E38 Lost DBW throttle getting Code 2138

[rtagg]

I've lost all throttle control on my LS2 E38, and I'm getting a DTC Code 2138 "APP1-APP2 Corr (correlation?) error. I know with that code the problem is usually a pedal position sensor, but I've tried a new AC Delco unit with no change. I've run some diagnostics I found in a Tech2 manual still can't find the problem. When I log it with my foot off the pedal the MVPI log shows APP1 100% at 4.8V and APP2 0% at 0.4V. If I slowly press the pedal down, APP1 values will decrease while APP2 values will increase with a crossover point of about 44-45% Shouldn't the APP1 and APP2 values be the same, or at least very close?

I can't find any shorted or broken wires in the DBW circuits (either pedal or TB), so I'm starting to think the problem may be with the ECU. Has anyone else ever come across this?

Thanks in advance.

[blindsquirrel]

Sure wish I knew the year/make/model I should be looking at in the manual! As a complete guess, I used 2006 Corvette. If that's not what you have the pinouts may be totally wrong.

Circuit/System Verification

Ignition ON, observe the scan tool APP sensor 1 voltage parameter. The reading should be between 0.32-4.75 volts, and change with accelerator pedal input.
Ignition ON, observe the scan tool APP sensors 2 voltage parameter. The reading should be between 0.32-4.75 volts and change with accelerator pedal input.
Ignition ON, observe the scan tool APP sensors 1 and 2 parameter. The scan tool should indicate agree.
Clear the DTCs with the scan tool. Operate the vehicle within the Conditions for Running the DTC, or within the conditions that you observed from the Freeze Frame/Failure Records.
Verify that DTC P2120 or P2125 are not the only throttle position DTCs set.
⇒ If DTC P2120 or P2125 are the only DTCs set, replace the ECM.
Verify that DTC P0641 or P0651 are not set.
⇒ IF DTC P0641 or P0651 is set, refer to DTC P0641 or P0651 .


Circuit/System Testing

Ignition OFF, disconnect the harness connector at the accelerator pedal. Allow at least 60 seconds for the ECM to power down. This can be verified by a loss of communication message on the scan tool.
Test for less than 5 ohms of resistance between each low reference circuit terminals A and D and ground.
⇒ If greater than 5 ohms, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
Ignition ON, test for 4.8-5.8 volts between each 5-volt reference circuit terminals C and F and ground.
⇒ If less than 4.8 volts, test the affected 5-volt reference circuit for an open/high resistance or short to ground. If the circuit tests normal, replace the ECM.
⇒ If greater than 5.2 volts, test the affected 5-volt reference circuit for short to voltage. If the circuit test normal, replace the ECM.
Ignition ON, verify the scan tool APP sensor 1 and 2 voltages are less than 0.1 volt.
⇒ If greater than 0.1 volt, test the APP sensor 1 and 2 signal circuits terminals B and E for a short to voltage. If the circuit tests normal, replace the ECM.
Install a 3A fused jumper wire between the signal circuit terminal E and the 5-volt reference circuit terminal F of the APP sensor 1. Verify the scan tool parameter APP sensor 1 voltage is greater than 4.8 volts.
⇒ If less than 4.8 volts, test the APP sensor 1 signal circuit for an open or short to ground. If the circuit tests normal, replace the ECM.
Install a 3A fused jumper wire between the signal circuit terminal B and the 5-volt reference circuit terminal C of the APP sensor 2. Verify that the scan tool parameter APP sensor 2 voltage is greater than 4.8 volts.
⇒ If less than 4.8 volts, test the APP sensor 2 signal circuit for an open/high resistance or short to ground. If the circuit tests normal, replace the ECM.
Ignition OFF, disconnect the harness connector at the ECM.
Test for less than 5 ohms of resistance on all APP sensor circuits between the following terminals:
? ECM C1 signal circuit terminal 29 to APP terminal E
? ECM C1 signal circuit terminal 32 to APP terminal B
? ECM C1 5-volt reference circuit terminal 36 to APP terminal C
? ECM C1 5-volt reference circuit terminal 56 to APP terminal F
⇒ If greater than 5 ohms, repair the affected circuit for an open/high resistance.
Test for infinite resistance between APP sensor 1 signal circuit terminal E and APP sensor 2 signal circuit terminal B.
⇒ If less than infinite resistance, repair the short between APP sensor 1 signal circuit and APP sensor 2 signal circuits.
If all circuits test normal, test the APP sensor. Refer to Component Testing for instructions.
⇒ If the APP sensor tests normal, replace the ECM.


