789D CMD Off-Highway Trucks (Command for Hauling) Power Train Electronic Control System Caterpillar


Switches

Usage:

789D SPD

Pressure Switch (Transmission Lube Filter)



Illustration 1g00304427
Pressure Switch (Transmission Lube Filter)

The Transmission Lube Filter pressure switch is an input to the ECM. The switch connects to J1-40. The switch has a plunger within the switch that closes the contacts with an acceptable pressure drop across the filter. During normal operation, the switch grounds the signal circuit. When the pressure across the filter is greater than the specified value, the switch plunger moves and the switch plunger allow the contacts to open. The open switch tells the ECM that the filter is restricted.

The switch has two contacts. One contact connects to J1-20 on the ECM. The other contact connects to frame ground.

Transmission Charge Filter Bypass Switch

The "Charge" pump provides hydraulic oil to the variable displacement pump and closed loop hydraulic circuit. The Charge pump is a gear pump. Excess fluid cools the variable displacement pump. The filter bypass switch relieves pressure in the hydraulic system when the filter is clogged or the hydraulic oil is cold. Excess charge pressure is directed to the hydraulic oil tank.

Torque Converter Bypass Switch



Illustration 2g03525817
Bypass Switch

The "Torque Converter Bypass" switch is an input to the Transmission ECM. When a restriction exists in the filter element, oil pressure increases at the filter. The increased oil pressure activates the bypass valve. The excess oil is returned to the hydraulic tank to relieve the pressure in the system.

Transmission Oil Level Switch



Illustration 3g00482124
Oil Level Switch
(1) Switch
(2) Schematic symbol

Oil level switch (1) is closed to ground during normal operation. The switch opens when the oil level is less than the minimum level. When the switch opens, the Transmission ECM warns the machine operator of the low oil level. Off the machine (on the bench), the switch is open with the float in the down position.

The switch has two contacts. One contact connects to J1-35 on the ECM. The other contact connects to frame ground.

Transmission Actual Gear Switch



Illustration 4g06361126

The transmission upshifts and downshifts automatically, in response to transmission gear changes. The "Transmission" ECM continuously sends the status of the "Transmission Current Gear" to the "Chassis" ECM via dedicated data link. The "Transmission Actual Gear" switch continuously updates the "Transmission" ECM on the "Current Gear" status of the transmission. The information is used to ensure that the truck is in the most efficient gear.

The Actual Gear Switch is connected to the following pin locations on the Transmission ECM:

  • Transmission Gear 1 (J2-37)

  • Transmission Gear 2 (J2-38)

  • Transmission Gear 3 (J2-39)

  • Transmission Gear 4 (J2-45)

  • Transmission Gear 5 (J2-46)

  • Transmission Ground Verify (J2-47)

Caterpillar Information System:

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