Hi, I got a 98 ML320 and I have following problems.

problem 1:
SOmetimes when I turn the key it does not start, sometimes it does.

problem 2:
After driving a while 'BRAKE' light comes on on the instrument cluster and transmission gets stuck in second gear. It feels like
its a fail safe mechanism to prevent you from going fast. What causes BRAKE light to come on? and why does it affect the transmission? If I shut down the engine and restart then it works fine.
problem 3:
AC is blowing hot air from all vents.

problem 4:
Power locks make ticking sound from driver door and unlocks and then
locks by itself all four doors.

Other then that its a great car.

Please help soon.

 

I am the original owner of a 99 ML320. At 154K, the 3.2L engine still drives like new, with regular synthetic oil changes.

I had the same starting problem (your Problem 1) about 4 years ago. After replacing the fuel pump and a sensor (might be the O2 sensor but I do not remember now), the problem is gone. A good and experienced mechanics can help diagnose the problem with meters. You can also go to a dealer, but it will be more expensive.

I can not help your other problems.