4x4 Won't Engage

This is a common question with early Dakota 4x4s that pops up
every couple of months on the various Dakota message boards. Here is a quick
explanation of what the problem is.  Take a look at this image,

My truck has a lift kit so yours will look slightly different. Look at the
connection just behind the tie rod end in the image. That electrical connection
is for the 4WD lamp. The front axle has a Central Axle Disconnect or CAD. The
CAD works sort of like an automatic hub and on early Dakotas works completely by
vacuum. There are two vacuum lines that connect to a small round canister on the
front axle that looks kind of like two small pie tins pressed together. You can
see it on the left side of this image.

The other end of the vacuum lines run to a switch on the transfer case. If you
look under the truck, you'll find it to the rear of the transfer case on the
driver's side of the truck. It will have 4 vacuum lines connected to it. Two
lines for the front axle, one from the engine and one to a ball canister
connected to the underside of the bed.





All the wire with the plug on the front axle does is turn the 4WD lamp on and
off. It is very common for the vacuum actuator on the front axle to stick.

Here's how to test it:

Disconnect the two vacuum lines from the front axle. With the truck running and
in two wheel drive you should have vacuum to only ONE of these two lines. Make a
note of which line has the vacuum. Now shift the truck into 4Hi and recheck the
lines. The vacuum should change to the OTHER line. If it does not, the switch on
the transfer case is bad and should be replaced. If I recall correctly, it will
cost around $30-$40 at the dealer. It's kind of like an oil pressure sending
unit and will screw into the transfer case. It has a small metal ball on the
threaded end and when the transfer case is shifted into 4WD, something inside
the transfer case pushes in the metal ball and changes the valve inside the
switch unit.





If the vacuum does change from one line to the other, the actuator on the axle
is stuck. Like I said earlier, this is very common. You can try to loosen it up
by spraying some WD40 into each vacuum tube the lines connect to. Don't be shy,
really try to flush the crud out of it with the spray. Reconnect the lines and
drive the truck forward shifting in and out of 4WD. Repeat the process rolling
backwards. If it doesn't come loose, you will have to replace the actuator. Once
it is working, repeat the flushing process about once every six months to
prevent it from happening again.





Also, sometimes when in 4WD, if you shift back into 2WD, the front actuator will
stick. Usually happens in cold weather. The transfer case will be in 2WD but the
CAD is still locked and the 4WD light will stay on. Shift the truck into reverse
and back up about 15-20 feet. That will usually pop it loose when stuck locked.