793D Off-Highway Truck Power Train: Electronic Control Module (Power Train)

Location of the Power Train Electronic Control Module

The Power Train Electronic Control Module (Power Train ECM) is the main component in the power train electrical system. The Power Train ECM is located at the rear of the cab. The Power Train ECM has several input signals and output signals. Refer to Illustration 1 for the input signals and the output signals.

The Power Train ECM receives current through a fuse on the fuse block. This current is used to activate the upshift solenoid, the downshift solenoid, and the lockup solenoid. These solenoids control the option of torque converter drive or the option of direct drive. These solenoids also control the direction and the speed. The Power Train ECM also controls the electrical system for the hoist.

The Cat Data Link is used to share information with other systems such as the Brake Electronic Control Module (Brake ECM), the Engine Electronic Control Module (Engine ECM), and the Vital Information Management System (VIMS). Signals that are sent to the Engine ECM result in reduced engine speed during upshifts and increased engine speed during downshifts.

The Brake ECM controls the following items:

• Traction Control System (TCS)
• Auto Retarder Control
• Rear axle cooling

The Power train ECM controls the following items:

• Top gear limit
• Reverse
• Load counter
• Neutral coast
• Body up gear limit
• Protection for starter
• Body hoist control
• Speed limiter
• Shift control
• Neutral start
• Backup alarm
• Overspeed protection
• CTS
• Engine prelube
• Directional shift management
• Autolube

System Operation

Table 1

Operation of the Torque Converter For Transmission Speed
Transmission Speed	Torque Converter Drive	Direct Drive with an Engaged Lockup Clutch
Reverse	X
Neutral	X
First	X	X
Second		X
Third		X
Fourth		X
Fifth		X
Sixth		X

Neutral (Torque Converter Drive)

This machine has two neutral positions on the rotary actuator. Position NEUTRAL-1 on the rotary actuator has a detent position on the rotary selector spool. Position NEUTRAL-1 is NEUTRAL on the operator console. Position NEUTRAL-2 on the rotary actuator also has a detent position on the rotary selector spool.

Position NEUTRAL-2 on the rotary actuator does not have a position on the operator console. The operator does not have control over NEUTRAL-2. The rotary actuator and the rotary selector spool automatically go through NEUTRAL-2 on all shifts to NEUTRAL and from NEUTRAL.

When the transmission control is in NEUTRAL position, the rotor for the shift lever switch is in NEUTRAL-1 position. With the shift lever switch in this position, current flows from the Power Train ECM through the shift lever switch to machine ground. The Power Train ECM then activates the downshift solenoid. When the downshift solenoid activates, hydraulic oil pressure goes to the rotary actuator. This causes the rotary selector spool to turn.

The rotor on the transmission gear switch is connected to the rotary selector spool by a flexible coupling. When the rotor on the transmission gear switch is in the NEUTRAL-1 position current will flow from the Power Train ECM through the transmission gear switch to ground. This current flow is a signal to the Power Train ECM of a completed shift. Normally, the downshift solenoid would be deactivated. However, the Power Train ECM keeps the downshift solenoid activated in the NEUTRAL-1 position. This holds the rotary selector spool in NEUTRAL position.

First (Torque Converter Drive or Direct Drive)

When the transmission control is moved to FIRST position, the rotor for the shift lever switch moves to FIRST position. Current then flows from the Power Train ECM through the shift lever switch to machine ground. This signal informs the Power Train ECM that the shift lever switch is in FIRST position.

The rotor on the transmission gear switch is connected to the rotary selector spool by a flexible coupling. When the rotor on the transmission gear switch is in the FIRST position, current flows from the Power Train ECM through the transmission gear switch to ground. This signal informs the Power Train ECM that the shift is complete. The machine is now in FIRST gear and the machine is in torque converter drive.

FIRST gear is torque converter drive at lower ground speeds. FIRST gear is direct drive at higher ground speeds. The transmission speed sensor transmits a signal to the Power Train ECM. This signal tells the Power Train ECM the ground speed of the machine. When the ground speed reaches approximately 6 km/h (4 mph), the Power Train ECM sends current to the lockup clutch solenoid. When the lockup clutch solenoid activates, hydraulic oil pressure goes to the lockup clutch. The machine is in FIRST gear and the machine is in direct drive.

The lockup clutch will stay activated until there is an upshift, a downshift, or a decrease in ground speed. If there is a sufficient decrease in ground speed and the machine is still in FIRST, the Power Train ECM will stop current to the lockup solenoid. The lockup solenoid will deactivate. Hydraulic oil pressure to the lockup clutch will stop and the machine will return to torque converter drive.

Fourth (Direct Drive)

When the transmission control is moved to the FOURTH position, the rotor for the shift lever switch is moved to the FOURTH position. With the shift lever switch in this position, current flows from the Power Train ECM through the shift lever switch to machine ground.

