Two Speed Rear Axle Shift System Description and Operation Air Operated

The two-speed rear axle air shift system has an electrical circuit that calibrates the speedometer to match the selected rear axle ratio. The main components of the system are air pressure-operated and not affected by this circuit. Problems with the electrical circuit will only affect speedometer operation. The AUX 2 (w/LG5) or CHASSIS (w/LP4/L21) circuit breaker supplies battery voltage during RUN or START through the ignition 3 voltage circuit to the two-speed rear axle air shift switch. When the vehicle is in low gear no air pressure should be present at the two-speed rear axle air shift switch. The two-speed rear axle air shift switch should be in a normally closed position with no air pressure present. This supplies voltage through the air shift switch signal circuit to the speedometer adapter (early LG5, LP4, or L21) or the powertrain control module (late LG5). The speedo adapter/powertrain control module changes calibration to match the low gear.

When the two-speed rear axle shift control valve is placed in the HIGH position, the shift systems air pressure increases to about 35 kPa (5 psi.) opening the two-speed rear axle air shift switch. Battery voltage is no longer present at the speedo adapter/powertrain control module. The speedometer then changes the calibration so that the speedometer reading matches road speed. As the shift system's air pressure increases, the two speed rear axle actuator shifts from low to high range.

When the two-speed rear axle shift control valve is placed back in the LOW position, the shift systems air supply is closed off and residual air pressure is vented to the atmosphere through an exhaust port in the two-speed rear axle quick release valve. As the shift systems air pressure decreases, the two-speed rear axle actuator shifts from high to low range. The two-speed rear axle air shift switch returns back to its normally closed position supplying voltage to the speedo adapter/powertrain control module and calibrates back to low gear.

View the list of major components that make up the air operated two-speed rear axle shift system below.

The two-speed rear axle actuator is mounted on the differential carrier. When air pressure is applied to the two-speed rear axle actuator, the shift fork is actuated shifting the rear axle into the high range. When the air pressure is exhausted from the rear axle actuator, a return spring moves the shift fork back allowing the rear axle to return to the low range.

The two-speed rear axle air shift switch is a pressure controlled switch mounted near the rear axle. The air shift switch with no air pressure applied to it, is normally in the closed position. The switch will open when the systems air pressure of 5 psi is applied. This switch controls voltage to the speedo adapter based on which gear the vehicle is currently in.

The two-speed rear axle pressure protection valve is mounted to the brake pedal bracket. The pressure protection valve is a normally closed pressure sensitive type control valve. It protects the vehicle's air system if a failure occurs in the two-speed rear axle air shift system.

The two-speed rear axle shift control valve is mounted to the shift lever or in the instrument cluster. The shift control valve is positioned by the driver to operate the two-speed rear axle actuator at the rear axle. The driver selects the rear axle range by positioning the two-speed rear axle control valve to the LOW gear axle range or the HIGH gear axle range. When the two-speed rear axle shift control valve is in the LOW range, the shift control valve is closed and no pressure is supplied to the system. When the two-speed rear axle shift control valve is placed in the HIGH position the shift control valve is opened allowing air pressure to the system.

The Bendix QR-N two-speed rear axle shift quick release valve is mounted on the frame side rail forward of the rear axle housing. The shift quick release valve quickly exhausts air pressure from the two-speed rear axle actuator when the two-speed rear axle is shifted from high range into the low range.

Two Speed Rear Axle Shift System Description and Operation Electric Operated

The two-speed rear axle electric shift system utilizes a two-speed rear axle shift motor to shift the axle range to either high or low. The shift motor is controlled by a two-speed rear axle shift relay which is located on the rear frame crossmember. Power flows from the battery to a junction block on the firewall on to the Maxifuse Block Assembly located under the hood. From the Ignition 3 Maxifuse power flows through the ignition switch when it is in the RUN position. The circuit continues on to the axle fuse in the Minifuse block and then splits to feed two identical Ignition 3 circuits at the two-speed rear axle relay. The ground side of the relay coil is controlled by a two-speed axle switch. The two-speed axle switch is located on the instrument panel on trucks with automatic transmissions and on the gear shifter for manual transmissions. When the driver pushes the Two Speed Rear Axle Switch the current is allowed to go to ground energizing the relay. When the relay is energized it sends current to the two-speed rear axle motor and the motor completes the shift. When the two-speed rear axle switch is in the LOW position, the switch is open, the relay is de-energized, and voltage is present at the powertrain control module calibrating it for low gear.

When the two-speed rear axle switch is placed in the HIGH gear position, the relay control circuit is closed to ground, removing voltage from the powertrain control module and energizing the two-speed rear axle shift relay. The energized relay allows current to flow through the high gear shift motor control circuit to the shift motor. Inside the shift motor, current flows through closed contacts of the high range switch and motor to ground which starts the motor rotating. Current also continues from common motor terminal through a low range switch and to ground through the resistor. As the motor begins to rotate the gear, a low range cam pin on the gear ramps up and opens the low range switch. The path to ground is now open and a diode prevents the powertrain control module from being energized. The motor rotation continues to shift the axle to high range. As the shift is completed, a high range cam pin opens the high range switch, opening the circuit and removing power from the motor. Motor rotation is stopped quickly by means of a dynamic brake. At this point the gear will have rotated 180 degrees. A break in torque by the driver allows spring force to move the swivel pin and shift fork in the appropriate direction for axle range change. When the opposite axle range is selected by the driver, the process is repeated by the motor turning 180 degrees in the opposite direction.

There are minor variations in harness wiring between trucks with Air-Operated systems and Electric Operated systems. Also wiring variations exist between trucks with automatic and manual transmission and hydraulic and air brake options. See the wiring diagram for details.

View the list of major components that make up the electric operated two-speed rear axle shift system below.

On manual transmission models, the two-speed rear axle driver control switch is mounted onto the manual transmission shift lever. On automatic transmission models, the two-speed rear axle driver control switch is mounted to the instrument panel. The two-speed rear axle driver control switch is positioned by the driver in order to operate the two-speed rear axle shift motor at the rear axle. The driver selects the rear axle range by positioning the two-speed rear axle driver control switch to the LOW or HIGH axle range.

The two-speed rear axle shift motor is mounted to the differential carrier. The two-speed rear axle shift motor consists of the following components:

The shift motor rotates a worm gear which turns another gear which moves an eccentric pin in the slot area of the slider block changing spring compression. The slider block uses spring compression to move a swivel pin and shift fork into the appropriate direction for axle range change.

The two-speed rear axle shift relay is mounted to the crossmember in front of the rear axle housing. When the two-speed rear axle driver control switch is in the HIGH range, a ground is supplied to the two-speed rear axle shift relay. The two-speed rear axle shift relay then actuates the two-speed rear axle shift motor to the HIGH range. When the two-speed rear axle driver control switch is returned to the LOW range, the two-speed rear axle shift relay is de-energized. The two-speed rear axle shift motor then returns to the LOW range. The two-speed rear axle shift relay has two ignition 3 wires that have voltage on them when the ignition switch is in the RUN position.