[Jarek]

Hello,

I have a situation where I want to remove a TCM from SS with A8 in order to unlock it and be able to minimize the shudder (by changing lock-up speed delta) however in the meantime I need to leave the car in at the shop to fix some body work issues.

Question can a car without TCM move around the parking lot at mechanic's (in an emergency or service mode)?
I have info that the older transmissions were able to work without TCM just allowing you using 3rd gear and reverse. Is that the case for A8?
I will appreciate a feedback.

BR,
Jarek

[dmcmahan60]

I just paid the extra money to buy the unlocked TCM from HPTuners because my car was my daily and could not have it down for a week waiting on the TCM to be unlocked. I don't think if it does not communicate with the TCM that it will move or even start? I could be wrong. I paid $400 for the TCM unlocked and programmed from HPTuners. But you have to copy some of the calibration files from the stock TCM and put them on the new one.

[Jarek]

Thank you dmcmahan60.
In my case (as I am based in EU) I am actually travelling to US with my TCM to send it to HPTuners (to avoid any customs/taxation discussions and delay) so my options are limited.

Like I have said in my 1st message - my mechanic told me that old transmission were able to go in emergency/service mode.
I am curious if anybody had a similar situation.

[Efi69Cam]

The a8 will not function without the tcm, similar to how the engine will not run without the ecm.

[Jarek]

Thank you Efi69Cam.
Again was that tested or is just assumed?
Sorry to be a bit harsh but looking for hard data.

[dmcmahan60]

I am pretty confident in saying that if the TCM is not there it will not start just like if there was no ECM it would not start.

[Jarek]

Thank you dmcmahan60.
In that case I will need to test that.

[Efi69Cam]

I don’t need to unplug my tcm to “prove” to myself the fact that an entirely electronically controlled device will not function without its controller.

[ctrlz]

Originally Posted by Efi69Cam

I don’t need to unplug my tcm to “prove” to myself the fact that an entirely electronically controlled device will not function without its controller.

While I absolutely LOVE this answer, I think there is a possibility there may be some default functionality.
From what I have read some cars will default to 3rd gear and limp mode when the ECM loses communication with the TCM.

EDIT: My above idea applies to a "lost communication" problem. Based on what I see in the repair manual, I think Efi is correct. The transmission needs the TCM connected to enter into its default modes. You can't remove the TCM completely. Below are the default modes, which all seem to show the TCM taking action:

DIAGNOSTIC INFORMATION AND PROCEDURES > DTC P0601-P0604, P0606, P062F,
P16F3, P16F4, OR P16FB: CONTROL MODULE > ACTION TAKEN WHEN THE DTC
SETS
P0601, P0603, P0604, P0606, P062F, P16F3 or P16FB
P0601, P0603, P0604, P0606, P062F, P16F3 and P16FB are Type A DTCs.
TCM turns Off power to all shift and pressure control solenoids.
TCC is disabled.
2nd gear is obtained if fault was detected with the default valve in the low state.
8th gear is obtained if fault was detected with the default valve in the high state.
TCM disables transmission adapts learning and freezes current adapt values.
TCM inhibits Tap Up/Tap Down.
TCM inhibits TCC.
TCM requests the ECM to disable powertrain braking.
TCM commands maximum line pressure for all shifts
TCM request the ECM to limit engine torque based on transmission input speed.
TCM request the ECM to disable neutral idle.
TCM requests the ECM to disable auto grade braking.
TCM request the ECM to disable auto stop function.
P0602 or P16F4
P0602 and P16F4 are Type A DTCs.
No other default action taken.

[Efi69Cam]

Originally Posted by ctrlz

While I absolutely LOVE this answer, I think there is a possibility there may be some default functionality.
From what I have read some cars will default to 3rd gear and limp mode when the ECM loses communication with the TCM.

EDIT: My above idea applies to a "lost communication" problem. Based on what I see in the repair manual, I think Efi is correct. The transmission needs the TCM connected to enter into its default modes. You can't remove the TCM completely. Below are the default modes, which all seem to show the TCM taking action:

DIAGNOSTIC INFORMATION AND PROCEDURES > DTC P0601-P0604, P0606, P062F,
P16F3, P16F4, OR P16FB: CONTROL MODULE > ACTION TAKEN WHEN THE DTC
SETS
P0601, P0603, P0604, P0606, P062F, P16F3 or P16FB
P0601, P0603, P0604, P0606, P062F, P16F3 and P16FB are Type A DTCs.
TCM turns Off power to all shift and pressure control solenoids.
TCC is disabled.
2nd gear is obtained if fault was detected with the default valve in the low state.
8th gear is obtained if fault was detected with the default valve in the high state.
TCM disables transmission adapts learning and freezes current adapt values.
TCM inhibits Tap Up/Tap Down.
TCM inhibits TCC.
TCM requests the ECM to disable powertrain braking.
TCM commands maximum line pressure for all shifts
TCM request the ECM to limit engine torque based on transmission input speed.
TCM request the ECM to disable neutral idle.
TCM requests the ECM to disable auto grade braking.
TCM request the ECM to disable auto stop function.
P0602 or P16F4
P0602 and P16F4 are Type A DTCs.
No other default action taken.

From what I can see, you are correct. These are actions taken by a tcm when loss of coms with the ecm or other fault occurs. Like the “limp home” mode built into most powertrain controllers since the beginning.

The solenoids that control the hydraulics in the trans obviously cannot do anything without control by the tcm. I don’t have the service info in front of me but my instincts tell me that the default/no control state of the transmission is essentially neutral.