The Power Train ECM determines that FOURTH gear is needed. The rotary selector spool turns the rotor on the transmission gear switch as the transmission shifts through the gears. As the rotary selector spool turns, the rotor on the transmission gear switch is turned by a flexible coupling. When the rotor on the transmission gear switch is in the FOURTH position, current flows from the Power Train ECM through the transmission gear switch to machine ground. The Power Train ECM determines that the shift is complete. The machine is now in FOURTH gear and the machine is in direct drive.

Shifting Procedures

The following sections will explain the operations of automatic upshifts and downshifts. Except for different electrical wires that must be grounded or activated, the shifting sequence is basically identical for all speeds.

Typical Automatic Upshift (SECOND to THIRD)

When the transmission control is put in THIRD speed position, the rotor for the shift lever switch is turned to the THIRD speed position. Current goes from the Power Train ECM through the shift lever switch to machine ground. This current flow and a signal from the transmission speed sensor will inform the Power Train ECM that an upshift is needed. The upshift to THIRD speed position from SECOND speed position is automatic when the correct ground speed signal from the transmission speed sensor is sent to the Power Train ECM, and the transmission control is in THIRD speed position or above THIRD speed position.

The Power Train ECM stops current flow to the lockup clutch solenoid when a shift is necessary. The lockup clutch solenoid deactivates. Hydraulic oil to the lockup clutch stops and the machine is in torque converter drive for a moment. At the same time, the Power Train ECM sends a signal to the Engine ECM. The Engine ECM will decrease the engine speed during the upshift.

The Power Train ECM sends current through the upshift solenoid to ground. When the upshift solenoid activates, hydraulic oil pressure goes to the rotary actuator which turns the rotary selector spool. The rotary selector spool turns the rotor on the transmission gear switch. When the alignment of the rotor on the transmission gear switch changes from SECOND speed position to THIRD speed position, current from the Power Train ECM will go through the transmission gear switch to ground. This new current flow tells the Power Train ECM that the transmission is now in THIRD speed position.

The Power Train ECM deactivates the upshift solenoid when the transmission gear switch and the ground speed is in the operating range for THIRD speed. The Power Train ECM will then activate the lockup clutch solenoid. The lockup clutch solenoid will send hydraulic oil pressure to the lockup clutch. At the same time, the Power Train ECM signals the Engine ECM to increase engine speed. The machine is now in direct drive THIRD speed.

Special Upshifts

The transmission switch will go through position NEUTRAL-2 when an upshift is performed from the NEUTRAL-1 position. This provides a delay that is needed by the hydraulics. The operator has no control over this occurrence.

During an upshift from NEUTRAL to REVERSE or from NEUTRAL to FIRST, no signal is sent to the lockup clutch solenoid. Both speeds are torque converter drive and the lockup clutch is not used.

An upshift from FIRST to SECOND is a typical automatic upshift.

Typical Automatic Downshift (FOURTH to THIRD)

When the transmission control is put in THIRD speed position, the rotor for the shift lever switch turns to the THIRD speed position. Current goes from the Power Train ECM through the shift lever switch to machine ground. This current flow and a signal from the transmission speed sensor will inform the Power Train ECM that a downshift is needed. The downshift to THIRD speed position from FOURTH speed position is automatic when the correct ground speed signal from the transmission speed sensor is sent to the Power Train ECM.

The Power Train ECM stops current flow to the lockup clutch solenoid when a shift is necessary. The lockup clutch solenoid deactivates. Hydraulic oil to the lockup clutch stops and the machine is in torque converter drive for a moment. At the same time, the Power Train ECM sends a signal to the Engine ECM. The Engine ECM will increase the engine speed during the downshift.

The Power Train ECM sends current to the downshift solenoid. When the downshift solenoid activates, hydraulic oil pressure goes to the rotary actuator and the rotary selector spool turns. The rotary selector spool turns the rotor on the transmission gear switch. When the alignment of the rotor on the transmission gear switch changes from FOURTH speed position to THIRD speed position, current from the Power Train ECM will go through the transmission gear switch to ground. This new current flow tells the Power Train ECM that the transmission is now in THIRD speed position.

The Power Train ECM deactivates the downshift solenoid when the transmission gear switch and the ground speed is in the operating range for THIRD speed. The Power Train ECM will then activate the lockup clutch solenoid. The lockup clutch solenoid will send hydraulic oil pressure to the lockup clutch. The machine is now in direct drive THIRD speed.

Special Downshifts

The transmission switch will go through position NEUTRAL-2 when a downshift is performed to the NEUTRAL-1 position. This provides a delay that is needed by the hydraulics. When a downshift from REVERSE to NEUTRAL occurs the delay is timed and slightly longer. The operator has no control over this occurrence.

During a downshift from REVERSE to NEUTRAL or from FIRST to NEUTRAL, no signal is sent to the lockup clutch solenoid. Both speeds are torque converter drive and the lockup clutch is not used.

A downshift from SECOND to FIRST is a typical automatic downshift.

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793D Off-Highway Truck Power Train: Electronic Control Module (Power Train) 2013-01-01T17:00:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown