I own a '94 Ford F800 with a diesel engine. Mine is shut down with a SOLINOID actuated fuel cut off valve.
On your shut down it sounds like you stall the engine rather than cut fuel flow??
I think I would look at installing a fuel bypass so that after your pump fuel is returned to tank.
I am thinking that any starvation of an older system may cause trouble recovering from a system shut down ie. drawing air into system.
OP, How easy is it to recover from running out of fuel with your old truck?
Bleeding air off would always be a bad thing. Your air horn would not use enough air to run your system down even at an idle IMO, and any time your air pressure drops to the point of setting he drive axle brakes, there is not enough pressure remaining to fully apply brakes should the need arise.
Killing the starting system could be overridden by rolling truck to start.
Is their any kind of emergency shutdown air valve???
Many are mechanically activated but could be rigged to trip via a SOLINOID.
Whatever you come up with, let us in on it! :dubious:
NO! Diesels (older ones, like the OP’s) do NOT depend on ANY electrical circuit to stay running, and the engine will NOT cease to operate no matter how many or which wires are cut.
Diesels are compression ignition engines. Electrical devices (starter, sometimes glow plugs) are used to get them running, but once they are running all they need to stay running is fuel. If the fuel is supplied by mechanical means, as in the OP’s vehicle, the battery itself can be removed and the engine will continue to run.
You are obviously familiar with spark ignition engines, which typically run on gasoline. On these your assumptions hold true. Not on diesels.
I asked for specifics to see if maybe you knew something about this that I didn’t know. I see now that you don’t.
Yes Rick, I read that. It wouldn’t be the first time someone didn’t know exactly what they were dealing with.
I only see 2 ways of doing this electrically. Cut off the air with an electrically driven throttle or cut off the fuel with an electrically driven fuel cut off valve. the fuel valve would be the cheaper route but that raises a question as to whether or not you can cut off fuel between a tank and a mechanically driven fuel pump. Without knowing the specifics about the pump I would guess it has to be a positive displacement device to generate the pressures needed at low engine rpm. Not sure what would happen if you starved it. The same applies if the valve is put between the pump and the injector body. It would have to recycle the fuel back to the tank.
so I’m thinking that a T be installed between the fuel pump and injector body with the cutoff fuel valve routing fuel back to the tank or a loop back to the pump inlet. that would keep the pressures equalized but starve the injector body.
My truck is similar to this one: http://www.toolsouq.com/files/images/original/1079/1.jpg
The only way to turn it off is by pressing the exhaust brake button. It is a button on the footwell that you operate with your left heel. Some pics here: exhaust brake operation | Mercedes-Benz Forum
I works by blocking the passage of exhaust gasses and makes the engine slow down, and if the vehicle is in neutral it will make the engine stall. Here’s a pic: File:Exhaust brake governor.jpg - Wikimedia Commons
The blackish pipe is part of the exhaust pipe. The silverish thing is the exhaust brake. When it is operated, a rod pops out and blocks the circular hole that is visible inside the black pipe.
By the way, on my truck it doesn’t work as a true exhaust brake. You can use it to turn the engine off when idling and in neutral but it is not powerful enough to brake the vehicle on the move.
I had for a time a smaller Mercedes truck of the same era (like this one: http://www.yauto.cz/includes/img/inzerce/Mercedes_Benz_208D_1.jpg)
At one time both the battery and the alternator were shot. We would start it with a heavy duty external battery charger and it would be good to go. Nothing electrical like lights or radio would work but the engine didn’t have problem running.
This one could be turned off with the key, but it had some vacuum operated mechanism that cut the fuel. (Unlike my other truck, this had a vacuum pump and hydraulic brakes). Actually it was quite a common problem with these trucks that some vacuum hose would go bad and the engine would keep running when the key was removed! The only way to turn off the engine then would be to stall it (put it in some high gear, push the brakes and release the clutch).
Fun times! 
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No, this one is electrically activated
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The parking brake in commercial trucks works in a fail safe manner. The parking brake pads are spring loaded and normally, when there is no air pressure in the system the springs push the pads against the friction area. When the pressure in the system exceeds 6 bars the springs are pushed back and the parking brake released.
If for whatever reason the pressure drops below 6 bars, the springs will make the parking brake automatically engage. (Working pressure is 8 bars, so that’s only two bars of safe operation. A good, not too hard but not soft either depress of the brake pedal will drop the pressure by one bar. So it is not a good idea to make small frequent uses of the brake pedal, instead it should be depressed and held there)
The horn consumes air in a faster rate than the compressor can replace it, but not very much faster. I believe this will make the parking brake engage gradually and not cause any accident.
You might want to give this some serious thought.
Picture the brakes bringing the truck to a stop on the railroad tracks. Truck thief dies. Or worse yet pieces of truck hit and kill bystanders.
The investigation shows you rigged the brakes to do that.
Things at that point might really get ugly for you with the legal system.
Rick, the device also shows the speed and position of the vehicle in real time, provided you have a smartphone or computer. Therefore I could start the horn when I see the vehicle is stopped at a red light.
If I may piggy-back a question - If you installed a fuel cut-off upstream of the injector pump, would it require the pump to re-suck the fuel from the tank the next time you tried to start the engine? I’ve operated old tractors where you had to pull the big red knob to kill the engine, but I never thought about it before. I know on my current tractor, whenever I bleed the filter that’s just before the injector pump, I better have a fully charged battery to get it started again.
Question for the OP. When you stop the engine normally, what do you do?
See post #7.
Now we’re getting somewhere. Was it the same type of fuel pump and mechanical fuel injection? Where was the fuel cutoff located? I have reservations regarding a positive displacement pump getting starved for fuel, thus the idea of introducing the cutoff in a T fitting so it doesn’t block the fuel as much as reroute it back to the pump inlet. The problem with this scenario is that most electrical fuel pumps would be power-open. You’d want one that was no power-open. A vacuum operated fuel mechanism would require an electrical vacuum switch.
Hello only a newbie, did you sort the Gps device ? the easiest way to disable the Mercedes or any truck is to purchase a 24v (or 12v if so wired) dump valve, the valve drops the air in the park circuit so the brakes are effectively “stuck on” if you do not fancy the fail safe mode of this (AN INTERLOCK RELAY) then you can as someone suggested interrupt the start relay but it will start if towed. A 24v blled valve can introduce air into the feed pipe from the lift pump which will stop the engine (shit to bleed) or an air valve can be added to the stop circuit (which effectively is like keeping your foot on the floor operated stop control, note no exhaust brake fitted, the fuel was cut off in addition where fitted, on the non turbo etc and smaller trucks the button simply operates an air actuator than manually turns the fuel off)
Hope you got sorted