I have no way of locking all the doors other than manually locking the rear doors and using my key on the front two. Any suggestions on what needs to be fixed/replaced/tweaked?

Most likely a leak in the vacuum locking system, and in most cases, the rubber diaphram of one or more of the elements is the culprit - possibily the actuator in the driver's door. There is a way to check the system, but I believe you need special equiptment to do so. The thing is; if one is bad, sooner rather than later the rest will go bad as well. The diaphrams aren't that expensive, so it's better to replace them all at once and have done with it.

thanks for the info. I will look into replaces them all.

Even if you are going to replace all of the diaphrams, you will still need to buy a mtivac to test the lines. A mityvac is an essential tool for these cars. You will probably use it more than a lug wrench.

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Dave
1983 300D Daily Driver

What does a mytivac run?

I have to disagree w/ blindwolf simply because the diaphrams are the only moving parts in the system. The rubber connectors - aside from the one under the hood - and plastic lines don't move and aren't exposed to heat. A few years ago, I replaced all of my diaphrams (just 1 was leaking) and have had no problem at all.

OK guys I don't follow your replies.

280EZRider. You are disagreeing with your own post?????
Also there are quite a few rubber connector and 3/4 way connectors throughout the entire system and the hard plastic vacuum lines can crack.

blindwolf is correct buy a Mityvac. You can get one at Harbor Freight for $30. And you will use this tool thought the entire car for diagnosis.

First you need to check the vacuum pump output. If OK then move onto the
door lock system in the cabin of the car.
The yellow/red and yellow/green plastic lines are the door lock lines.
Under the drivers carpet and under the black plastic trim piece are the vacuum lines. Remove the carpet and plastic trim piece. With the Mityvac you will check the operation of the individual and collective system.

At the drivers position notice that the 2 yellow based lines come from the firewall to the driver door and back to the driver floor position. From that position the lines go to the driver door element. Disconnect the yellow lines one at a time ( red then green ) and test it by applying vacuum. If it holds then that element is OK. Check the 3/4 way connector. If old or cracked replace it. Check both the yellow/green ( green=go means doors unlocked Red=stop doors locked ) and the yellow/red lines. Even though you are checking only one diaphragm per element, you are checking the lock and unlock lines and connections.

Now go to the yellow lines going back towards the drivers side passenger door. Test both sides (yellow/green and yellow/red)

Now notice the line going in front of the drivers seat support. These lines control the 2 rights side doors, the gas filler door and the trunk.
If the 2 left side doors are fine then the problem is on the right side.
Button up this side and go to the front passenger floor. Remove the carpets and notice the routing of the yellow lines. One set going to the front passenger door jamb and the other going to the rear. Test both sets of lines.

If the back lines are producing the leak then go to the individual element to determine the faulty element.

In the right rear passenger door
In the trunk, remove the right side plastic liner and check the gas door line ( only one for the gas element)
Check the trunk element
Check the connector to the vacuum reservoir above the gas tank.

Follow this post and let us know what you find.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Sears sells a Mity Vac equivalent for about the same price.

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83 SD

84 CD

When mine started acting up, I just replaced all 5 too.

Mine was a bad fuel lid element...fairly common failure point I have read.

__________________
Dale

Pull up your carpet. Look for the rubber tees. Note the red lines for lock and green for unlock. Get a diagram from this website and understand how those lines work. Those actuators are Falsely blamed 99.9% of the time.

The tee connections deteriorate over a period of time.Especially in the winter. Finally replace the master door lock switch. Its a smaller job and also make sure your accumulator tank has agood seal because that deteriorates. You would be surprised, by just pushing and shoving on rubber connectors fixes things a lot of the time.

Check the actuators last as a last resort.

I've replaced my fuel door actuator twice, it does seem like a weak spot. I still think it's easy/cheap to replace them all the first time the act up, just to get a fresh start.