3126B Marine Engines Caterpillar

Engine Wiring Information


3126B 3GS

The wiring schematics are revised periodically. The revisions reflect the updates to the engine's wiring harness. For the most current information, always check the revision number of the schematic. Use the schematic with the latest revision number.

Harness Wire Identification

Caterpillar identifies all wires with eleven solid colors. The circuit number is stamped on the wire at a 25 mm (1 inch) spacing. Table 1 lists the wire colors and the color codes.

Table 1
Color Codes for the Harness Wire    
Color Code     Color     Color Code     Color    
BK     Black     GN     Green    
BR     Brown     BU     Blue    
RD     Red     PU     Purple    
OR     Orange     GY     Gray    
YL     Yellow     WH     White    
        PK     Pink    

For example, a wire identification of A701-GY on the schematic would signify a gray wire with the circuit number A701. A701-GY identifies the power circuit for the No. 1 Injector solenoid.

Another wire identification on the schematic is the size of the wire. The size of the wire will follow the wire color. Wire size or gauge is referred to as AWG (Americal Wire Gauge). AWG is a description of the diameter of the wire.

For example, a code of 150-OR-14 on the schematic would indicate that the orange wire in circuit 150 is a 14 AWG wire.

Requirements for Wire Size

The wire size to all Caterpillar components must have an adequate size in order to handle the maximum current of the circuit. Table 2 shows the various AWG wire numbers that are used. The metric equivalents for the diameter of the AWG wire numbers are also shown.

Table 2
Metric Equivalents for AWG Wire Numbers    
AWG Wire Number     Diameter (mm)     AWG Wire Number     Diameter (mm)    
20     0.8     14     1.6    
18     1.0     12     2.0    
16     1.3     4     3.2    

Follow these requirements for wire size:

  • Make all positive battery connections and all negative battery connections to the customer connector with 14 AWG wire.

  • Make all negative battery connections from the bridge displays to the negative bus bar with 12 AWG wire minimum.

  • Make all other connections with 16 AWG wire minimum.

  • Connect only the recommended components to the negative battery wiring for the bridge displays.

Wiring for the Cat Data Link

Make all connections for the Cat Data Link with 143-5018 Wire .

Battery Connections

Caterpillar recommends bus bars which are dedicated solely to the electronic devices that are connected to the engine Electronic Control Module (ECM). This ensures that the ECM, the switches, the sensors, and the electronic display modules have a common reference point.

Make the battery connections according to Illustration 1. This ensures that all of the components in the system have the negative battery bus bar as a common reference point.

Illustration 1g01126072

Battery connections for the operator station (two engine arrangement)

All negative battery connections must begin at the negative battery bus bar and the wiring must terminate at the negative battery connection on the component that is being wired. Make the connection from the negative battery bus bar to the displays on the bridge with a minimum of 12 AWG wire. No other components should be connected to the negative battery bus bar.

Welding on an Engine that is Equipped with an Electronic Control System

Proper welding procedures are necessary in order to avoid damage to the engine's ECM, sensors, and associated components. Remove the component that requires welding from the engine, when possible. If you cannot remove the component from the engine, the following procedure must be followed. This procedure provides the minimum amount of risk to the electronic components.


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. Stop the engine. Turn the keyswitch to the OFF position.

  1. Refer to Illustration 2. Disconnect the negative battery cable from the battery. If a battery disconnect switch is installed, open the switch.

    Note: Disconnecting the positive battery cable and the negative battery cable is recommended. Using the battery disconnect switch to open one side of the battery circuit and removing the battery cable for the remaining side of the battery circuit is acceptable.

    Illustration 2g01126278

    The welding current will not cause damage to any of the associated components.

  1. Connect the welding ground cable as close as possible to the area that will be welded. Components which may be damaged by welding include bearings, hydraulic components, and electrical/electronic components.

  1. Protect the wiring harness from welding debris and spatter.

  1. Weld the materials by using standard welding methods.

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