What is a CDL?
Your Perentie is fitted with three differentials. One on each axle (front and rear) and one in the transfer box/case. On a standard perentie, only this third diff in the transfer case can be locked, and is referred to as the Centre diff lock.
When driving on a sealed road, the centre diff allows varying torque to be transmitted to the front and rear axles. This is important when turning, as the front wheels travel further than the rear and the Centre diff allows the front and rear prop shafts to turn at slightly different speeds.
This is a problem off road, with all the torque going to the axle offering least resistance. Let’s say we lose traction to the left rear wheel; the Centre diff allows all the torque to flow to the rear prop shaft which simply spins our traction-less wheel. We’ve gone from a four wheel drive to a one wheel drive, with that wheel spinning around doing nothing useful other than flicking mud and making for a potentially good photo #doingitforthegram
Engaging the CDL ensures equal torque to each axle. As long as you have drive to both wheels on either axle, you will be making progress. In our example above, the left rear starts to spin, the CDL is locked and keeps the front and rear prop shafts rotating at the same speed. As long as both front wheels have traction, we will have drive to those wheels. If however, we lost traction to one of our front wheels as well, the CDL can no longer help us. Both prop shafts are now rotating at the same speed, but all torque is directed to the spinning wheel on each axle. It’s time for you to consider installing a front and/or rear diff locker!
Caution: Do not drive for long periods on sealed roads with your CDL locked.
Appropriate torque vectoring?
How is my CDL actuated?
While they could have opted for a nice simple lever, the futuristic engineering boffins at 1980’s Land rover decided a pneumatic actuation system was the way to go. When you pull your ‘off road’ switch out, pressurized air is pushed into one side of a rubber diaphragm, driving a pin into the gears of the Centre diff locking it in place.
How can I be sure my CDL is locked or unlocked?
Engage your CDL by pulling out the off road pneumatic switch on the dash. You should see the CDL dash light come on (assuming your light and indicator switch are functioning). Jack up either of the front wheels. You should not be able to spin that wheel by hand, as the CDL is preventing your front front prop shaft front turning independently of the rear. If you can spin that wheel freely, your CDL is NOT locked.
Trouble shooting your CDL
The pneumatic actuation system in vulnerable. First up, check you have good airflow to your diaphragm by getting under the car and locating the air hoses running into it. This diaphragm is attached to the transfer case and the little air hoses should pull off by hand. Block them with your finger and feel for suction.
If you have no pressure you may only need to run new rubber lines back up to the switch, an easy job that you may as well do anyway while you’re there!
More commonly, the diaphragm has perished and is leaking, meaning your actuation pin won’t go in or out of the CDL. This happened to me with the CDL engaged, leaving me stuck as I had a long freeway drive home. If you can get a new diaphragm, it is accessed by removing the floor pan on the driver’s side.
The CDL pneumatic switch. the two rubber hoses to the right run down to your actuator diaphragm.
The CDL actuator. The small cylinder houses the diaphragm. Here you can see the inlets for the air hoses.
Getting at the actuator was a pain… Maybe I could have cleaned her up a bit for the photo?
Have you had trouble with your CDL? Let us know in the comments below. Cheers legends happy adventures.