3126E Truck Engines Caterpillar

Engine Description


3126E HEP

The Caterpillar 3126E Truck Engine is designed primarily for on-highway trucks with medium-duty applications. The engine has the following characteristics: air-to-air aftercooled, direct fuel injection, four stroke cycle and in-line 6 cylinder.

Engine Specifications

Note: The front end of the engine is opposite the flywheel end of the engine. The left and the right sides of the engine are determined from the flywheel end. The number 1 cylinder is the front cylinder.

Illustration 1g00407141

Cylinder and valve location

(A) Exhaust valve

(B) Inlet valve

Table 1
3126E Truck Engine Specifications    
Arrangement and Cylinders     In-Line 6 cylinder    
110 mm (4.3 inch)    
127 mm (5.0 inch)    
Aspiration     ATAAC (1)    
7.2 L (440 in3)    
Firing Order     1-5-3-6-2-4    
Rotation (flywheel end)     Counterclockwise    
Valve Lash (inlet)    
0.38 mm (0.015 inch)    
Valve Lash (exhaust)    
0.64 mm (0.025 inch)    
( 1 ) Air-to-air aftercooled

Electronic Engine Features

The Caterpillar 3126E Truck Engine is designed for electronic controls. The integral on board computer controls the operation of the engine. Current operating conditions are monitored. The Engine Control Module (ECM) controls the response of the engine to these conditions and to the demands of the operator. These conditions and operator demands determine the precise control of fuel injection by the ECM. The electronic engine control system provides the following features:

  • Engine speed governor

  • Automatic air/fuel ratio control

  • Torque rise shaping

  • Injection timing control

  • System diagnostics

The following programmable features are included in the electronic control:

  • Cruise control

  • Governing of PTO

  • Vehicle speed limiter

  • Idle shutdown

  • Compression brake operation control

For more information on electronic engine features, refer to the Operation and Maintenance Manual, "Engine Features and Controls" topic (Operation Section).

Additional Features

The following additional features provide increased engine fuel economy and serviceability:

  • Cold starting capability

  • Tampering detection

  • Diagnostics

  • American Trucking Association (ATA) data link ("SAE J1587")

Hydraulic Electronic Unit Injectors

Hydraulically Actuated Electronic Unit Injectors (HEUI) perform the following functions: pumping of fuel, fuel metering and fuel timing.

The unit injectors are controlled by the ECM which uses the camshaft position and the engine speed signals from the engine speed/timing sensors and the inlet air pressure sensors. The engine's rated rpm is identified on the Information Plate.

Engine Diagnostics

The engine has built-in diagnostics in order to ensure that all of the components are functioning properly. In the event of a deviation from the programmed limits, the operator will be alerted to the condition by a "DIAGNOSTIC" lamp that is mounted on the dashboard. Under certain conditions, the engine horsepower and the vehicle speed may be limited. A Caterpillar electronic service tool or the vehicle's cruise control switches (if equipped) may be used to display the diagnostic code.

There are three types of diagnostic codes: active, logged and event.

Most of the diagnostic codes are logged and stored in the ECM. For additional information, refer to the Operation and Maintenance Manual, "Engine Diagnostics" topic (Operation Section).

The ECM provides an electronic governor that controls the injector output in order to maintain the desired engine rpm. The functionality of electronic governor is similar to the Caterpillar mechanical governor, but the electronic governor includes additional features.

Engine Cooling and Lubrication

The cooling system consists of the following components:

  • Centrifugal pump that is driven by belts

  • Water temperature regulator which regulates the engine coolant temperature

  • Oil cooler and radiator which incorporates a shunt system

The engine lubricating oil that is supplied is cooled. The engine lubricating oil is also filtered. Bypass valves provide unrestricted flow of lubrication oil to the engine components during the following conditions:

  • High oil viscosity

  • Plugged oil cooler or plugged oil filter elements (paper cartridge)

Engine Service Life

Engine efficiency and maximum utilization of engine performance depend on the adherence to proper operation and maintenance recommendations. In addition, use recommended fuels, coolants and lubricants. Use the Operation and Maintenance Manual as a guide for required engine maintenance.

Expected engine life is generally predicted by the average power that is demanded. The average power that is demanded is based on fuel consumption of the engine over a period of time. Reduced hours of operation at full throttle and/or operating at reduced throttle settings result in a lower average power demand. Reduced hours of operation will increase the length of operating time before an engine overhaul is required. For more information, refer to the Operation and Maintenance Manual, "Overhaul Considerations" topic (Maintenance Section).

Aftermarket Products and Caterpillar Engines


In order to maximize fuel system life and prevent premature wear out from abrasive particles in the fuel, a two micron absolute high efficiency fuel filter is required for all Caterpillar Hydraulic Electronic Unit Injectors. Caterpillar High Efficiency Fuel Filters meet these requirements. Consult your Caterpillar dealer for the proper part numbers.

When auxiliary devices, accessories, or consumables (filters, additives, catalysts, etc) which are made by other manufacturers are used on Caterpillar products, the Caterpillar warranty is not affected simply because of such use.

However, failures that result from the installation or use of other manufacturers' devices, accessories, or consumables are NOT Caterpillar defects. Therefore, the defects are NOT covered under the Caterpillar warranty.

Welding and Caterpillar Electronic Engines


Because the strength of the frame may decrease, some manufacturers do not recommend welding onto a chassis frame or rail. Consult the OEM of the equipment or your Caterpillar dealer regarding welding on a chassis frame or rail.

To help avoid damage to the electronic controls, proper welding procedures are necessary. Before welding on a vehicle that is equipped with an electronic engine, observe the following precautions:

  1. Turn off the engine. Place the key start switch in the OFF position.

  1. If the vehicle has a battery disconnect switch, open the switch. Otherwise, disconnect the negative "-" battery cable from the battery of the vehicle.


    Do not ground the welder to electrical components such as the ECM or sensors. Improper grounding can cause damage to the drive train bearings, hydraulic components, electrical components, and other components.

    Clamp the ground cable from the welder to the component that will be welded. Place the clamp as close as possible to the weld. This will help reduce the possibility of damage.

  1. Clamp the ground cable from the welder to the component that will be welded. Place the clamp as close as possible to the weld.

  1. Protect wiring harnesses from welding debris and spatter. Use proper welding procedures.

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