Two Speed Rear Axle Shift System Description and Operation (Air-Operated)

Two Speed Rear Axle Shift System Description and Operation (Air-Operated)

Two Speed Rear Axle (Air-Operated)

The two speed rear axle air shift system has an electrical circuit that re-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 AXLE (w/RQ2) or CHAS (w/RQ3) 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) or the powertrain control module (late LG5/L18). 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 re-calibrates back to low gear.

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

Rear Axle Actuator - Two Speed

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.

Rear Axle Air Shift Switch - Two Speed

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/powertrain control module based on which gear the vehicle is currently in.

Rear Axle Pressure Protection Valve - Two Speed

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.

Rear Axle Shift Control Valve - Two Speed

Two 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.

Rear Axle Shift Quick Release Valve - Two Speed

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.

Speedometer Adapter

The speedometer adapter is used to match the ratio of the speedometer to the ratio of the rear axle. The speedometer adapter gives an accurate speedometer reading when the rear axle is shifted from one range to the other. When voltage is present at the speedo adapter it calibrates for low gear and with the voltage removed the speedometer adapter reads the high gear of the rear axle.