Cruise Control Problems: Troubleshooting Guide

Hello to all you diesel mechanics!
I'm having a problem with the cruise control in my car! I drive a Seat Toledo 1.9 TDI.
My cruise control is not working! And when I read the error codes, it only shows "speed signal implausible."
Please help!
Best regards, Christian!

... there are many 1.9 TDI Toledos available.

Please provide the engine code. It should be listed in the service booklet.

Or at least the year it was built.

PS: Without the signal, the cruise control won't work.
Does the speedometer show the correct speed?
Is the sender unit on the transmission working correctly?
What speed does the engine control unit (ECU) display when the speedometer shows a certain value?



Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

Hello and thank you for your help already!
Okay, so that's a BMW 1 Series from the year 2001.
The speedometer is working and displaying the correct speed. However, the automatic climate control system is not receiving a speed signal, but it *is* receiving a RPM (revolutions per minute) signal.
!!

Check the fault memory of ALL control units (ECUs). If you have a broken wire, it's possible that communication between the ECUs is not working. For example, my transmission ECU writes an error to the memory if the engine control unit is incorrectly coded or has been removed.

"I've already had it checked. I can clear the error code, but then when I drive and try to activate the cruise control again, the error reappears! It says 'speed implausible'! That's all it says! At least, that's what a VW diagnostic tool shows. I don't have a wiring diagram either! And you can't even get a repair manual for this vehicle!"