CX31 and CX31 HT On-Highway Transmissions Transmission Operation Caterpillar

Transmission Operation
1.1. Power Take-Off (PTO)
2.1. Driveline Power Take-Off (DPTO)
3.2. Nomenclature
4.2. Features
5.1. Locations for the Support Brackets

Transmission Cooling (General Recommendations)

Automatic transmissions generate heat due to torque converter slippage, charging pump losses, and friction in rotating components. The transmission fluid absorbs this heat, and an oil cooler is required to dissipate the heat. Keeping the oil cool maintains the lubricating properties of the oil. Keeping the oil cool allows the transmission to operate properly. Also, cooled oil extends the life of the transmission components.

The engine coolant (antifreeze) and the automatic transmission fluid (ATF) absorb the heat generated by the engine and transmission. The transmission fluid carries the heat to a heat exchanger (radiator or cooler) and the oil or antifreeze is cooled.

Fluid Requirements

AVSpare recommends a 1:1 ratio of ethylene glycol to water and the proper anti-corrosive additives as a cooling medium for the radiator. If the vehicle operates below minus 1.7 °C (35 °F), a 60:40 ratio of ethylene glycol to water is recommended. See the Truck Application and Installation Guide for additional information.

Do not use water without using the proper cooling additives. Using water alone does not provide adequate boiling and freezing protection. Also, using water without additives is corrosive to the cooling system of the engine.


Torque Converter Cooling

AVSpare On-Highway Transmissions have lockup clutches that reduce the heat generated by torque converters in all gears except first gear and reverse. Under normal operating conditions, the transmission will upshift and downshift to the proper gear ratio and spend most of the operating time in the converter lockup clutch mode. The lockup clutch reduces the torque converter cooling requirements. If the driver selects first gear hold, the lockup clutch is disabled. This mode of operation will result in continuous torque converter usage. As a result, torque converter slippage will generate higher heat loads. The following conditions will dictate transmission cooling requirements: the application, ambient temperatures, the horsepower being transmitted and the length of time in converter drive.

Temperature Operating Limits

The maximum allowable transmission oil temperatures for normal (continuous) and intermittent (testing) conditions are the following:

Continuous sump ... 110° C (230° F)

Continuous transmission out ... 121 °C (250 °F)

Intermittent sump ... 121 °C (250 °F)

Intermittent transmission out ... 148° C (298° F)

Transmission Oil Temperature Gauge

The transmission oil temperature gauge located on the truck dash has an amber LED light that will be turned on when the transmission sump oil temperature reaches 110° C (230° F).

Transmission Overheating

In the unlikely event that the transmission does over heat, the transmission should be shifted to neutral and engine speed should be elevated to high idle until the transmission temperature returns to the proper operating temperature. Elevating the engine speed to high idle causes the engine fan to provide the most airflow across the radiator and maximizes oil flow through the cooler.

Towing and Coasting

Since the transmission hydraulic pump is driven by the engine, the rpm of the pump will vary with the speed of the engine. One of the many functions of this pump is to provide lubrication. If the engine speed is non-existent (towing) or at a lower level than intended (coasting), considerations must be taken into account. If the CX31 is installed in a wheeled vehicle that requires towing, the drive shaft must be disconnected from the transmission or the drive axle or axles lifted off the ground. Coasting in NEUTRAL is not recommended by AVSpare. Although, if coasting in neutral situation should occur, the engine speed must be increased in order to provide the proper lubrication to the transmission.


Power Take-Off (PTO)


The CX31 Transmission has three PTO mounting pads. There is one pad on each side of the transmission near the input end. Also, there is one pad at the rear of the transmission. Bolt-on PTO units are available from aftermarket suppliers. The bolt-on PTO units can be specified in order to adapt to various output options. Specifying and designing the installation of the PTO devices are the responsibility of the OEM, body builder, or the end user.

Both side PTO's are standard on the CX31 Transmission. When the side PTO positions are viewed from the rear, side PTO positions are 8 o'clock and 1 o'clock. In addition, there is an optional rear power take-off (RPTO) that is positioned on the lower right, next to the transmission output shaft.

------ WARNING! ------

To avoid personal injury due to entanglement with rotating shaft, keep PTO master cover and all power drive system covers in place. Install PTO shaft guard when the system is not in use.

