May 20, 2023
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Trying to help out a friend. Vehicle is 08 silverado with 4.8 motor. P2138 code is present. He has previously prior to asking me to help swapped out TB and Accel pedal and still is getting P2138. I went over today to diagnose and get this with scanner. First pic is no throttle. 2nd pic is full throttle. Pedal does absolutely nothing engine rpm wise. You can floor it and there is no increase. In the process of troubleshooting I notice the temp is climbing on the engine to dangerous levels and shut it down. I'm guessing this might be related ... Looking through other threads I'm thinking maybe the fan clutch is bad and is pulling down the 5v to make the pedal look bad? Does that make sense? I would assume to diagnose I would need to pull the fan clutch connector and see if the problems go away? Any other things to try or check? Thanks in advance for any help!
 

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mrrsm

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Welcome to GMT Nation...

 
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mrrsm

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wardmichael

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doing some more reading and seems the D and E readings in the pictures above should be same or close to same value? Am I correct?
 

mrrsm

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mrrsm

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Ordinarily... Diagnosing GM APP Pedal Sensor issues using just a DMM will not work. This is because the Signal Sample Rate going TO and FROM the PCM is TOO HIGH as the APP Pedal moves Up & Down and causes subtle variations in 'Voltage over Time'.

Thus, the average DMM won't be able to isolate them in Very Fine Detail; especially when trying to find any momentary *BLIND SPOTS* or fragmented *DROP OUTS* as they are happening that can only be resolved at this level of detail, either by using ANY Oscilloscope (...or a Graphing Multi-Meter like the Snap-On Model# MT-2400).

At around 6:00 and then again at 7:00 into THIS Video... the Trainer describes what SHOULD be happening when APP#1 Signal and APP#2 Signal are working properly (But Inversely Proportional) when viewed as Channel #1 and Channel #2 on his Oscilloscope Screen.

He also shows what a momentary *Glitch* or *Blind Spot* will look like when these events occur to Trip the PXXXX Code and prompt the PCM to place the Vehicle into a "Reduced Power Mode" rather than risk having a "Runaway" condition occur. This Video is worth watching from the very beginning for gleaning more Diagnostic Instructions:

 
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wardmichael

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May 20, 2023
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Haven't had a chance to get back to this unfortunately. This weekend most likely. I have been thinking about it though. In this situation, the P2138 seems to be a hard set code. the code has been cleared and instantly reappears (i believe) with just a turn of the key. I'd assume that should point towards something - but not being familiar with the system isn't obvious to me.
 

mrrsm

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The *Delicacy* involving ANY Module required to behave in combination with each other will trigger an instantaneous "Reduced Power Mode" if the PCM senses that anything is likely to allow Un-conrolled Air to enter through the Air Filter and eventually cause the Engine to create a *Runaway RPM* event.

Follow this Simplified Automotive *Boolean* Logic:

(1) IF... the PCM can NOT determine the Weight & Temperature of the in-coming Air in Grams Per Second via the MAF-IAT...

(2) And IF... the PCM can NOT determine Precisely what the Angle of the Throttle Butterfly Plate is to allow a Controlled amount of Air to pass through the Intake Manifold...

(3) And IF... the PCM can NOT establish the Precise Position of the APP Pedal and Establish *Driver Intent*... OR Senses Brake Pedal Actuation for the same reason.

(4) And IF... the PCM Can NOT Determine the signal coming from the VSS (Vehicle Speed Sensor) to know the Speed of the Moving Vehicle...

(5) THEN... the PCM will Shunt the Engine Operation down to a *Reduced Power Mode" every time.until ANY or ALL of those Modules and Dangerous Conditions are FIXED...

(6) AND... THEN ...After Clearing the Codes... subsequent to these involved Modules report to the PCM in combination via Bi-Directional Class 2 Network Communications that things are now stable, reliable and produce measurable conditions...

(7) THEN... the PCM will give permission to operate the Engine Power Train in a Normal, Power Producing Mode.

(8) GOTO (1)
 
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