p1793 - Transmission Control Module Stack Overrun

[Spykem4e]

hi guys, my 05 cts went at the garage to TRY to know what is going on with the transmission (its been years like that only durong summer time) when only the a/c is on and when coasting downhill between 45-55mph, the rpm bounces from 900rpm to 1600 rpm until I press the gas or the downhill stops.

The garage put a scan on it and they saw that the tcc unlock and lock and unlock and luck again. They just dont know why it does that but they're guessing that the programmation is made like that when there's a big load on the engine/transmission. They tested the a/c and they couldn't believe how fast it was to cool down etc.

They checked for some dtc code and nothing except:

p1793 - Transmission Control Module Stack Overrun.

what is this? even them doesn't know!

 

[brianl703]

A stack overrun is the result of a software bug or (rarely) a hardware problem.

Have they checked with the Caddy dealer to see if there's any software updates available for the transmission control module?

 

[wirelessF]

Wow. Your transmission just BSOD. Just wow. Bring it to a computer technicion and have em reformat it

Try disconnecting the battery. Hopefully this will reset the computer and relearn again. If the code comes up again, check it with your dealership.

 

[Colt45ws]

Yeah, thats a programming bug, or if it only happens in the summer the heat may be getting to the TCM (or it only triggers the bug when the A/C is on)...

 

[GMBoy]

Here you go....



xTooltipElement
Service Information
2005 Cadillac CTS | CTS (VIN D) Service Manual | Document ID: 1394229
--------------------------------------------------------------------------------

DTC P1793
Circuit Description
The engine control module (ECM) sends wheel speed data to the transmission control module (TCM). The data is sent to the TCM through a communication network called the controller area network (CAN). Two circuits are used to communicate CAN data between the ECM and TCM. A fault in the CAN will not cause DTC P1793 to set by itself. If a CAN fault occurs, other DTCs will set before DTC P1793.

When the TCM receives invalid wheel speed data from the ECM, then DTC P1793 will set. DTC P1793 is a type C DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:

DTC P1793 Wheel Speed Signal

Conditions for Running the DTC
• The engine run time is at least 6 seconds.

• No other CAN errors are present.

Conditions for Setting the DTC
The TCM receives no valid wheel speed data from the ECM for 2 seconds.

Action Taken When the DTC Sets
• The TCM does not request the ECM to illuminate the malfunction indicator lamp (MIL).

• The TCM inhibits Autograde braking.

• The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as a Failure Record.

• The TCM stores DTC P1793 in TCM history.

Conditions for Clearing the DTC
• A scan tool can clear the DTC.

• The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without a non-emission related diagnostic fault occurring.

• The TCM cancels the DTC default actions when the ignition is OFF long enough in order to power down the TCM.

Step
Action
Value(s)
Yes
No

1
Did you perform the Diagnostic System Check - Vehicle?
--
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2
Install a scan tool.
Turn ON the ignition with the engine OFF.
Important: Record all Failure Records before clearing the DTC. Using Clear Info erases the Failure Records from the ECM and TCM.

Record the DTC Failure Records.
Did you record any ECM Failure Records?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information
Go to Step 3

3
Clear the DTC.
Turn the ignition OFF for at least 30 seconds.
Start, and allow the engine to idle.
Did DTC P1793 reset?
--
Go to Step 4
Go to Intermittent Conditions in Engine Controls - 2.8L or Intermittent Conditions in Engine Controls - 3.6L (LY7)

4
Did any other DTCs set?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information
Go to Step 5

5
Replace the TCM.

Refer to Control Module References in Computer/Integrating Systems for replacement, set-up and programming.

Is the action complete?
--
Go to Step 6
--

6
Perform the following operation to verify the repair:

Select DTC on the scan tool.
Select Clear Info.
Start, and allow the engine to idle.
Select Specific DTC.
Enter DTC P1793.
Has the test run and passed?
--
Go to Step 7
Go to Step 2

7
With the scan tool, observe the stored information, capture info and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information
System OK

© 2009 General Motors Corporation. All rights reserved.

