Hello and thanks for trusting me to help you today. I am a licensed diagnostic and repair technician, I look forward to working with you.
Yes, This is a chat where we can talk back and forth.
This is a strange problem you have, the fact it works when jacked up is very strange.
That was my first theory. When the front of the car is lifted the differential will transfer the power to the wheel with least resistance. I would be inclined if not already done to stop one wheel from turning while it is running, see if the speedo stops, then stop the opposite wheel. If it works only when one of the wheels are turning but not both I suspect it could be the differential in the gearbox, I would also remove the sensor and take a good look down inside the hole at the diff and the drive teeth on the sensor in case they are worn, although I'd imagine your mechanic has already checked the sensor.
Otherwise, the wiring on the sensor circuit can sometimes cause problems on these and having the front of the car off the ground could cause enough flex to get a good enough connection though a bad wire connection. Its a long shot to be honest but sometimes these things happen and this is certainly a strange problem and may well have a strange solution.The gearbox seems the obvious problem, but I would check the wiring and signal from the sensor on diagnostics before going down the gearbox route.
No problem. Please do keep me updated and come back here if you need any further assistance.. I will be happy to continue my assistance! If you are happy with my help, please kindly rate my service. Thank you, Ross.
Yes I would lift the whole car so it is raised on a lift and flat. Ideally plug in the diagnostic system and read the sensor output, this would rule out an issue with the instrument cluster.
It is possible, although I would expect the ABS warning lamp to be on, Many vehicles use the abs speed sensors to provide the speed signal to the dashboard. These use a separate speed sensor on the gearbox, but saying that if your model has traction control, it will also use the speed sensors on the wheels and require these to input signal to the main control unit.The only way to get a better idea is to carry out diagnostics into the ABS system to ensure the sensors are all giving a good clear signal and that the ABS system can communicate with the instrument cluster. There should be a fault code stored if there is a problem with the signal. Im actually wondering if there is a loss of comms between a control unit that is causing the problem. In which case it may be a bad connection at the ABS Module or a wheel speed sensor.
Just come back here if you have any problems. If your mechanic finds a fault code and is still unsure of the cause, if you can provide the code I can look further into the problem for you.
Best of luck getting it fixed ! Hopefully is a simple bad connection somewhere.