My 1994 420 S Merecedes changes gear but when I drive it it doesn't change to another gear

Have you checked the transmission fluid level? it could be low.

We put fluid in it. Any other suggestions?

Someone on the Mercedes forum said they had the same issue, they cleaned the Mass Airflow Sensor and the problem went away. Im not shure if this will work for your car but it could be a cheap resolution if it fixes it.

mine was acting up the same way and was putting also. i had to clean the wheel sensors located by the hub of each wheel. its for anti skid stuff and when it gets dirty from brake dust and years of not being cleaned the sensor cant read the magnetic pulses the hub is sending by rotating. it then thinks the wheels are not spinning the same and wont allow you to give it gas let alone switch gears to go faster. also, i had the gas evap tube which is black plastic/rubber melt through. it goes into the air intake and has like 3 pieces to it. about a dime/10cents in diameter? nickel maybe? i couldnt get the strap off so i peeled as much as i could off ( where it connects to air intake. backside. you will see a hole when you take off the Mass airflow sensor.) and stuffed a slightly smaller tube in there. I used a small radiator tube so it could withstand the heat better. s420 1995