 

[GMBoy]

Sorry the above did not format correctly in the cut and paste - but looks like the TCM module or a wheel speed sensor is at fault. I could email you the diagnostic chart if you PM me.

 

[Colt45ws]

Kewl, there you go. Their interpretation of the code was wrong. You'll probably have to visit a dealer with a GM Tech II.

 

[Gary Allan]

You've just gotta love the Information Age.

 

[Spykem4e]

I tried 3 different cadillac dealers and they all say that it could be normal and found no code at all and its the first time they saw that.

I try 2 independant transmission shops and they say its not suppose to do that but due to a large amount of load when the a/c is on, the tcm, ecm or tps has been programmed like that but shouldn't. Then before saying its one of the tcm, ecm or tps, they checked for some codes.. well well.. both of them, found the p1973.

They had both something very similar than a gm tech II. both said something about its a transmission code but more something about wheel speed sensor that maybe try to change to fix the problem. I dont know why the 3 dealers didn't say anything....

 

[JTK]

I had a similar situation with the 4T45E auto in my 2007 Cobalt. Mine would flutter out/in of TC lockup under light throttle and continue to do so until you changed throttle position. The newer 4T45E's use a pulse-width modulated TC lock solenoid (heavily computer controlled). I assume your Caddy does too. Supposedly a software patch is needed, but as far as I know has never been rolled-out. My Chevy dealer cleared the shift adapts in the ECU/TCM and the issue never happened again. (knock on wood?)

Joel

 

[GMBoy]

I sent you an email with all the repair info.

 

[Spykem4e]

Just an update: it was my front left speed sensor that was faulty.

The wheel speed sensor is integrated in the hub with the abs sensor and wheel bearing. I changed it and I have to wait until a lot of rain to see if it works 100%.

I took some bottles of water and splash it on the new hub and everything is perfect. With the hold hub with some water getting in it, it would light up the abs and t/c warning light on the dash.

Thanks GMBoy.

 

[GMBoy]

Originally Posted By: Spykem4e
Thanks GMBoy.




You are very much welcome.

 

[JTK]

Originally Posted By: Spykem4e
Just an update: it was my front left speed sensor that was faulty.



I'm glad to see your issue is resolved, but I don't get it. A faulty WSS generally causes your ABS and other associated systems to throw fits and light up dummy lights. They either work or they don't AFAIK. Also, why did it happen only with the A/C on? Like I say, with my 4T45E that had the same exact symptoms and a supposed software glitch, a clearing or the shift adapts eliminated the problem.

Joel

 

[Spykem4e]

JTK, the faulty wheel speed sensor did light up the dummy lights on my dashboard.

The problem with the A/C at the on position while decelarating with the foot off the gas pedal making the rpm bounce from 1800 rpm to 1000 rpm and so on, is another problem.

Unfortunately, the problem with the a/c is not resolved. I drive my car without the a/c now because I'm afraid to toast the transmission (I dont know why but I'm afraid). I tried with the defrost, heated seats on etc and no problem until I activate the a/c.

Is there a relation with the a/c voltage + idle speed sensor or I dont know what? I'm going to borrow a tech2 very soon and If there's a place I can try to look for, tell me!

I'm sure its not a transmission problem because the transmission works perfectly when the a/c is not working. Below 70km/h or higher than 95 km/h with the a/c on, the transmission works very good. its just when coasting down a hill or taking my foot of the gas between those speed that the transmission seems to hunt.

I checked the rpm with my foot off the gas at those speed before the tcc lock when the car is cold and the transmission doesn't drop to 1000 rpm so the problem is that the transmission is disengaging when the a/c is on.

Dealers (independants transmission shops + 3 caddy dealers) saw the complaint but all said its normal because it could be the programmation that is like that. Is there a place to see and maybe change the programmation?