Saturday, December 1, 2012

795F AC Off-Highway Truck: Machine ECM Switched Inputs

Key Start Switch Input

Key Start Switch "ON" input connections to the electric drivetrain controls and the other control components on the truck

The Key Start Switch provides one system voltage input to the following electric drivetrain controls:

Drivetrain ECM
Motor 1 ECM
Interface Module 1
Motor 2 ECM
Interface Module 2

When the Key Start Switch is in the ON position, a system voltage input is provided to all of the control modules on the machine.

The presence of this voltage input will cause each ECM and control to transition from the "sleep mode" (inactive) to the operational mode. The voltage input will cause each ECM and control to initialize and then become operational.

When the Key Start Switch is moved to the OFF position, the system voltage is no longer present at the ECM "Key Switch - ON" input. The absence of the system voltage signals to the ECM to transition from the operational mode to the sleep mode. The ECM will perform some internal power down procedures and then enter the sleep mode.

The transition period from the sleep mode to the operational mode happens in a short time.

The key start switch ON circuits are not monitored for diagnostics by the ECM.

Throttle Lock Switch


Location of the Throttle Lock Switch (arrow) on the cab center console.


Throttle Lock Switch connections

The momentary Throttle Lock Switch provides one switch to ground input to the Drivetrain ECM.

When the truck is in the travel mode and the Drivetrain ECM detects the grounded switch input circuit, the ECM will set the machine travel speed at the current travel speed. Once the function is set, the speed will be maintained regardless of the position of the throttle pedal.

The Drivetrain ECM will disengage the throttle lock function if a brake pedal or retarder lever is moved, the shift lever is moved or if a drivetrain-related fault occurs.

If the Drivetrain ECM has detected a fault condition in the throttle pedal position sensor or a sensor circuit, the ECM will allow the Throttle Lock Switch to be used as a backup throttle.

In order to move the truck under this condition, the operator will hold the momentary Throttle Lock Switch in the ON position. When the ECM detects the grounded switch input circuit, the ECM will maintain the Engine speed at low idle. The ECM will send torque commands to each motor control ECM that will result in limited machine travel in the LOW gear range.

The ECM will maintain the limited travel function until the Throttle Lock Switch is released.

Since the switch provides only one input to the Drivetrain ECM, the ECM cannot monitor the switch circuits for diagnostics.

Motor ECM Enable Feedback


Motor ECM enable feedback circuit connection to the Drivetrain ECM

At startup, the Drivetrain ECM will use a sinking driver to ground an enable circuit for each motor ECM.

Each motor control ECM will perform system checks in order to determine the status of the components that are under control. If no fault conditions are present, each ECM will use a sinking driver circuit to ground an enable acknowledge circuit. The grounded circuit to the Drivetrain ECM that indicates that each motor ECM has received the grounded enable signal and is ready for operation.

In addition, each motor ECM will send enable information to the Drivetrain ECM on the CAN B datalink. The data link provides redundancy if a feedback enable circuit signal between a motor ECM and the Drivetrain ECM is interrupted or lost.

The enable and the feedback circuits will remain grounded at all times during system operation. If the enable circuit that the Drivetrain ECM uses to enable each motor ECM is interrupted or lost, the Drivetrain ECM will disable drivetrain operation. A loss of the feedback signal on the wired circuit will not result in a drivetrain shutdown. The Drivetrain ECM will use the data that is received on the data link.

MARYGAR