yikes!!!

[Tony LEE]

"Wouldn't the shutdown solenoid work from the power supplied by the alternator, even with the batteries disconnected? "

In an MCI with belt driven alternator or belt driven AC compressor, the alternator field won't be energised until the air pressure is high enough to tension all the belts correctly.

[Stan]

"Wouldn't the shutdown solenoid work from the power supplied by the alternator, even with the batteries disconnected? "

All the older buses with the big Delco alternator used an external voltage regulator which has to have some minimum input voltage before they can supply field current to the alternator. No field current = no output voltage.

[Hi yo silver]

Thanks Stan,
Now I get it.  You have solved a mystery.
Dennis

[gus]

Stan,

On my 671 the elect solenoid and air valve are bolted together, there is nothing on the governor.

The only time the air valve is open is when the master switch is turned off - then the solenoid opens the air valve to the air cylinder which shuts off fuel via a rod directly into the cyl head and injectors.

So, the air valve is normally closed as long as there is air pressure.

Lin,

Thanks, you have saved me the trouble of the experiment. I thought that would happen, I couldn't see anything to keep it from starting since the fuel is always on after air pressure is gone.

[Stan]

Gus: A solenoid valve is an integral unit as you describe, a solenoid screwed to the top of a valve body. It requires power to pull the solenoid (A Skinner V5 valve draws 10 watts) so you cannot have a valve under power when the engine is shut down or the batteries will go dead. This is also a failsafe device, in that if you lose electrical power, the engine will stop.

I am not familiar with the shutdown system you describe, but it is the same principle as the one I described. Air, from the normally open shut down valve, pushes the piston out of a cylinder which pushes the injector rack to the stop position. It doesn't really matter where the cylinder is mounted. On the old truck engines it was all manual - a cable up through the cab floor with a knob you pulled to stop the engine.