View Full Version : What is limp mode? Is my controller bad?

10-26-2008, 08:52 AM
The transmission control module (TCM or TCU) for my 1998 A4 V6 30V 2.8L automatic w/tiptronic is going on and off while the ignition is ON. Even parked in the garage with the engine not running, though it does happen while driving too.

When the TCM switches itself off, the shift lock light goes out with a click, the trip computer display on the instrument cluster doesn't have the transmission gear select information, and VCDS/VAG-COM can't access the automatic transmission module and the ECU has a 18034/P1626 thrown (Powertrain Data Bus: Missing Message from TCU). The reverse happens when it comes back online. No fault codes are inside the transmission controller. I've checked maybe 85% of the wires going into the TCM (all except the multifunction switch wires, the CAN bus and K wires, and the ECU signal lines), everything looks good and there doesn't appear to be any shorts, though this would be an intermittent short if it was a short.

I guess I am in "limp mode" when the TCM switches itself off while driving? Or, if I take out the TCM and drive without it, I'd always be in limp mode, correct? Is driving in limp mode very bad? Do I need a new TCM? I really don't need any more problems with this car...