Tuesday, December 25, 2012

797B Off-Highway Truck: Switches

Switches provide the following types of information to the inputs of the chassis ECM: open signals, ground signals and + battery signals. Switches are open or closed.

When a switch is open, no signal is provided to the corresponding input of the ECM. This "no signal" condition is also called "floating".
When a switch is closed, a ground signal or + battery voltage signal is provided to the corresponding input of the ECM.

Start Switch (Key)

The key start switch is an input of the ECM. The switch connects to connector contact J1-58. The switch tells the ECM that the operator wants to start the engine. The ECM then initiates the neutral start function. See Systems Operation, "Neutral Start Function" for more information. The signal is open during normal operation. The switch closes to the +battery when the switch is in the START position.

The switch has a connector with four contacts. Contact 3 connects to the ECM. Contact 4 connects to battery voltage. Contact 2 connects to the engine shutdown control. Contact 1 connects to the main relay.

Note: After the switch is initially turned to the START position, the switch will not return to the START position from the ON position. The switch must be turned to the OFF position. The switch can then be turned to the START position.

The ECM does not record faults that occur with the key start switch.

Pressure Switch (Hoist Screen Bypass)

Pressure Switch Hoist Screen Bypass(1) Switch. (2) Schematic symbol.

The pressure switch (1) has a plunger inside the switch that closes the contacts with an acceptable pressure drop across the screen. The pressure switch grounds the signal circuit during normal operation. The pressure switch plunger moves in order to allow the pressure switch contacts to open. The pressure switch contacts open when pressure across the screen is greater than the specified value. If the screen is restricted the open pressure switch sends a signal to the ECM. The ECM informs the operator that the screen is restricted or dirty. The normal state of a pressure switch that is not installed is normally open.

The hoist screen bypass pressure switch has two screw terminals. One of the terminals connects to the chassis ECM at contact J1-40. The other terminal connects to J1-12.

The machine has two screens. The pressure switch for each screen is wired in series, and the resulting signal is sent to the ECM. If either screen is plugged, a warning will be sent to the VIMS.

Switch (Case Drain Hydraulic Oil Filter)

Switch (Case Drain Hydraulic Oil Filter)(1) Schematic symbol. (2) Switch.

The filter bypass switch is an input to the ECM. The bypass switch is located on the housing of the filter for the case drain hydraulic oil. The bypass valve will open in order to protect the system if the filter is restricted. The bypass switch provides an input signal to the ECM. The ECM determines if the filter is restricted and the ECM informs the VIMS. The filter bypass switch connects to the ECM through contacts J1-57 and J1-12.

Note: The machine has two filters. The bypass switch for each filter is wired in series, and the resulting signal is sent to the ECM. If either screen is plugged, a warning will be sent to the VIMS.

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