The Word On The Street - April - 2016

New But Familiar: Introducing the New Mercedes Benz 7G DCT

Mercedes Benz technology is usually a little ahead of its time, so, when I heard Mercedes was using a new direct shift gearbox, I thought, “well it’s about time.” Let’s face it: DSG transmissions have been out for quite a while.

This new Mercedes DSG (figure 1) is designated the 7G DCT (dual clutch transmission 724.0; (figure 2).

It’s a 7-speed, front wheel drive transaxle that replaces the 722.7 5-speed front wheel drive, similar to a Honda design (figure 3) and the 722.8 front wheel drive CVT (figure 4).

The reason you probably haven’t heard of these two units is they’re used in the A and B Class vehicles sold in Europe. Unless one of our service boys stationed overseas were to bring one of these vehicles stateside, you wouldn’t have worked on one. The new 7G DCT is found in the 2011-on A and B Class sold in Europe.

So what does this unit have to do with us here in the States? Well, the 7G DCT is also found in the 2013-on Mercedes Benz CLA vehicles, which are sold on this side of the Atlantic.

So it isn’t all that new. It won’t be too long before one of these vehicles shows up in your shop. This transmission can be found behind several engine sizes and is available in all wheel drive.

When you take a closer look, this DSG is a little different: It has a three-shaft layout without a fixed reverse gear. Mercedes designed this transmission without a separate fixed reverse gear to create less drag and noise.

There’s one internal, hollow shaft assembly and two separate output shafts, each with a fixed gear (figures 5 and 6). The internal shaft is splined to the K1 outer clutch and the hollow shaft is splined to the K2 inner clutch.

If you look at the power flow for first and reverse, it works much like any other DSG except for reverse. In first gear engine torque is transferred through the applied K1 (outer) clutch, driving the internal shaft with a fixed gear (1). The 1st/5th shift fork (sliding sleeve) engages with the 1st gear idler. The 1st gear idler drives output shaft 1 with a fixed gear to turn the differential pinion gear (figure 7).

In reverse, the K2 (inner) clutch is applied and drives the hollow shaft with its fixed gear (2), driving the reverse/3rd idler gear. The dual, 6th/ reverse shift fork (sliding sleeve) engages with the 3rd gear idler. The 3rd gear idler drives the internal shaft with its fixed 1st gear, driving the first gear idler. The 1st/5th shift fork (sliding sleeve) engages with the 1st gear idler.

The 1st gear idler drives output shaft 1 with a fixed gear to turn the differential pinion gear (figure 8). A little different than your normal reverse setup; I never said Mercedes wasn’t complicated.

Let’s talk about the all wheel drive that Mercedes refers to as their new 4Matic system (figure 9). It’s somewhat different from what Mercedes has used in their earlier 4Matic all wheel drive system, but not very complicated and actually similar to what we’re used to seeing in most other vehicles. It’s just new compared the earlier 4Matic system.

The new 4Matic system uses a typical power takeoff unit (PTU) flanged to a driveshaft going to the rear differential (figure 10). The new 4Matic system weighs 154 lbs (70 kg); 25% lighter than many other manufacturers’ systems.

The PTU is more compact with tapered bearings, and integrated into the main transmission, which supplies it with its own oil circuit. Other systems branch off power via an add-on PTO component.

Which takes us to the rear differential. As mentioned before, this new Mercedes torque-on-demand rear differential is similar to what other manufacturers have been using for some time. It consists of an electrohydraulic, multidisc clutch set and rotor type pump down, driven off the rear driveshaft. (figure 11).

The multidisc clutch pressure is controlled by an electronic solenoid. With the clutch open (slipping), the car behaves just like a front-wheel drive vehicle, with all torque sent to the front axle. With the clutch closed (holding), the rear axle is also driven, with torque balanced between axles according to overall road conditions.

The ABS system monitors the tire speed to the front and rear powertrain. Drive torque to front and rear is used to counter under- or oversteer conditions under load, with chassis electronic systems used in a secondary role.

In Eco mode, the front axel gets the most torque in normal conditions, while in Sport and Manual modes, activation time to the rear axel increases, providing more torque to offer greater rear wheel dynamics.

Well, there you go: something new but still quite familiar.