2G Limp mode

[bring_1]

I've got a 97 Tsi AWD Auto. Go to drive it one day and i've got nothing but 3rd and reverse. It's in limp mode. Also when reving the engine the A/T temp light comes on dimly. So far I've tried checking all the plugs on the trans, they're not mismatched the car was driving fine as is and all are still connected. Pulled tcu codes and got no signal. Ran CEL codes at autozone, got P1600 Serial Communications Link failure. Checked for voltage into tcu, all fine there. Checked the wires that go between the ecu and tcu they were fine as well. Swapped with a working tcu. nothing changed. Where do I go from here?

 

[Altoids94]

Check if the MAS is properly connected.

 

[jayson427]

If it was in limp mode it would be stuck in second gear.

 

[bring_1]

No jayson427 limp mode is 3rd gear.

 

[H@xtGSX]

This is what I found on your P1600:

The ECM receives various messages from the TCM via communication line.
The ECM monitors an abnormal message signal due to open circuit in communication line and TCM failure.
DTC SET CONDITIONS

Check Area

Two seconds or more have passed immediately after the engine started.
Judgment Criteria

Abnormal communication line (TCM to ECM) with the TCM.
TCM failed.
TROUBLESHOOTING HINTS
The most likely caused for this code to be set are:

Automatic transmission control system failed.
Open or short circuit or loose connector in communication line between the ECM and TCM.
TCM failed.
ECM failed.


....so, if the engine seems to still be running fine, but the transmission is unresponsive, I'm guess that your TCU is toast or you have a short between it and the ECU that you can't see... or the ECU trace for it is burnt. You'll probably need to open both of them up and look, then do continuity checks on the TCU wires to the ECU.

 

[bring_1]

I have checked the comm. wires between the ECU and TCU they are fine. I have also swapped the TCU for a know working unit. I have also opened up my ECU and the caps are all fine, smells fine, and there are no noticeable shorts.

I'm really at a loss here. No signal from the obd port screams dead computer or bad wires. I still get no signal at the obd port with a replacement TCU and knowing the wires have continuity.

 

[H@xtGSX]

Check the OBD connector too. The pins in it are notorious for coming loose and not connecting well. I know that all too well...

What happened between when it worked and it taking a dump on you? Did you do any work under the hood? Have any accidental battery groundings that arc'd to the body? Super heavy rains and water in the engine bay? Attacked by ninjas hell bent on pulling a wire loose from a socket just far enough to make it not work, but not so far that your notice?

 

[bring_1]

Ninjas for sure. Nope didn't do any messin' around just parked it after a spin one day and about a week later it was limp mode. One thing that is really odd is the A/T temp light coming on dimly while reving the engine. The only info I could find on this is someone told me the A/T temp light is controlled by the ecu so maybe I should suspect that.

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