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Tractor Trailer ISO 3731 Cable Standards and Cab Switched Auxiliary Devices

The addition of the ABS light presents a challenge for powering cab switched auxiliary devices.

Beginning with the 1998 year model production vehicles, the standard tractor/trailer cable and connectors (SAE J560) incorporate a DOT requirement. The Number 7 Blue wire is now a key on 12.5 Volt 10 Ampere power wire which powers the trailer ABS module. There are limits to additional circuits that can be powered from this wire. Modification of the J560 connector, cable or wiring can effect DOT regulation compliance.

To meet the needs of those who require 12-volt cab switched power to various auxiliary devices on trailers, manufacturers are recommending the installation of a second vehicle connector cable, ISO 3731.

Parts Needed:

Other related components (as required)

Action Required:

Determine the voltage and amperage requirements of the auxiliary device(s) to be controlled. It is recommended to use relay(s) for devices requiring more than 10 Amperes.

Locate unused circuit breaker(s) and relay(s) module sockets.

Construct a wiring harness to accommodate the switching, breaker and relay control circuit. Use appropriate connectors, soldering, and heat shrink methods to construct the harness. Refer to Table 1 for recommended usage of pins.

Locate a suitable area on back of cab at least 10' from the existing J560 connector. Mark this area with a template of the ISO 3731 socket.

NOTE: Some models are equipped with a predrilled hole for this socket.

Carefully drill the appropriate holes to install the ISO 3731 socket, making sure not to damage any existing wiring, air lines, etc that may be present.

Remove all burrs left from the drilling or hole saw operations.

Apply a thin (1/8') bead of clear silicone sealant around the holes, install and bolt in the ISO socket. Use any gasket(s) supplied with the socket assembly.

Construct a second harness from the relay(s) to the back of cab area allowing sufficient length to install into the ISO 3731 socket. Allow for strain relief measures at the socket.

Using proper soldering, heat shrinking, and connection protection methods, wire the harness to the ISO 3731 socket in accordance to the switched control circuits required for your application per item three (3) above.

Test all circuits, controls, switches, etc for correct operation with VOM.

Using the pin out diagram developed during the installation of the ISO 3731 socket, wire the trailer auxiliary devices accordingly.

Using a VOM, test all newly wired devices and controls prior to hooking up to tractor.

Make a final test of all ISO 3731 related circuits and devices. Ensure no problems, such as wire chaffing, rubbing, pulling, or connection problems exist before releasing to customer.

For future reference it is a good idea to make up a wiring diagram used for the ISO 3731 configuration from the dash control switches to the trailer devices and place the vehicle file.

ISO 3731 Receptacle, Supplemental Circuits
Pin No.ColorCircuit
1WhiteGround Return
2 BlackCab Switched Aux. Power, 12 Volt
3YellowABS Malfunction Signal to Cab
4 RedConstant Ignition Switched Aux. Power
7BlueOpen, alternate ABS when specified

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