1985 300D shut down problem
 

Hi guys, I am new to the Mercedes world. However, I am a diesel guy. With that said I was wondering if one of you guys have come across this problem.

I just picked up a 1985 300 D turbo that will not shut down using the key. I can only shut it down buy pushing down on the brake pedal firmly.

I know the 123's systems are mostly vaccumed controled including the shut down. It just seems odd that I can not use the key to shut it down but I can using the brake pedal. Have anyone seen this problem before?

Well this is the first I have read that stepping on the Brake would shut it off when the Key would not.
But, note there is a Manual Shutoff Lever on the Linkages where the come together on the Valve Cover.

The Question is; is there any other Vacuum powered items you have having problems with. Are Power Brakes working OK, the Door Locks, The climate Control, and is your Transmission shifting OK?

It is a good Idea to put a Vacuum Gauge on the Main Line or at the Vacuum Pump to see how much actual vacuum you have.
Also do a visual check of all of the rubber Vacuum Connectors under the Hood. Bend them slightly to see if the Rubber is cracking.
Be careful of the little plastic nipples on the Main Vacuum line as they snap off easy.

In order to quickly troubleshoot the Vacuum System a Hand Held Vacuum Testerd (it has a hand pump as well as a Vacuum Gauge) is valuable (a.k.a. Mighty Vac; although the Mighty Vac is a plastic one).

If you look at where the hard Plastic Vacuum Tubes go through the Fire Wall you will see that they are color Coded.
You will see a Brown Line; that is the Vacuum Supply line to a Vacuum Switch that is on the Steering Colum; the one your Key actuates.
You will also find Brown Line with a blue stripe. That is the Vacuum Line from that Switch to the Shutoff Valve/Servo on the Fuel Injection Pump.

There is a Black Tubing and that is simply a Vent line that goes inside but connects to nothing.

If you happen to have a blue Line it goes to some sort of Vacuum Adjustable Seat.

The 2 lines connected to a common Check Valve or 2 separate Check Valves are the Locking System Lines and the other line Green with a Stripe on mine goes to the Climate Control.

One at a time disconnect the Climate Control Line and Plug the Vacuum supply end (some people use Golf tees as the plug) and see if it will shutoff normally. If it does you have a Vacuum leak in the Climate Control.
Leave the Vac Control Vac Supply Line Plugged and do the same thing to the Locking System Vacuum Supply Line.
If your shutoff is normal after that you know you have a leak in the Locking System.

If the Engine will not shutoff after doing the above and it still shuts off when you step on the Brake I would be thinking there is some sort of Vacuum leak in your Brake Booster.
But that could be something a simple as the O-ring where the Master Cylinder Bolts up to the Brake Booster.

Checking to see if the Brake Booster leaks is more difficult as you need to fabricate a connection for that.
Or do as I did and I replace the Main Line Plastic Tubing with I think it was 1/2 inside diameter Emission/Vacuum Hose. The Rubber hose makes it easier to connect Plastic Fittings you can buy at the Auto Parts store to allow you to connect the Mighty Vac or a Vacuum Gauge.

Thanks Diesel 9112, I really appreciate the response. Thisproblem really has me scratching my head. I have done diesel work for most ofmy life (mostly CAT, CUMMINS, and DETROIT)but not so much on Mercedes. I’m in the process of getting the service manual soI can learn the Mercedes. But I don’t think the service manual will tell me theanswer to this one.

So, again thank for the tips/and info. It may take me a fewdays to get to it as I can hardy get my wife out of the car. She really loves it. I find it had to believehow well the car rides and rattle free it is being it is over 30 years old. Iwill let you know what I find out.