Component Test

Important: The Circuit/System Testing must be performed before proceeding with the Component Test.

Dynamic Test

Install a 3A fused jumper wire between the 5-volt reference terminal of the applicable APP sensor and 5 volts. Install a jumper wire between the low reference terminal and a ground.
Sweep the sensor through the entire range while monitoring the voltage between the signal terminal and the low reference terminal with a DMM. The voltage should vary between 0.30-4.98 volts without any spikes or dropouts.
⇒ If the voltage is not within the specified range or is erratic, replace the accelerator pedal assembly.

[blindsquirrel]

You are correct that APP1 and APP2 should return similar values (P2138 sets with a 5% or greater difference between APP1/APP2 at idle). Your symptoms match what would happen if 5v ref & low ref to APP1 were swapped around.

[rtagg]

blindsquirrel, your second post pointed me in the right direction and I have fixed the problem, Thank You!

I was going through some pinout diagrams earlier today, and found that the one I had for the pedal position sensor was for the wrong year, and once I located the one for my engine (2007 Corvette), it showed the APP1 low ref and 5V ref were indeed crossed. I swapped the wires and now the 2138 DTC is gone and I have throttle control again.

Thanks again for your help, I really appreciate it!

[blindsquirrel]

No problem. I accept burnout videos as payment.

[rtagg]

I'm just curious, do you have any ides why the two APP channels are not equal through the entire pedal travel? The schematic I have shows what looks like an additional resistor in the APP 1 circuit which would explain how it happens that way, but I would have thought that GM would want the two channels to be equal at all times as a safety check.

[blindsquirrel]

They don't explain why, exactly, but I assume it's a safety thing - two APP output values that are identical wouldn't be the safest way to do it. Two distinct output values allows the ECM to be sure the APP signal outputs aren't shorted together and reading just one sensor.

"The APP sensors are used to determine the pedal angle. The engine control module (ECM) provides each APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors provide the ECM with signal voltage proportional to the pedal movement. The APP sensor 1 signal voltage at rest position is less than 1 volt and increases to above 4 volts as the pedal is actuated. The APP sensor 2 signal voltage at rest position is near 0.5 volt and increases to more than 2 volts as the pedal is actuated."

[rtagg]

What you're saying makes sense, thanks again for your input.

[blindsquirrel]

They did it the logical way for the TP1/TP2, those are inverse of each other. Dunno why they chose the weird way for the APP. Except maybe that they cross at the midpoint with inverse sensors. But if that's why, that would make the TP arrangement less failsafe than the APP arrangement. 'Ees a mystery, I guess.

[rtagg]

Yes, I guess some things are better left unknown....

[Pistolpetep250]

I'm having the same issues. Mine is in a 09 Silverado 6.0. I have replaced the TPS about a year ago when it popped the DTC. NOT much changed in the since of better after the new one other then the check engine light went off. FAST FORWARD..While scanning my truck today i noticed the volts for the TPS was at 3.74 volts at idle and 26% throttle position. When I floored the pedal it went down to .8 volts. The throttle position only goes to 80.3% when floored as well. Should i try a new Throttle position senor to see if this one is defected? I keep having weird issues that come and go that point to a faulty ECM. What would be the best test above to determine my ECM is bad?

[blindsquirrel]

Use the non-SAE channels and you'll see numbers that make more sense.

Accelerator pedal position
Accelerator pedal effective position
Throttle commanded position
Throttle position

If the TB opens all the way when you mash it and it's not setting any codes I don't think there is a problem.

[Pistolpetep250]

Use the non-SAE channels and you'll see numbers that make more sense.

Accelerator pedal position
Accelerator pedal effective position
Throttle commanded position
Throttle position

If the TB opens all the way when you mash it and it's not setting any codes I don't think there is a problem.

Ok I'll try that later after work! Thank you for your help. I also don't think there is a problem with the sensor or TB because nothing really changed on how it ran after new sensor and cleaning. Did the relearn procedure a few times. I need to figure out how to program a new ECM with HP Tuners if possible. GM wants $305 for the ECM and $145 to program it. Hoping I can do it myself for about half that. Is there anything you can enlighten me on programming a new ECM? I'm just beyond burned out with the gremlins in this truck and can only assume the ECM now.