Hello all

I recently purchased a 1993 400SEL and I love it, but I notice some times that it takes a few seconds to shift from first gear to second which causes the engine to rev for that period sending the RPMs a bit high.

I am concerned that my Transmission is slipping, but I also know that there is a Transmission Up-shift Delay Solenoid Valve, so that can be operating incorrectly.

I have been driving it for about a month and it happen three times only.

I am also thinking to replace the original radio but I am not sure of any complications to the Boss system that is already in the car.
I only plan to replace the head unit and not any other parts or add any other equipment.

Any ideas and/or advice will greatly be appreciated.