For those who own trucks with a CAD (center axle disconnect) front axle, this article will explain the benefits of remotely controlling the CAD operation and will also provide information on how to make the modification.
For those who are not familiar with a front axle CAD unit, the following description may help.
OEM vehicle - manually shifted transfer case (with CAD) - Theory of Operation
The CAD front axle has three axle shafts - a driver side axle shaft, a passenger side axle shaft, and a short center axle shaft. The passenger side axle shaft and the center axle shaft can be connected or disconnected by the CAD unit. The center axle shaft is called the intermediate axle shaft in the photo below. The CAD unit is a vacuum motor that controls a splined sliding collar to lock the right axle shaft to the center axle shaft. There is a switch at the end of the CAD unit that allows illumination of the 4WD indicator lamp in the dash.
2WD Selected - front driveshaft does not rotate and CAD is disconnected. This feature minimizes front axle friction at highway speeds.
4WD HI Select on-the-fly - As the transfer case shifter is being pulled into the 4-HI position, a synchronizer in the transfer case brings the front driveshaft up to speed. As the shift is completed, a vacuum valve inside the transfer case actuates the front axle CAD unit and locks the passenger side axle shaft directly to the center axle shaft. 4WD indicator lamp is illuminated.
4WD LO Select - Vehicle must be stopped. Select 4WD LO. 4WD indicator lamp is illuminated.
Something worthy of noting - when 2WD is selected, the front drive shaft stops rotating, the left and right axle shafts are still rotating (driven by the wheels), and the center axle shaft is still rotating, BUT in the opposite direction (because of the differential). This is the reason that when 4WD Hi is selected, the synchronizer in the transfer case brings the front driveshaft up to speed first. When the front driveshaft is brought up to speed first, then all three front axle shafts are rotating at the same speed and in the same direction. Now the CAD unit can connect the passenger side axle shaft to the center axle shaft.
2WD-LO Conversion (parts needed)
Toggle switch - two position maintained switch with physical lockout
Humphrey 4 way / 2 position, 1/8" NPT, spring offset, vacuum solenoid valve Model 410/12 VDC
Appropriate length of vacuum tubing and 16 gauge automotive wire and wire connectors.
This conversion separates the operation of the transfer case manual shifter and the CAD vacuum controlled shifter. The external vacuum ports of the vacuum valve (inside the transfer case) will be disconnected and plugged. A new vacuum solenoid valve for the CAD unit operation will be installed in a location of choice. A fused switch will be installed to operate the new CAD vacuum solenoid valve. A vacuum supply line will be routed to the CAD vacuum solenoid valve. Two vacuum lines will be installed to connect the new CAD vacuum solenoid valve to the CAD unit.
Instructions for operating the new conversion are as follows:
2WD High - The transfer case selector must be in 2WD and the CAD switch must be turned off and locked out.
4WD High - Vehicle must be stopped. The CAD switch must be turned on. The 4WD indicator lamp will illuminate (truck may have to be moved slightly to engage 4WD lamp). From this point forward the transfer case can be shifted from 2WD to 4WD or 4WD to 2WD on-the-fly.
2WD Low - Vehicle must be stopped. Shift transfer case into Low Range. Leave the CAD switch turned off and locked out. The 4WD lamp will not illuminate.
4WD Low - Vehicle must be stopped. Shift transfer case into Low Range. Turn on CAD switch. The 4WD lamp will illuminate (truck may have to be moved slightly to engage 4WD lamp).
Tips
The 2WD Low Range can be very useful, especially for backing trailers in tight spaces, whether loaded or not. This is true for manual and automatic transmissions. You can basically idle the rig while backing very slowly in tight turns without the wheel hop associated with 4WD.
Also, when traveling off-road on very steep and twisty terrain with varying traction conditions in low range, you can shift from 2WD to 4WD or 4WD to 2WD on-the-fly. Just leave the transfer case in 4WD and operate the CAD switch - "Off" for 2WD, "On" for 4WD. Always unload the engine (ease up on the throttle) when making the changes to allow the CAD unit to slide the shift collar easily.
The CAD switch has a physical lockout to reduce the chance of accidental operation. When the switch not going to be used, always engage the lockout with the switch in the "Off" position.
