Not all coach issues are open-and-shut cases. Some, on the other hand, are.  The entrance door system seems to be one of the things we take for granted because it seems so simple.  But looks can be deceiving, and when something does go wrong, operators are often not familiar enough with the system to quickly and accurately diagnose even the simplest of problems.

This month's "Maintenance Matters" will try and wipe away some of the fog of D coach entrance door functions with a direct, no-frills look at just what the door control system does. A few years ago, we touched on the  hard-wired version of the door controls; this article will focus on the multiplex control circuits common to later-model doors.

First, we need to consider just what we want, or need, the entrance door system to do — and what we don't want it to do. Obviously, we want the door to open and close smoothly and quietly every time and then lock when shut. We also want it to disable coach motions such as kneeling while people are in the stairwell. We do not want the door to close on a passenger when the wind gusts, or to slam open with a jolt that damages the mechanism itself.

So we see that the door system does more than just open and shut the door. Each task is driven by an electrical and pneumatic action, and each has been designed into the operation and control of the entrance door system.

The door system operates exactly as the hard-wired coaches of earlier years. Air flow to open and close the door is delivered and exhausted through the silver and purple (actual color of the lines on the coach) air lines from the control module to the door cylinder. Air is normally applied to the close side of the door cylinder (purple air line) all the time.

The white and orange lines pass a 50/50 mix of anti-freeze and water through a control valve as the cylinder piston is moved by the air side of the piston. This control valve (4K-22-3) is operated by air pressure from what is known as the "skinner" valve (16B-10-57). When air is applied from the skinner valve to the door stop valve, the 50/50 mix is prevented from flowing, and the door position stops where it is. This prevents the door from moving as passengers enter or exit the coach, even with air applied to the close side of the cylinder. Loss of this 50/50 mix can allow the door to move unexpectedly.

Testing system function is easy with the built-in test lights supplied to us by the multiplex system. The table below shows which schematic is needed for each coach system, along with the most common inputs and outputs to look for.

DOOR CLOSED AND LOCKED
Pressing either the interior or exterior door "open" switch: (Active input to multiplex module, see table below)

• Removes power from the door latch solenoid, removing air from the locks and unlocking the entrance door.
• Removes power from the skinner valve which in turn removes air from the door stop valve, allowing the hydraulic lock to release. The door is now free to move.
• Supplies power to the door "open" solenoid (norgren valve, 4L-22-55), reversing the "close" air pressure to apply the "open" air pressure (white air line).
• As the door clears the closed position, the micro-switch on the door cylinder rod sends an active input to the multiplex, and the step-well light turns on.

When the door swings open air is being pushed into the door cylinder, pushing on both pistons in the door cylinder. The 50/50 mix flows through the stop control valve and regulators (12C-7-16), which dampens the door swing.

RELEASE OF DOOR SWITCH
When the door switch is released:

• Module input goes inactive
• Power is applied to the skinner valve, which applies air to the stop control valve, preventing the 50/50 mix from flowing. The door is locked into position.
• Power is removed from the door's norgren valve, returning air to the "close" side of the cylinder.
• Locks stay retracted as door is open.

DOOR OPEN
Pressing either the interior or exterior door "close" switch: (Active input to multiplex module, see table below)

TESTING
Testing system function is easy with the built-in test lights supplied to us by the multiplex system. The table below shows which schematic is needed for each coach system, along with the most common inputs and outputs to look for.

	I/O T-1	I/O T-2	VANSCO
Schematic:	7L-14-0173 7L-14-0309	7L-14-0777 07-14-1753	07-14-1593
INPUTS
Ent. Door OPEN Switch	A4-22	A4-22	I3-3
Ent. Door CLOSE Switch	A4-23	A4-23	I3-4
DOOR Micro Switch (OPEN position)	A4-20	A4-20	I3-2
OUTPUTS
SKINNER Valve (output to release)	A4-02	A4-02	O3-14
Door LATCH Solenoid (output)	A4-04	A4-04	O3-15
Door OPEN Solenoid 'A' (Bi-Part door)	A4-01	A4-01	O3-13
Door OPEN Solenoid 'B' (Bi-Part door)	A4-08	N/A	N/A

Still have questions? Ask your Fleet Support Manager or contact MCI technical support at 800-241-2947.

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