Note: Serious injury can be prevented with proper drive shaft guards. The OEM or the person who installed the auxiliary PTO is responsible to provide guards on the exposed rotating parts.

Direction of Rotation

Engine (Rotation SAE J824 - Standard)

  • Counterclockwise as viewed from the flywheel end of the engine

Transmission (Output Rotation)

  • Forward Same as Input (Converter)

  • Reverse Opposite of Input

PTO (Rotation)

  • Transmission side PTO drive gear rotates at engine rpm and same direction as transmission input.

  • Rear PTO shaft rotates at 1.25 X Engine rpm and in the opposite direction of transmission input.

  • PTO adaptors from various third-party suppliers, will determine PTO shaft direction and speed.

Transmission PTO Control

The CX31 Transmission ECU may be used to program PTO functions or the PTO supplier may provide separate PTO controls. The programmable features may include, but are not limited to the following conditions:

  • PTO operation in Neutral only

  • PTO engagement below a specific engine speed

  • PTO overspeed protection

Side PTO Mounting Bolts

  • M10 x 1.5 externally threaded fastener

  • Minimum fastener Grade 10.9 in accordance with SAE J1199 (1040 MPa min)

  • Minimum quantity of eight bolts are required to secure the PTO to the transmission. Alignment dowels are optional hardware that may be provided by the PTO supplier.

Side PTO Electrical Connections

  • Refer to Special Instruction, REHS2884, "Application and Installation Guide (Electrical)" for specific OEM electric interface definitions and requirement.

  • Refer to the supplier of the PTO attachment for installation instructions of the PTO electrical connections.

Rear PTO Interface

Removal of the rear PTO cover after the transmission has been properly filled with oil may cause the loss of some fluid. The amount of oil lost will depend on the installation angle of the transmission and the surface plane of the vehicle when the cover is removed. (Some PTO component suppliers recommend draining the transmission oil prior to installation of PTO components). Check the transmission oil level after installing the PTO components in order to make sure that any oil that may be lost has been replaced. Also, check the oil level in order to make sure that the added component did not increase the oil requirements of the transmission. The installation dimensional drawings provide additional specifications for the rear PTO mounting pad.

Note: Rear PTO will accept a four bolt SAE J744 “B” pump drive and pilot. A supplier adapter is required to mate with the 17 tooth AVSpare spline.

Rear PTO Bolt Requirements

  • M12 x 1.75 externally threaded fastener

  • Minimum fastener Grade 10.9 in accordance with SAE J1199 (1040 Mpa min)

  • Minimum quantity of eight bolts required to secure the PTO to the transmission.

Rear PTO Sealing Requirements

  • The rear PTO is designed for a standard 241, 6V-1197 O-Ring Seal.

  • The O-ring is intended to provide a static radial sealing over the pilot diameter of the unit mating to the rear PTO pad.

  • O-ring Seal material is FKM (Viton)

Side and Rear PTO Torque Limits

Table 1
Maximum Output Torque Permitted at the Side and Rear PTO Couplings 
CX31 PTOs  Maximum Torque 
Single Side PTO  700 ft lb 
Single Rear PTO  800 ft lb 
Maximum Total PTOs  1200 ft lb 


Driveline Power Take-Off (DPTO)



DPTO is a feature that allows the transmission to deliver power to a PTO load through the transmission output shaft instead of delivering power to the vehicle wheels.


Illustration 1g01382463
(1) Pump
(2) Transfer case (split shaft PTO)

The objective of this section of the Application and Installation Guide is to supply the OEM builder with information needed to design a DPTO system. The Special Instruction, REHS2884, "Application and Installation Guide (Electrical)" provides programming and wiring information about the DPTO. This Mechanical Application and Installation Guide outlines features and the guide sets the reflected inertia limits for the transmission brake. Programmable options include the following options: the selection of the transmission operating gear, lockup clutch engagement, disengaging engine speeds and appropriate on and off features.

The ECU provides a fuel efficient mode of operation for the DPTO feature when activated. Typical applications use fourth gear, which is 1.0:1.0 ratio (engine speed) as the DPTO output. The torque converter lockup function is normally set to engage at engine speeds above 1,000 rpm. The torque converter lockup function is normally set to revert to converter drive below engine speeds of 800 rpm. Converter drive provides higher starting torques for DPTO applications and converter drive allows the engine to run with a stalled output DPTO.

