1983/08/31 Caterpillar

3406 Truck Engines

Description of Change: A new filter orifice is now available for parts service in the governor and fuel injection pump group on 3406 Truck Engines. The new filter orifice can be installed in the oil supply line for the governor servo. The filter will keep the orifice free of debris. The orifice controls the movement of the governor servo to prevent sudden changes in the engine rpm (governor surge). Formerly, it was possible for governor surge to cause too much cab vibration during operation at engine speeds between 1600 and 1700 rpm.

Adaptable To: The 8N4622 Filter Orifice (black) can be used on engines with the 8N2898 Fuel Pump Housing. The 8N4621 Filter Orifice (yellow) can be used on engines with the 7N7334 Fuel Pump Housing. Check the outside of the pump housing for the part number.

The servo valve oil passages in the two pump housings have a different diameter. Because of this, the filter orifices have a different diameter seal lip and must be used in the correct pump housing.

An analysis of a vibration problem is difficult because there are many possible causes that have no relation to the engine. These causes cannot be corrected with the use of a filter orifice. A filter orifice can correct a rough ride or cab vibration problem under the two conditions that follow:

1. if the vibration occurs under a low load condition, at partial throttle, when there is no change in the throttle position, and

2. when the throttle is moved to the full-on or full-off position the vibration stops.

If there is still vibration when the throttle is moved to the full-on or full-off position or when a shift of the transmission to neutral is made, the use of a filter orifice will not correct the problem.

Two primary causes of governor surge are cab/throttle movement or a change in the vehicle drive train load. Therefore, changes in these areas can also decrease or correct a vibration problem. Improvements have been made on some truck models by the installation of cable throttle controls, by the addition of bellcranks to the throttle linkage with the mounting brackets fastened to the engine, by the use of less flexible engine mounts or by a change in tires, etc.

For installation of the filter orifice, it is not necessary to remove the fuel injection pump from the engine. Remove the lower 1N5999 Plug on the side of the fuel pump housing near the engine block. See the illustration. Put the filter orifice in oil to make installation easier. Install the orifice, small end first, in the oil passage as far as it will go. Install the 1N5999 Plug.

On engines that have a noise suppression panel, remove two or three bolts that hold the panel to the block. The panel can then move down to give enough clearance to install the filter orifice. Install the noise suppression panel.