How does the W123 shutoff system work - PeachParts Mercedes-Benz Forum

psaboic · #1 05-28-2014, 08:10 PM

Hi folks,

I know that when you turn off the ignition key on a gasoline powered MB it cuts the spark, thus turning off the motor. On our W123's, what does turning off the key cut off to stop combustion? Fuel flow, or air flow? Also, I'm assuming whatever gets cut off it is vacuum operated, correct?

Just trying to better understand the ignition system and engine stop system of the car, and wondering if there is something in that circuit that if it ever broke, would it prevent the car from starting.

Glenn

uberwasser · #2 05-28-2014, 08:16 PM

It cuts off fuel flow at the injection pump via a vacuum shutoff actuator (one of its names).

There is a vacuum line that enters the cabin through the firewall and travels under the dash to the ignition. It is the vacuum "feed" for the shutoff mechanism. When you turn the key to "OFF" the ignition passes vacuum from this feed out a vacuum line that goes back out in to the engine bay and connects to said actuator. I don't remember if this actuator pushes, or pulls, but it actuators a lever inside the pump that cuts off fuel.

It's the same movement as is made by the "STOP" lever on top of the valve cover.

It's why a bad vacuum leak in the system leads to no or slow shutdown.

BWhitmore · #3 05-28-2014, 08:45 PM

It is my experience that a broken shutoff actuator will not prevent the car from starting.

uberwasser · #4 05-28-2014, 09:00 PM

I missed that part. Yeah, not related to the vacuum side. I suppose it's possible there could be a problem with the linkage side in which the lever isn't returning to open.

jay_bob · #5 05-28-2014, 10:31 PM

In this thread I wrote a description of the operation for a forum member who had inadvertently swapped the key switch lines under the dash.
http://www.peachparts.com/shopforum/diesel-discussion/346853-1987-300d-shut-off-lever-not-resetting.html

There are 2 ways to kill a Diesel engine. You can only control 2 sides of the fire triangle.
Cut the fuel or Cut the air

(Modern gas engines not only kill the spark but the fuel supply also.)

The IDI engines (OM60x or OM61x) cut the fuel supply by pulling the rack to the minimum fuel position with the actuator. The actuator has a hook on it that attaches to the rack and pulls in when vacuum is applied. That is, the rack moves towards the rear of the vehicle to stop.

When the key is placed in the run position, the remaining vacuum is bled off the actuator (if it hasn't all leaked out already from sitting too long) and the rack is relaxed and allowed to be under the control of the driver's foot and the governor.

MB designed the system to be as failsafe as possible. Failure of the vacuum or failure of the actuator results in the engine not being able to be shut off. Better than the engine dying on the Autobahn in the event of a failure. Failure will not stop the engine from being started either.

The only caveat with this system is that if you ever remove the actuator, it is extremely critical that the hook on the end of the actuator is properly attached to the rack. If it is placed wrong, the rack gets jammed to full fuel position and as soon as you crank the engine it will go to redline and you have no way to stop it. The only way to stop it is to have a piece of wood ready to slap over the intake. So if you ever have to change that actuator be prepared by exposing the air intake (removing the filter temporarily if necessary) and have a piece of wood ready to go to choke the air if needed.

Zacharias · #6 05-29-2014, 12:48 AM

Quote:

Originally Posted by jay_bob

The only caveat with this system is that if you ever remove the actuator, it is extremely critical that the hook on the end of the actuator is properly attached to the rack. If it is placed wrong, the rack gets jammed to full fuel position and as soon as you crank the engine it will go to redline and you have no way to stop it. The only way to stop it is to have a piece of wood ready to slap over the intake. So if you ever have to change that actuator be prepared by exposing the air intake (removing the filter temporarily if necessary) and have a piece of wood ready to go to choke the air if needed.

Just to be clear, if it were me, I would have someone else start the car and I would stand with the piece of wood just over the exposed intake, ready to stamp it down at a split-second's notice.

Removing the air cleaner to expose a nice even hole to cover, with minimal intake air left after closure, would be a requirement, I would say. Trying to close off the intake any other way is not guaranteed to shut it down (BTDT).

WMO Madness · #7 05-29-2014, 02:46 PM

My apologies if I missed it, I scanned through the reply. I had the same problem with my 1984 300D, at first it did not want to shut down. I had to force it via the linkage, then it is also hard to start like it was not getting fuel. Did the vacuum test on the shut valve, it did not hold vacuum. Replaced with a spare I had, car starts and shuts off normal. By the way, it is trying to remove the old one than catching the hook to catch the lever in the IP. Just insert it into the cavity, it somehow align itself and catch the level inside. Good luck.

Diesel911 · #8 05-29-2014, 08:17 PM

Quote:

Originally Posted by psaboic

Hi folks,

I know that when you turn off the ignition key on a gasoline powered MB it cuts the spark, thus turning off the motor. On our W123's, what does turning off the key cut off to stop combustion? Fuel flow, or air flow? Also, I'm assuming whatever gets cut off it is vacuum operated, correct?

Just trying to better understand the ignition system and engine stop system of the car, and wondering if there is something in that circuit that if it ever broke, would it prevent the car from starting.

Glenn

If you think it is the Shutoff Vacuum Circuit preventing you from shutting down remove the Vacuum Hose from Vacuum shutoff Valve/Servo/Actuator on the Fuel Injection Pump which is Brown with a Blue Strip on My Year and model.
That keeps the vacuum from reaching it and causing it to shut off.

If it still will not start like that get a section of vacuum Hose and connect it to the Vacuum shutoff Valve/Servo/Actuator on the Fuel Injection Pump and blow into the Tube. That should push the stuff inside of the Valve so it would not interfere with starting.

I am going to give My description of the Vacuum shutoff System.

When you turn the Key off all of the Vacuum should bleed out of the System.
Trace the Vacuum Lines from the Nipple or Nipples away from the Main Vacuum Line. Eventually you should see a solid Brown Vacuum Line that goes through a Grommet in the Fire Wall and it ends up at a Mechanical Vacuum Switch/Valve that is on top of the Steering Wheel Lock Housing.
That is what gets actuated when you turn the Key.

From that Vacuum Switch/Valve a Brown Vacuum Line with a Blue Stripe goes to the Vacuum shutoff Valve/Servo/Actuator on the Fuel Injection Pump.

If there is a problem with the Vacuum Switch/Valve on the Steering Wheel Lock Housing that can also keep you from starting. If that is the problem Disconnecting the Brown Vacuum Hose with a Blue Stripe at the Valve/Servo/Actuator on the Fuel Injection should allow you to start.