by Dennis Madden

Over the past 10 years, four-wheel-drive vehicles have become more and more popular. For a while there, everything from Ford Escorts to Pontiac 6000s had a 4X4 option. The Mercedes 4X4 hit the scene in 1990. They called it the 4Matic, and it was available in two models: the 300E and 300TE.

The 4Matic is a fully automatic transmission control system. That is, it senses the need for better traction, and engages the front drive axle as needed. So what's the big deal, you ask? Normally I'd agree; fully automatic four-wheel-drive systems are nothing new. However, Mercedes did it without electromagnetic clutches: they did it hydraulically! And to make things even more interesting, the pump sits outside the transfer case. Not just outside the transfer case, the pump actually sits about four feet away from the transfer case! I think the engineers at Mercedes may have gotten a little carried away with this one, but hey, it works.

In this edition of "Shop Talk," we'll cover a basic overview of the 4Matic system: its theory of operation, including both the electronic controls and hydraulic components.

The 4Matic system has four phases of operation: 0, 1, 2 and 3. Phase 0 is the two-wheel drive mode. During phase 1 operation, the front drive shaft is engaged, but with only about 35% of the available torque. In phase 2, the transfer case locks the front and rear drive shafts together. With this phase, there's no slip between the front and rear axles. Finally, phase 3 operation locks the rear differential so both rear wheels receive equal torque. This is in addition to the phase 2 operation of the transfer case. Figures 1 through 4 illustrate the four phases of the 4Matic system.

To get a clearer understanding of how this system works, let's break the description into four sections: electronic inputs, hydraulic components, transfer case operation, and rear differential operation.

Electronic Inputs
The 4Matic system incorporates five electrical inputs: three wheel speed sensors (one on each front wheel and one for both rear wheels), a brake switch, and a steering angle sensor. The purpose of these sensors is pretty straightforward.

Naturally, the wheel speed sensors allow the 4Matic computer to identify wheel spin, which in turn enables the computer to determine which phase of traction control to use.

The steering angle sensor lets the computer know whether the car is going straight, or around a turn. If you think about it, this makes sense: If the car is going around a corner, the outboard wheels will turn faster than the inboard wheels. When the computer sees this, it allows for a certain degree of wheel slip without engaging the transfer case.

Finally, the brake switch lets the computer know when the car is coming to a stop. The 4Matic computer returns to phase 0 operation anytime the brakes are applied.

Other electronic components include the 4Matic computer, the ABS computer (which receives the wheel speed sensor inputs, and then communicates them to the 4Matic computer), three warning lights (ABS, 4Matic Warning, 4Matic Function), and finally, the electrical actuators found on the 4Matic hydraulic unit. Figure 5 illustrates all of these components.

Hydraulic Components
The 4Matic system consists of four hydraulic components: an oil reservoir, an oil pump, a service valve, and the hydraulic unit.

The oil reservoir is obvious; it holds the oil for the system. The oil pump is also obvious; it develops the hydraulic pressure for the entire system.

The service valve (figure 6) has a series of passages that connect several of the hydraulic components. The service valve has two positions: One is for normal operation; the other is a test position. While in the test position, the entire system is depressurized.

Finally, the hydraulic unit (figure 7) engages the transfer case and rear differential. Figure 8 illustrates the location of these components.

Transfer Case
As mentioned before, the 4Matic system has four phases of operation; three of them are in the transfer case. The transfer case consists of two clutch packs and a compound planetary gear set (figure 9). The planetary gear set has three main components: the ring gear, which is connected to the output shaft of the transmission; the planetary carrier, which is connected to the rear drive shaft; and the sun gear, which is connected to the front drive shaft.

The two clutch packs are used for phases 0, 1 and 2. That is, one clutch pack locks both the front and rear drives together. The other clutch pack connects the front drive of the transfer case to the front drive shaft. I know this sounds a little confusing, so let's cover each of these three phases, one at a time.

Phase 0 is rear wheel drive only. The central differential locking clutch is applied to lock the planetary gear set (figure 10). Since the front drive shaft is released, all the torque goes to the rear drive shaft.

Phase 1 engages the front axle clutch for all wheel drive operation (figure 11

). However, during phase 1, the central differential locking clutch is released. This allows the planetary gear set to act as a differential, so the front and rear drive shaft can turn at different rates.

Phase 2 reapplies the central differential locking clutch, which bypasses the differential action of the planetary gear set. During this phase, the front and rear drive shafts are locked together, offering equal torque to both front and rear differentials.

Rear Differential Operation
Believe it of not, the differential has two clutch packs built into the carrier (figure13). This allows the 4Matic computer to lock the right and left rear axles together. This action only occurs during phase 3 operation.

Let's look at figure 14 and see how these components interact. The ABS computer monitors wheel slip and relays that to the 4Matic computer. Based on the degree of slip, the 4Matic computer signals the hydraulic unit to pressurize one or more of three hydraulic systems: two in the transfer case and one in the differential. These two components, working together, provide the various phases of 4Matic operation. Naturally, there are checks and balances in the system that monitor proper operation.

Figure 14

As with any electronic control system, the key to diagnosis is in understanding how the system operates. From there, every electronic control system is very similar. Only time will tell how thoroughly we'll need to understand the 4Matic system.