Bob -
Think SIAM from the outside towards the coupler:
S - sand (usually on older units with pneumatic sander; both sides must
be coupled for
forward & reverse sanders to work) normally 0 psi in these hoses
I - independent brake, application and release (this is the engine brake
only) 45 psi with
engine brake applied
A - actuating (used to release engine brake during a train brake
application [power braking
normally 0 psi
M - main reservoir equalization (130-140 psi at all times)
Electro-Pneumatic sanding eliminated the need for the two outside MU hoses.
Passenger capable units had a second hose under the coupler which was the
signal line. E-units used in suburban service had a third hose under the
coupler which was for "straight air" train brake applications from a 24RL
brake
valve.
Amtrak units have a second hose under the coupler which is MR pressure used
on the passenger cars for the water raising system (toilets, sinks, showers,
diners, lounges)
I have been told by an EMD electrical engineer that the SD-70MACs can be
entirely MUed thru the 27 point MU cable and not use any of the MU hoses.
Hope this answers your questions.
Bob Campbell
[Non-text portions of this message have been removed]
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