The software provides a driveline brake that can be used in applications to stop the rotation of the transmission output shaft. The driveline brake feature uses two of the stationary clutches engaged at 40 psi that act as the brake on the transmission. The clutches have energy limits that must not be exceeded. Therefore, the braking software will only allow the braking feature to become active below a transmission output shaft speed of 175 rpm.

The program also causes the brake to disengage (time out) after 10 seconds if the rotating inertia has not come to a stop. A cooling off period for the clutches is also provided in the software that will only allow the braking function to be reapplied after a 30 second cool down. The driveline brake must not be used for large generators or to decelerate large pumps.

The brake feature is limited to inertias that are reflected at the transmission output shaft of less than 58 lb ft2.

Illustration 2g01391211

A formula for calculating the inertia limits is given in the following example:

(3) Drive Shaft Inertia = 5.73 lb ft2

(4) Input Shaft Inertia = 3.77 lb ft2

(5) Gear Ratio = 1.96

(6) Intermediate Shaft Inertia = 2.86 lb ft2

(7) Pump/Rotor (wet) Inertia = 9.87 lb ft2

Total Reflected Inertia Limit = TRIL#

Maximum Inertia Example (Worst Case)

Note: All inertia units are: lb ft2 = #

TRIL# = 5.73# + 3.77# + 1.962(2.86# + 9.87#) = 58.4#


Locations for the Support Brackets



If bolts are removed from the CX31 transmission to add mounting brackets, and so on, the bolts must be replaced with longer bolts so that thread engagement is maintained. The replacement bolts must be of the same grade and torqued to the required specifications.

Thread engagement length and bolt torques are critical on the Aluminum Case and Cover of the CX Transmissions. The bolts selected must have a thread engagement length of twice the bolt diameter. Standard torques on short bolts will strip the threads. The bolts must not be too long because the bolts will bottom out and crack the case.

Illustration 3g03449096
(53) Bolt locations for support brackets

Illustration 4g03449102
(54) Bolt locations for support brackets



Tightening torques are called out in the installation dimensional drawings. Tightening torques that are called out in the installation dimensional drawing take precedence over the standard torque values. The dimensional drawings are in the Appendix of Special Instruction, REHS2790, "Application and Installation Guide (Mechanical)".

Standard Bolt Torques

Note: The torques in the following tables are based on metric Grade 10.9 or higher fasteners (SAE Grade 8 or higher inch fasteners). Use hardened washers. Split lock washers are not permitted.

Table 2
Inch Nuts and Bolts 
Thread Size (Inch)  Standard Torque 
1/4  12 ± 3 N·m (9 ± 2 lb ft) 
5/16  25 ± 6 N·m (18 ± 4 lb ft) 
3/8  47 ± 9 N·m (35 ± 7 lb ft) 
7/16  70 ± 15 N·m (50 ± 11 lb ft) 
1/2  105 ± 20 N·m (75 ± 15 lb ft) 
9/16  160 ± 30 N·m (120 ± 22 lb ft) 
5/8  215 ± 40 N·m (160 ± 30 lb ft) 
3/4  370 ± 50 N·m (275 ± 37 lb ft) 
7/8  620 ± 80 N·m (460 ± 60 lb ft) 
900 ± 100 N·m (660 ± 75 lb ft) 
1 1/8  1300 ± 150 N·m (960 ± 110 lb ft) 
1 1/4  1800 ± 200 N·m (1320 ± 150 lb ft) 
1 3/8  2400 ± 300 N·m (1780 ± 220 lb ft) 
1 1/2  3100 ± 350 N·m (2280 ± 260 lb ft) 

Table 3
Metric Nuts and Bolts 
Thread Size (Metric)  Standard Torque 
M6  12 ± 3 N·m (9 ± 2 lb ft) 
M8  28 ± 7 N·m (21 ± 5 lb ft) 
M10  55 ± 10 N·m (41 ± 7 lb ft) 
M12  100 ± 20 N·m (75 ± 15 lb ft) 
M14  160 ± 30 N·m (120 ± 22 lb ft) 
M16  240 ± 40 N·m (175 ± 30 lb ft) 
M20  460 ± 60 N·m (340 ± 44 lb ft) 
M24  800 ± 100 N·m (590 ± 75 lb ft) 
M30  1600 ± 200 N·m (1180 ± 150 lb ft) 
M36  2700 ± 300 N·m (2000 ± 220 lb ft) 

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