2011 Chevy 3500, Mystery problem.

MilwaukeeHoarder

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First time poster here, I got a 2011 Chevy 3500 w/ allison @ 100k miles.

Last week my truck got stuck in the backyard when parking my trailer. the dash showed “Shift inhibited return to neutral” and threw a CEL, it finally budged after cycling 20 times. Once I got it going it kept bouncing between gears and going into neutral randomly. Scanned it and it was showing low voltage on fluid Pressure switch so I dropped the pan and replaced that. After replacing the truck became drivable except one little thing.
When you are taking off from a stop sign it will start to move then throw itself into neutral(just revs and rolls), then limp mode (3rd gear) and stay there until I clear the CEL it throws everytime it does this.

Here’s the part that confuses me, if I set my tuner to automatically clear codes it won’t bounce into neutral and it drives fine. I read a forum on here from 10 years ago with a guy having the same problem but no resolution was ever posted.
I’m just hoping someone here knows what’s going on because I’ve called 10 transmission shops around here and none of them have ever worked on an Allison because “they don’t break”.

Any advice is appreciated! Thank you.

The code was P0700 on engine side and P0842 on the transmission side, nothing else.

Edit: I forgot to mention this only happens when the tranny is cold. If I keep clearing it until it reaches above 119 degrees it will take off just fine once that temp is passed, no codes.
 

2004LB7

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DTC P0842

Circuit Description

The pressure switch manifold (PSM) is a multiple-switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. The C pressure switch monitors C shift valve positioning and relays it to the transmission control module (TCM). When the C pressure switch is in the open state, C shift valve should be in the unstroked position.

DTC P0842 is a type A DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0842 Pressure Switch C Circuit Low/Stuck Closed

Conditions for Running the DTC
•The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.
•Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.
•C shift valve is commanded to the OFF, unstroked, position.

Conditions for Setting the DTC

DTC P0842 sets when C solenoid is commanded OFF and pressure switch C status remains ON for 2–6 seconds, depending on fluid temperature.

Action Taken When the DTC Sets
•The TCM illuminates the malfunction indicator lamp (MIL)
•When in a forward range and shifting is completed, the transmission will fail to another forward range, unless the transmission is compromised by over speeding or direction change, then the transmission will shift to NEUTRAL.
•When in a forward range and the shift is in process, the transmission will return to a previous range, except for 1–2 shifts and 2–1 shifts which will be completed.
•When in NEUTRAL or REVERSE, the transmission will fail to NEUTRAL.
•While diagnostic response is active, the shift selector is moved from a forward range to NEUTRAL or from a forward range to REVERSE, then the transmission will shift to NEUTRAL.
•DTC P0842 will be stored in TCM history.
•The TCM freezes shift adapts.
•The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the DTC

A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
•Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
◦A bent terminal
◦A backed-out terminal
◦A damaged terminal
◦Poor terminal tension
◦A chafed wire
◦A broken wire inside the insulation
•When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
•You may have to drive the vehicle in order to experience a condition.
•This DTC may indicate that C solenoid is mechanically defective or the shift valve is stuck in the stroked state.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

5.This step tests the TCM response function.

6.This step tests the transmission wiring harness integrity.

7.This step tests and evaluates the internal wiring harness.

8.This step tests the PSM for a failed condition.

Step

Action

Value

Yes

No

Schematic Reference:Automatic Transmission Controls Schematics

1

Did you perform the Diagnostic System Check – Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Measure the transmission fluid level. Refer to Transmission Fluid Level and Condition Check.

Is the transmission fluid level correct?

Go to Step 3

Go to Transmission Fluid Level and Condition Check

3

Measure line pressure. Refer to Line Pressure Check.

Is the pressure within the specified value?

Go to Step 4

Go to Symptoms - Automatic Transmission

4

•Install the scan tool.
•Turn ON the ignition, with the engine OFF.

Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data.
•Record the DTC Freeze Frame and Failure Records.
•Clear the DTC.
•Start the engine.
•Using the scan tool, monitor TFP SW. C status.

Does the scan tool indicate TFP SW. C status is ON?

Go to Diagnostic Aids

Go to Step 5

5

•Turn the ignition OFF.
•Disconnect C2 connector (RED) at the TCM.
•Install the J 39700 breakout box and the J 43799 breakout box adapter at C2 connector.
•Connect terminal 1 to a known good ground.
•Turn ON the ignition, with the engine OFF.

Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open?

Go to Step 6

Go to Step 15

6

•Turn the ignition OFF.
•Connect the C2 connector (RED) to the TCM.
•Disconnect the transmission 20-way connector.
•Turn the ignition ON. Additional DTCs might be set.
•Connect terminal D to a known good ground.

Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open?

Go to Step 7

Go to Step 14

7

•Remove the oil pan. Refer to Oil Pan Replacement.
•Disconnect the AT internal wiring harness from the PSM and ground terminal A.

Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open?

Go to Step 8

Go to Step 13

8

•Remove the PSM from the control valve body and leave the internal harness connected. Refer to Pressure Switch Manifold Replacement.
•Provide a ground connection for the PSM to either the control valve body or the transmission main case.
•Using the eraser end of a pencil, push gently on the pressure switch C membrane.

Does the scan tool indicate TFP SW. C status is ON when grounded and OFF when open?

Go to Step 9

Go to Step 12

9

Remove the control valve body and inspect for a stuck or sticking C shift valve. Refer to Control Valve Body Disassemble.

Was the C shift valve stuck or sticking?

Go to Step 11

Go to Step 10

10

Replace the C shift solenoid and install the control valve body. Refer to Control Valve Body Replacement.

Did you replace the solenoid and install the control valve body?

Go to Step 16

11

Replace the control valve body assembly. Refer to Control Valve Body Replacement.

Did you complete the replacement?

Go to Step 16

12

Replace the PSM. Refer to Pressure Switch Manifold Replacement.

Did you complete the replacement?

Go to Step 16

13

Replace the AT internal wiring harness. Refer to Wiring Harness Replacement.

Did you complete the replacement?

Go to Step 16

14

Repair the engine wiring harness circuit for an open or short. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 16

15

Important: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM.

Replace the TCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and progr
amming.

Did you complete the replacement?

Go to Step 16

16

Perform the following procedure in order to verify the repair:
•Clear the DTC.
•Start the engine.
•Use the scan tool, monitor TFP SW. C status.
•Select Specific DTC.
•Enter DTC P0842.

Has the test run and passed?
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Sounds like the solenoid has given up the ghost but you need to follow the diag flow chart posted a few posts above
 

MilwaukeeHoarder

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took it upon myself to remove the tcm to check wires and realized it wasn’t secured in the proper place. It was just bouncing off the belt pulleys and getting gouged (didn’t penetrate) . I tested resistance at the plug for shift solenoid 1 and got 0 ohms. Plugged TCM back in and now it won’t go into reverse (flashes) and will only BANG into drive.

Im starting to think it might be the tcm. We got FOUR codes now after plugging it back in….
(p2669,p0752,p0842,p0972)
Part number for the TCM is AC DELCO - 24256861
 

TheBac

Why do I keep doing this?
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Im really surprised the guys didnt say TCM first after reading its an '11 truck. Those were infamous for their TCMs just failing out of the blue. I would be doing my diagnostic, aiming at that.
 
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2004LB7

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I'd look over that harness real good for any breaks or damaged insulation.

But as Tom pointed out, those TCM have a weird tendency to crap out. With yours banging around like you described, I'd consider it suspect at best. If you have a buddy or someone else that has the same truck, maybe try swapping the TCMs and test it out. Otherwise we won't know if it's a bad TCM or something else wrong
 
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MilwaukeeHoarder

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Nov 27, 2024
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Connecticut
I'd look over that harness real good for any breaks or damaged insulation.

But as Tom pointed out, those TCM have a weird tendency to crap out. With yours banging around like you described, I'd consider it suspect at best. If you have a buddy or someone else that has the same truck, maybe try swapping the TCMs and test it out. Otherwise we won't know if it's a bad TCM or something else wrong
Can I just swap a TCM plug n play? No tuning or what-not?

I currently have a tune for towing on the truck and I’m pretty sure it messes with shift points. I’m not sure how any of that works at all so I figured worth the ask.
 

2004LB7

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Can I just swap a TCM plug n play? No tuning or what-not?

I currently have a tune for towing on the truck and I’m pretty sure it messes with shift points. I’m not sure how any of that works at all so I figured worth the ask.
You can swap without issues