For those who own trucks with a CAD (center axle disconnect) front axle, this article will explain the benefits of remotely controlling the CAD operation and will also provide information on how to make the modification.
For those who are not familiar with a front axle CAD unit, the following description may help.
OEM vehicle - manually shifted transfer case (with CAD) - Theory of Operation
The CAD front axle has three axle shafts - a driver side axle shaft, a passenger side axle shaft, and a short center axle shaft. The passenger side axle shaft and the center axle shaft can be connected or disconnected by the CAD unit. The center axle shaft is called the intermediate axle shaft in the photo below. The CAD unit is a vacuum motor that controls a splined sliding collar to lock the right axle shaft to the center axle shaft. There is a switch at the end of the CAD unit that allows illumination of the 4WD indicator lamp in the dash.
2WD Selected - front driveshaft does not rotate and CAD is disconnected. This feature minimizes front axle friction at highway speeds.
4WD HI Select on-the-fly - As the transfer case shifter is being pulled into the 4-HI position, a synchronizer in the transfer case brings the front driveshaft up to speed. As the shift is completed, a vacuum valve inside the transfer case actuates the front axle CAD unit and locks the passenger side axle shaft directly to the center axle shaft. 4WD indicator lamp is illuminated.
4WD LO Select - Vehicle must be stopped. Select 4WD LO. 4WD indicator lamp is illuminated.
Something worthy of noting - when 2WD is selected, the front drive shaft stops rotating, the left and right axle shafts are still rotating (driven by the wheels), and the center axle shaft is still rotating, BUT in the opposite direction (because of the differential). This is the reason that when 4WD Hi is selected, the synchronizer in the transfer case brings the front driveshaft up to speed first. When the front driveshaft is brought up to speed first, then all three front axle shafts are rotating at the same speed and in the same direction. Now the CAD unit can connect the passenger side axle shaft to the center axle shaft.
2WD-LO Conversion (parts needed)
Toggle switch - two position maintained switch with physical lockout
Humphrey 4 way / 2 position, 1/8" NPT, spring offset, vacuum solenoid valve Model 410/12 VDC
Appropriate length of vacuum tubing and 16 gauge automotive wire and wire connectors.
This conversion separates the operation of the transfer case manual shifter and the CAD vacuum controlled shifter. The external vacuum ports of the vacuum valve (inside the transfer case) will be disconnected and plugged. A new vacuum solenoid valve for the CAD unit operation will be installed in a location of choice. A fused switch will be installed to operate the new CAD vacuum solenoid valve. A vacuum supply line will be routed to the CAD vacuum solenoid valve. Two vacuum lines will be installed to connect the new CAD vacuum solenoid valve to the CAD unit.
Instructions for operating the new conversion are as follows:
2WD High - The transfer case selector must be in 2WD and the CAD switch must be turned off and locked out.
4WD High - Vehicle must be stopped. The CAD switch must be turned on. The 4WD indicator lamp will illuminate (truck may have to be moved slightly to engage 4WD lamp). From this point forward the transfer case can be shifted from 2WD to 4WD or 4WD to 2WD on-the-fly.
2WD Low - Vehicle must be stopped. Shift transfer case into Low Range. Leave the CAD switch turned off and locked out. The 4WD lamp will not illuminate.
4WD Low - Vehicle must be stopped. Shift transfer case into Low Range. Turn on CAD switch. The 4WD lamp will illuminate (truck may have to be moved slightly to engage 4WD lamp).
Tips
The 2WD Low Range can be very useful, especially for backing trailers in tight spaces, whether loaded or not. This is true for manual and automatic transmissions. You can basically idle the rig while backing very slowly in tight turns without the wheel hop associated with 4WD.
Also, when traveling off-road on very steep and twisty terrain with varying traction conditions in low range, you can shift from 2WD to 4WD or 4WD to 2WD on-the-fly. Just leave the transfer case in 4WD and operate the CAD switch - "Off" for 2WD, "On" for 4WD. Always unload the engine (ease up on the throttle) when making the changes to allow the CAD unit to slide the shift collar easily.
The CAD switch has a physical lockout to reduce the chance of accidental operation. When the switch not going to be used, always engage the lockout with the switch in the "Off" position.
Enjoy the conversion!
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