1984 300CD vacuum problems - PeachParts Mercedes-Benz Forum

bcinbigd · #1 09-07-2006, 10:53 PM

I recently bought a 1984 300CD which would only shut off if the blower motor was on. The A/C did not work and the key switch was balky. When the key switch finally froze up, I had to have the switch replaced. The mechanic charged the A/C, and it started to work. The car was always quite slow, shifting quickly to high gear. Soon after getting the repairs done, the A/C suddenly quit, the car would not shut off, the central locking system quit. I bought a repair manual for the 123 chassis, but it was next to worthless. The few photos in it were nothing like my car, which has automatic climate control (III), and I haven't found a way to identify the components of the vacuum control system. Also, there is a light blue plastic (vacuum?) tube bundled with the other tubes coming from the firewall that is not connected to anything. It seems to have a slight vacuum when I put my finger over the end of it. Any help here would be appreciated. My wife is seriously on my case for buying an old M-B.

#2 09-07-2006, 11:08 PM

You have a vacuum leak someplace that is affecting all those systems. Now it's just a matter of systematically going through the car and Isolating the leak. Start by taking a look at these vacuum diagrams, yours will be very similar to the 300D:

Vacuum Diagrams

Once you know which line is which you can try blocking off parts of the system to determine where the leak is. If you do a search on vacuum leak hear you will find lots of info.

I don't know of any light blue lines offhand, but if you post a photo, we can probably tell you what you're looking at. There is a black vent line that's supposed to be open under the dash, but that's the only one I know of.

Samuel M. Ross · #3 09-08-2006, 06:29 AM

http://peterschmid.com/vacuum/1977_1985/616_617_91/616_617_91.jpg
assuming yours has the 617.91 engine. Check out the links for "Vacuum Schematics" at http://peterschmid.com

If you have trouble getting a readable printout, get back to me on this FORUM's Private/Personal Mail.

If you don't already have a hand vacuum pump with gauge [aka "MityVac"], then having one is a MUST for you to work on the multiple vacuum systems on these cars... and getting them balanced is CRITICAL for proper and reliable functioning of the BRAKES, and tranny shifts.

Sam Ross

JEREMYC · #4 09-08-2006, 09:10 AM

Welcome to the forum!

Maroon 300D · #5 09-08-2006, 12:23 PM

Quote:

Originally Posted by bcinbigd

I recently bought a 1984 300CD which would only shut off if the blower motor was on. The A/C did not work and the key switch was balky. When the key switch finally froze up, I had to have the switch replaced. The mechanic charged the A/C, and it started to work. The car was always quite slow, shifting quickly to high gear. Soon after getting the repairs done, the A/C suddenly quit, the car would not shut off, the central locking system quit. I bought a repair manual for the 123 chassis, but it was next to worthless. The few photos in it were nothing like my car, which has automatic climate control (III), and I haven't found a way to identify the components of the vacuum control system. Also, there is a light blue plastic (vacuum?) tube bundled with the other tubes coming from the firewall that is not connected to anything. It seems to have a slight vacuum when I put my finger over the end of it. Any help here would be appreciated. My wife is seriously on my case for buying an old M-B.

Welcome to the forum and don't give up the faith!

The blue line you speak of may be a vent line. I say may because I'm not sure, but I believe most of these old MB's do have a vent line in the place you describe. That would mean it shouldn't be connected to anything. The color of the line should give away its function, though. If you search I'm sure you can find some information on which colored vacuum lines go where, also the diagrams in the links above might tell you.

Do your locks not work at all, even when the car is running? If that's the case the line to the yellow dashpot under the hood may be disconnected. They didn't change the oil while doing these other repairs, did they? Vacuum lines are frequently knocked loose that way since several vacuum lines are right next to the oil filter.

Vacuum problems can be kind of a pain but there's no reason you can't find and fix the problem yourself. It's not difficult, just a bit time consuming if you don't know what you're looking for. There's tons and tons of info. about vacuum diagnostics on this site and at www.dieselgiant.com. He has a great pictorial on how to do it. If you don't already have a handheld vac pump, you will need one. They should be included with the cars!

Your shifting problem may be the same one I have. I'm in 4th gear by 22 mph, which is not an ideal situation. Unfortunately it appears that there is no easy fix for this, however you can 'manually' shift by using the 'S' and 'L' functions, which makes it bearable. This will not hurt your transmission.

I thought this was an issue primarily with '85's but maybe you have the same tranny.

In any event, wear out that 'search' function and ask questions when the answers don't seem to be forthcoming. People here are extremely helpful and many are very knowledgable.

Good luck!

pwogaman · #6 09-08-2006, 08:57 PM

Get yourself one of these vacuum pumps, it will make the trouble shooting easier:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92474