check the fluid levels. that's the only thing I can think of.
check your transmission fluid, if it is low it will slip
does the speedometer effect the transmission
overheating of the fluid. This causes the transmission to slip. Slipping causes damage as friction material wears out quickly and even more overheating. This eventually causes failure.
Low transmission fluid will cause this and also weak bands inside the transmission can cause this.
You might need transmission fluid !?!?
a blown transmission
Your transmission might slip on your ML320 because the transmission fluid is low or because the transmission is damaged. The most common cause of transmission slip is wearing of the transmission.
Try to replace the oil filter if the transmission is not engaging when it's cold.
The bands may be weak, it may be low on fluid.
If it's only when you attempt third gear the problem is in your transmission.
bad linkage in shifting cable
Try changing you oil and if that does not help you will need to rebuild the tranny or replace it.