I have a 81 300D with 305K miles on her. She runs great except the delay between all gears causing me to let off the accelerator allowing it to shift. This happens in all gears and at all temps. I was thinking it might be a vacuum problem, as was suggested by the guy I got it from. Is vacuum something which controls the tranny? The door locks and moonroof also don't work until the car is running, unlocking the door in the morning won't unlock all doors, and moonroof won't function until the car runs a little while. If vacuum runs these things, as I am thinking it does, maybe it also controls the shifting????This is my first MB and I like it a lot, but the shifting isn't very flattering . Any advice is welcome.

Welcome, it sounds like you are on track by considering the vacuum system.

Do a search on "vacuum transmission locks" etc. There is much good info on Diesel vacuum on this site, so a search, investigate and write back when you get stuck. That's what I do.

Good luck.

John

Hi opie, the engine in your car has a mechanical vacuum pump bolted to the front. Looking down at the engine from the front, it has a metal tube attached to the top of it. It supplies ALL the vacuum for EVERYTHING. There are small check valves which fail with age, that can be replaced quite easily. Before you proceed though, check to see if the pump is indeed working correctly. From the sound of your transmission complaint it sounds as though the bowden pressure cable may be too tight. You didn't say whether the shift was firm or soft either. This is important . First things first though. 1. ATF level and condition. 2. Adjust bowden pressure cable. These two things take only a matter of minutes. 3. Check vacuum supply at the control valve located on the injection pump. These control valves are also a popular(and cheap)repair. Keep in mind that a big enough vacuum leak can affect several systems such as central locking, climate control, and trans shift characteristics. There's a vacuum reservoir you should check for leaks as well. This stores vacuum to operate the door locks when the engine isn't running. The sunroof is electrically operated, so don't waste time with it. You need to perform a complete function test of the above mentioned systems to better evaluate your situation. Good luck and keep me posted.

The shifting is quite hard and jarring. I am going to go through everything I have been told here over this weekend. I will do a filter and tranny fluid change and check for any vacuum leaks. Is there an easy way to check for vacuum leaks, whether by pinching a line closed and listening for something to change or some other means? Or do I have to go get a tester or guage of some sort?

Hi opiewann, the trans vacuum test will require you to accurately measure the vacuum going to the trans at a certain throttle position. This position is determined by installing a small disc( about the size of a nickel ) on the vacuum valve to act as a stop for the linkage. This is done whenever a control valve is replaced and it's also adjustable, allowing you to tweak the shift quality to your liking. As far as checking the supply pump you'll at least need a vacuum guage. There are also several check valves in the vacuum system that restrict vacuum under certain conditions. An accurate pneumatic diagram will allow you to eliminate/isolate sections of the system in order to diagnose accurately. Remember, first things first! Is the pump working? Does the reservoir leak? If I remember correctly, it's beneath the rear shelf, accessible from in the trunk. Also, look closely at the lines near the oil filter housing. It's easy to accidently unplug them during service. LOOK closely at all the lines you can see under the hood. The answer to your problem may be looking you right in the face! One more thing, does the engine shut off as soon as you turn the key? If not, that's another clue you have to work with. Get a pneumatic diagram and a vacuum guage and go for it. Good luck