How a Four-Wheel Drive System Operates

No doubt the worst time for your four wheel drive system to stop working is when you have twenty plus inches of snow on the ground.

This particular vehicle is a Suzuki Vitara and the way it's 4x4 system works is similar but not exactly like some other suv's. And the reason I say this is to point out that most suv's do the same thing, only using slightly different methods to get the job done. And that job, is to allow us to enter four wheel drive without having to get out of the vehicle and lock in the hubs on the front end.

On the older Vitara's this shift on the fly system works like this. When you shift from 2 wheel high to 4 wheel high you set in operation a sequence of events that result in your vehicle entering the four wheel drive mode. Here is that sequence of events as they should occur and what I had to do when my system failed.

Located on the transfer case of the older Vitara (1999) is a switch that gets closed when you shift to four wheel high. By moving the transfer case shift lever from 2H to 4H you are literally and mechanically moving a gear from one position to another. This gear is then engaged and causes the front drive shaft to begin rotating. That's all the transfer case does, it applies power to the front drive shaft. So what about that switch then? That switch is really what puts you in four wheel drive, not the transfer case. Here's why.

When you shift from 2H to 4H, you engage that switch and a voltage is then applied to a control module up under your dashboard someplace. This control module then applies voltage to an air pump located inside the front bumper. This pump in turn begins building pressure up. At some point, roughly between 5 and 10 psi a pressure relief valve will open within the pump and this air pressure is applied by way of a tube to a device called an "actuator" located within the front differential case. This actuator, also called a "dog clutch" is made to expand like a balloon. This expansion then presses up against the bearing surfaces of the internal gears forcing them to slide over on their pins and thus engage the other gears that are attached to the axles. With all the gears thus engaged, you have entered four wheel drive. The internal pressure within the air pump also causes another switch to close and on your dash you will see a 4WD light illuminate. Everything working together as it should.

My system stopped working when the temperature turned very cold. With no four wheel drive I was limited in how much traveling around I could do as I live in a rather mountainous area of the country. This had to get fixed as soon as possible. As things would have it, the weather warmed for a few days and my four wheel drive came back as though it had never left. And as soon as the temps dropped, I was back in two wheel drive.

A local garage inspected the system and announced the problem lay with the transfer case. However that did not explain how the four wheel drive could come and go like it had been doing. When a transfer case goes..it stays gone. It is just a gear box after all. The problem seemed temperature related. I knew that putting the transfer case in 4 high only engaged the drive shaft which in turn was causing the ring and pinion gears with in the front differential to turn. With out the spider gears engaged and driving the front axles, which is what the actuator was supposed to do, I would have no four wheel drive. Having had a look under the car I knew the front drive shaft was indeed turning, therefore the transfer case was not the problem. I then decided to do the work myself and leave the mechanic who obviously did not know what he was talking about to bamboozle some other customer.

Knowing how the system was supposed to work is a huge advantage of course and I began my investigation at the air pump located inside the front bumper. I reasoned that if the pump was working, then the transfer case position switch and control module were also working or else the pump would not come on. This is the divide and conquer system. With the ignition switch in the on position and the transfer case placed in 4 High spot, I uncovered the pump and used a pressure gauge to determine if there was air pressure at the pump. There was and it was pressurizing at about 8 pounds per square inch. I found it would cycle up to 8 pounds and the pump would shut off at that point. This told me that the pump was working as it should. It would pressurize and then the pressure relief valve would dump the pressure and the pump would turn off.

With the pump known to be in good working condition and with it the transfer case position switch and the control module, that left the actuator within the differential and the pressure tubing leading up to the differential to be investigated. I didn't get too far in this because I immediately found a crack in the tubing leading up to the differential. This crack bleeding off the air and was not allowing the system to ever pressurize and the actuator wasn't able to do it's job and engage the front axles. I found the cracked tube by leaving the pump running and allowing cigarette smoke to drift over the tubing under a strong light. When the smoke suddenly blew to one side, I had found the crack. As things will do, the very cold temperatures had caused contraction and opened the crack wide enough to lose pressure. When the weather warmed the tubing expanded again and closed back up. Cause and effect.

I fitted a small piece of flexible tubing over the crack and used small clamps on either end to cinch it down and that fixed the pressure loss problem. After tightening the last clamp I glanced inside at the dashboard and sure enough, the 4WD lamp was illuminated and shining brightly up at me.